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Permit L93-0063 - LOWE LEROY - APPEAL
L93-0063 S 160TH ST - SLADE WAY LEROY LOWE APPEAL . .�..�- ............._..�... �o�, � - City of Tukwila Washington 98188 ' • John W. Rants I. �� i' 2 ; 6200 Southcenter Boulevard • Tukwila, Wash! Rants, Mayor .., I .a . 1908 NOTICE OF DECISION March 30, 1992 LeRoy Lowe P.O.'Box 3972 Bellevue, WA 98009 RE: Notice of Decision by the Planning Commission File Number: 92 -01 -APP: HILLCREST 90- 13- BLA /91 -03 -APRD This is to confirm that the Planning Commission denied the project's design as presented on March 26, 1992. (The Conditions are listed on Attachment A.) The Planning Commission also adopted the findings and conclusions contained in the Staff Report dated March 19, 1992. The decision of the Planning Commission is not final until the appeal period has elapsed, which is ten calendar days after the above date of the decision. Appeals must be filed in writing to the City Clerk by 5:00 p.m. on the final day of the appeal period. Where the final day of the appeal period falls on a weekend or holiday the appeal period will be extended to 5:00 p.m. on the next work day. If you should have any questions regarding this project please feel free to write or call. Sincerely, Darren ^i on Assistant Planner tJ : tp 11 ka M 30 92 Phone: (206) 433 -1800 • City Hall Fax (206) 433 -1833 • City of Tukwila John W. Rants, Mayor Department of Community Development Rick Beeler, Director PLANNING COMMISSION MINUTES MARCH 26, 1992 Mr. Malina called the work session to order at 6:20 p.m. Members present were Messrs. Malina, Knudson, Haggerton, Flesher, Clark, and Meryhew. Representing the staff were Vernon Umetsu and Sylvia Schnug. 90 -3 -CA: Multi - Family Review Standards Vernon Umetsu discussed the review process for the Standards. The Planning Commission agreed by consensus to hold an information meeting prior to the public hearing, to answer any questions citizen's may have regarding the Multi - Family Design Standards. Mr. Meryhew asked that on page 9 of the Design Standards, the last sentence of the second • paragraph, the word "serves" be deleted. The Planning Commission agreed by consensus. Mr. Meryhew noted that pages 11 and 12 were out of order and needed to be re- numbered. The Planning Commission agreed by consensus. Mr. Meryhew also noted that on page 14, paragraph "D ", the letter "t" in the word "the" should be capitalized. With regard to page 29, the Planning. Commission agreed to change the word "may" to "shall ". The Planning Commissioners directed that staff should hold a public information meeting on approximately April 30th and agreed to hold the public hearing on the Multi- Family Design Standards, tentatively for May 7th with the understanding that there be 20 days public notification. Vernon Umetsu noted that this is a legislative action. Commissioners are therefore free to discuss issues and voice opinions outside of their formal meetings. 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665 Planning Commission Minutes Page 2 March 26, 1992 GMA STATUS REPORT: Jack Pace discussed the review process for GMA. MR. MERYHEW MOVED THAT TWO COUNCIL MEMBERS, TWO PLANNING COMMISSION MEMBERS AND THREE OR FIVE CITIZENS SERVE ON THE COMM TI'EES FOR GMA. Representing the staff at the public hearing were Jack Pace, Darren Wilson, Gary Schulz, and Sylvia Schnug. There were no citizen's comments. MR. KNUDSON MOVED TO APPROVE THE MINUTES OF THE FEBRUARY 27, 1992 MEETING. MR. HAGGERTON SECONDED THE MOTION; THE MOTION WAS UNANIMOUSLY APPROVED. 91 -9 -DR: Tukwila Dental Center Darren Wilson presented the staff report. Staff stated that this project was carried over from the previous month for the purpose of having the applicant provide additional modulation and detail to the north perspective of the building. The applicant has done this and staff recommends that based upon this new information, the project be approved. Mr. Mauna closed the public hearing at 8:15 p.m. MR. FLESHER MOVED TO APPROVE 91 -9 -DR: TUKWILA DENTAL CENTER, MR. KNUDSON SECONDED THE MOTION; THE MOTION WAS UNANIIVIOUSLY APPROVED. P92 -0023: Non - conforming, Setbacks Jack Pace presented the staff report. He noted that this was an amendment to the non- conforming section of the Zoning Code which deals with existing single family uses. Those residences which were built legally are now non - conforming due to the annexations into Tukwila. This amendment applies to the single family dwelling unit, not to accessory structures. The amendment reads as follows: "Single family structures and uses which have non - conforming setbacks may expand along existing building lines of the existing distance to the property line is not diminished." Bill Holstein, 4251 S. 160th: Mr. Holstein stated that he's been trying to get a permit to expand his one -car, detached Planning Commission Minutes Page 4 March 26, 1992 92 -01 -APP: Hillcrest Mr. Leroy Lowe, architect for the project, handed out Exhibits A, B, and C. He stated that there are water seeps on the site which are undermining Slade Way and the adjoining down- hill property. He said Exhibit A outlines the slides that occurred in 1960 and 1961, along with the geo- technical reports and recommendations. In all cases the recommendations have been to de -water the site in order to stabilize the site and Slade Way. Dennis Joule, 32729 SE 44 He stated that he was a geo - technical engineer. He went on to say that in order to stabilize this site, the site needs to be de- watered. He said that in order to de -water a site, the spring needs to be removed which in turn removes the wetland; that is where the difficulty lies. In this case Slade Way•is involved which lies between the original landslide area and this site. Some documentation indicates that some instability exists with regard to Slade Way in the form of movement. If Slade Way is moving, then in order to stabilize it, the area needs to be de- watered. The proposed site was investigated by Cascade Geo- technical in 1990 and it was their opinion it was a stable site, but in order to maintain its stabilization it should be de- watered. Mr. Malina asked where this water would go if it were removed. Mr. Joule stated that it would somehow need to get to the storm drain system. Currently the water is going down hill and in removing it it would be collected in a pipe system and would still end up down hill. He stated that additional studies would be needed to determine the stability of Slade Way and how the proposed site and hillside tie into it. Dick Steuth, 17815 SE 146, Renton: He stated that his area of expertise lies in the field of hydrology. He stated that currently, the water is running down the hill and it could be changed so that the water is stored for a desired amount of time and released at a controlled rate so that it has less impact on the downstream properties. Mr. Clark asked what the total square footage of the parcel is. Mr. Lowe responded that the site was 107,000 sq. ft. Wes Jennings, 6219 S. Alder, Tacoma: He stated that he has been asked to serve as a consultant to Mr. Lowe and that he is a certified soil scientist. He stated that there is a wetland on the site, however, there has been a lot of junk dumped into the wetland. He stated that all the reports that have been completed regarding this area have all come to the same general conclusions; this area has been unstable in the past and there is potential for instability in the future. Also, if the surface waters and ground waters are intercepted, the stability will be significantly enhanced. Planning Commission Minutes March 26, 1992 Page 6 Gary Schulz, Urban Environmentalist, stated that Jack Pace handed out a set of criteria for studies of wetlands that staff has worked up They are based on other jurisdictions and supported by the SAO. He noted that the applicant had received a copy of this handout. He noted that the letter that was submitted by the applicant's consultant for wetlands, was not intended to be a study, but it was a review of the site. The Ordinance is clear in that a study needs to be completed and a waiver cannot be issued because it is already known that there will be impacts. Mr. Schulz stated that he reviewed the site and in his opinion it would be considered a Type 2 wetland. The site has mature trees and the cover is significant to call it a forested wetland. The wetland study should include an accurately mapped wetland boundary, a wetland rating type, and it should include data of the vegetation, soils and hydrology on the site. This would be a standard wetland delineation report. The Ordinance does not rate wetlands based on value, that's a judgmental call, but there are many different functions of wetlands and the Ordinance does not allow one to evaluate that. This wetland is a discharge wetland, however, to remove vegetation from the site would create more water because much of the water is being taken up by the vegetation. In conclusion, Mr. Schulz stated that staff is following the regulations of the ordinance in requiring a wetland delineation study, therefore staff would recommend that the appeal be denied. Mr. Clark asked why it was a Type 2 wetland versus Type 3. Mr. Schulz stated that the Type 2 wetland rating allows a greater value or consideration for forested wetlands. Forested wetlands are defined as having at least 20% cover, and this wetland meets that description. He went on to say that this wetland has emergent plants, shrubs and trees and therefore has three classes, although which is more dominant is unclear. The tree cover is greater than 20 %. Mr. Haggerton asked if there was a specific form or format that the City requires for a wetland delineation study, and does it have to be provided by a certain professional title. Mr. Schulz stated that issue is not covered clearly in the Ordinance and that is why the criteria were handed out. This list of criteria' follows closely with what King County and Pierce County is using. The study should be completed by a wetland biologist. Mr. Clark stated that he could understand Mr. Lowe's hesitation in completing a wetland delineation study because if you admit that this is a wetland then implicit in that is the fact that the wetland cannot be destroyed. Mr. Schulz stated that if we don't know enough about the wetland it's hard to say that there's even a possibility of getting a home in there with some variance on buffer. We cannot decide that right now without the study. :_U41_1,nVJ::, : LviAn i,, bl itW!;). ..:i fee, .+`.i"rfktWA.V+, n.v ..w w .w.ar. �w.✓n w-.m v�.vrnw�ww..w...._..... Planning Commission Minutes 0 Page 8 March 26, 1992 distance from Southcenter Parkway, which is approximately 330 feet. The applicant is doing some improvements to their building and would like to put up a new sign. Given the setback distance from Southcenter Pkwy., and the size of the building, staff feels that it would be appropriate to grant the increase in the size of the sign. The applicant is not in conflict with the Comprehensive Plan or the Sign Code. Staff recommends that the sign request be approved. Richard Vandenberghe, WBDC Group He stated that the sign is being cleaned up and the size has been increased. Mr. Malina closed the public hearing at 10:40 p.m. MR. HAGGERTON MOVED TO APPROVE L92 -0014: TOYS -R -US BASED ON THE STAFF'S FINDINGS AND CONCLUSIONS. MR. CLARK SECONDED THE MOTION AND THE MOTION WAS UNANIMOUSLY APPROVED. The meeting was adjourned at 10:45 p.m. ZJ City of Tukwila ukwlla John W. Rants, Mayor Department of Community Development Rick Beeler, Director STAFF REPORT TO THE PLANNING COMMISSION Prepared March 17, 1992 HEARING DATE: March 26, 1992 FILE NUMBER: 92 -01 -APP: Hillcrest 90- 13- BLA/91 -3 -APRD LOCATION: Slade Way & S. 160th Street Tukwila, WA 98188 APPELLANT: LeRoy Lowe A.I.A. Architect P.O.Box 3972 Bellevue, Wa. 98009 REQUEST: Appeal from the decision of the Planning Director regarding the Sensitive Areas Ordinance requirements (TMC. 18.45.125). STAFF: Darren Wilson, Assistant Planner Gary Schulz, Urban Environmentalist ATTACHMENTS A. Planning Division Letter 1/30/92 B. Appellant's Letter 2/18/92 C. Appellant's Letter 2/21/92 D. Site Plan E. Cascade Geotechnical Summary Deport 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (2061 431-3665 Staff Report to the 92 -01 -APP: Hillcrest Planning Commission Page 2 FINDINGS BACKGROUND On July 2, 1990, a public hearing was held for a waiver request by the applicant from the City's Sensitive Areas Moratorium. The petitioner proposed to replat /short plat 5 single -family lots for development. The moratorium Ordinance No. 1544 precluded filing of new development- related applications in sensitive areas, including sub- divisions or short sub - divisions, without a waiver. The applicant's property has slopes over 15 percent. The waiver was granted with conditions by the City Council. Those conditions allowed Mr. Lowe to submit a Boundary Line Adjustment application. On October 16, 1990, the City received applications for a short plat and a boundary line adjustment from LeRoy Lowe. After reviewing both applications, staff concluded that only a boundary line adjustment is required. On June 28, 1991 the SAO became effective. Until this time, staff could not finalize the boundary line adjustment submitted by Mr. Lowe. As a requirement of the SAO an APRD (Administrative Planned Residential Development) was required concurrently with the existing application. (See attached SAO). The APRD application was submitted on September 9, 1991. On September 16, 1991 the applicant was informed that Public Works requested additional information from the applicant prior to completing their review. Based upon the applicant's additional information, staff met with the applicant on January 13, 1992 to discuss development issues as it relates to the Sensitive Area Ordinance. On January 30, 1992 staff provided a letter with all the requirements necessary for completion of this project. The applicant objected to staff's requirements. Under section 18.45.125, if you object or disagree with DCD decision or conditions for development in a sensitive area, you may appeal to the Planning Commission. DECISION CRI1'1~RIA Administrative decisions by the Department of Community Development may be appealed as follows: TMC 18.45.125: Appeals from decisions or interpretations of the Planning Director. Any person aggrieved by any interpretation of this title by the Planning Director may present to the Planning Commission a petition Staff Report to the Planning Commission 92 -01 -APP: Hillcrest Page 3 requesting review of such decision. Any such petition shall be made within ten days of the interpretation being appealed, the interpretation being sought, and the reasons why the Planning Commission should support the appeal. The action of the Planning Commission shall be final. The SAO contains the following requirements for interpretation. TMC 18.45.030: Interpretation. The provision of this chapter shall be held to be minimum requirements in their interpretation and application and shall be liberally construed to serve the purposes of this chapter. As noted in the applicant's appeal letter (See Attachment C) he has stated: "I appeal your letter directing me to preserve and protect the seeps on my property for that action would perpetuate a dangerous situation placing Slade Way in peril." "The expenditure for wetland delineation is unreasonable in light of the City's deficiency in handling the Slade Way concerns." "We appeal to you the staff and to the 'City to work with us to provide for reasonable development of our property and the city protection of Slade Way, the sanitary sewer, and the utilities therein." For further details of the applicant's appeal, review attachments B and C. The question before the Planning Commission is whether DCD staff has correctly interpreted the Sensitive Area Ordinance. Staff has identified three areas of concern; 1. Wetland Delineation Study 2. Geotechnical Study 3. Reasonable Use Exception (TMC 18.45.115 C.) in order to alter Type 2 wetlands. I. WETLAND DELINEATION STUDY The applicant's consultant and staff conducted site investigations and identified on site wetland. The wetland is a type 2 rating; however, was not mapped on the City's Sensitive Areas Map Inventory. As noted in the background, the applicant has submitted on applications for a lot line adjustment. TMC 18.45.060 procedures requires the applicant to provide "the location at any sensitive area and buffer on the site shall be indicated on the plans submitted." The applicant has noted in his two attachments B and C, objections to this requirement. Staff Report to the 92 -01 -APP: Hillcrest Planning Commission Page 4 The Sensitive Areas Ordinance does provide for *a process for a waiver from sensitive area studies. TMC 18.45.020.F.2 states the following: TMC 18.45.020 F. 2 Sensitive Areas Special Studies If there is agreement between the Director of DCD and the applicant concerning the sensitive area classification and type, the Director of DCD may waive the requirement for sensitive area studies. There must be substantial evidence that the sensitive areas classification is correct, that there will be no detrimental impact to the sensitive areas or buffers, and that the goals, purposes, objectives and requirements of this ordinance will be followed. Based upon site investigations, the sensitive areas classification is not correct, the City's maps have not identified wetlands on this site. As shown on attachment D, the site plan; there is potential for detrimental impacts on the wetland on this site. If we do not do the study, staff can not make any factual determination if the goals, purposes, objectives and requirements of this ordinance will be followed. II. GEOTECHNICAL STUDY The applicant has provided adequate geotechnical soil reports for the initial review. The geotechnical reports performed by Cascade Geotechnical dated May 30, & August 27, 1990 indicated that the site is currently stable. The applicant has submitted an additional geotechnical letter prepared by Dennis Joule, P.E. on 2/18/92. This report discusses the probable relationship between dewatering the Hillcrest site and improving the stability of Slade Way. A decision to de -water the site and alter wetlands cannot be made prior to the completed wetland study. A geotechnical peer review conducted by the City, may be necessary after the wetland area has been mapped. In addition further review is needed as to where the water drained from this site can be drained to. M. REASONABLE USE EXCEPTION The applicant feels staff will not allow the development of this property under a reasonable use exception. Staff nor the Planning Commission can make this decision until the wetland delineation study has been conducted. The following applicable section of TMC 18.45.115 apply to the reasonable use exception as follows: Staff Report to the 92 -01 -APP: Hillcrest Planning Commission Page 5 TMC 18.45.115 C. Reasonable Use Exception 1. If application of this Chapter would deny all reasonable use of the property containing wetlands, watercourses or their buffers, the property owner or the proponent of a development proposal may apply for a reasonable use exception. 2. The application for a reasonable use exception shall be in a format specified by and filed with the Department of Community Development. Requirements may include an environmental impact statement pursuant to Washington Administrative Code 197 -11 -400. Reasonable use exceptions shall be decided by the Planning Commission following a public hearing noticed as specified in 18.92. 3. If the applicant demonstrates to the satisfaction of the Planning Commission that application of this Chapter would deny all reasonable use of the property, development may be allowed which is consistent with the general purposes of this ordinance and the public interest. 4. The Commission, in granting approval of the reasonable use exception, must determine that: a. No reasonable use with less impact on the sensitive area and its buffer is possible; b. There is no feasible on -site alternative to the proposed activities, including reduction in size or density, phasing of project implementation, change in timing activities, revision of road and lot layout, and/or related site planning activities that would allow a reasonable economic use with fewer adverse impacts to the sensitive area and its buffer, c. As a result of the proposed development there will be no increase or unreasonable threat of damage to off -site public or private property• and no threat to the public health, safety or welfare on or off the development proposal site; d. Alterations permitted shall be the minimum necessary to allow for reasonable use of the property; e. The proposed development is compatible in design, scale and use with other development with similar site constraints in the immediate vicinity of the subject property; Staff Report to the 92 -01 -APP: Merest Planning Commission Page 6 f, Disturbance of sensitive areas has been minimized by locating the necessary alterations in the buffers to the greatest extent possible; g. The inability to derive reasonable use of the property is not the result of actions by the applicant in segregating or dividing the property and creating the undevelopable condition after the effective date of this Chapter, and h. Any approved alteration of a sensitive area under this section shall be subject to conditions as established by this chapter and will require mitigation under an approved mitigation plan. If a development is approved as a reasonable use, Board of Architectural process, review and standards shall be applied. Before this section can be address by staff and Planning Commission a wetland study is needed to define the extent of this issue. The Planning Commission nor Staff can address TMC 18.45.115 due to the lack of information. In addition, the certain activities are permitted in Type 2 wetlands. The allowable uses under TMC 18.45.080 include construction of new essential roads, repair of existing uses and facilities, right -of -ways and utilities. CONCLUSIONS 1. Wetland Delineation Study Based upon TMC 18.45.060 the applicant is required to do a wetland study as.part of the boundary line adjustment review. The applicant does not meet the waiver criteria contained in TMC 18.45.020.F.2. The proposed development plan could have detrimental impacts on the wetland. 2. Geotechnical Study As noted in the report, the applicant has provided adequate geotechnical soil reports for the initial review. Further work may need to be done once the wetland study has been completed. In addition, further stormwater studies need to be done to determine the impacts on down -slope properties. 3. Reasonable Use Exception At this time, the applicant has not provided the needed information to address this issue. Once the wetland study has been completed, there may be justification to use this section of the ordinance. Staff Report to the Planning Commission. RECOMMENDATION 92 -01 -APP: Hillcrest Page 7 Based on the information submitted, staff recommends the appeal be denied. DRAFT CODIFIED VERSION OF SENSITIVE AREAS ORDINANCE NO. 1599, INCORPORATING AMENDMENTS MADE • IN ORDINANCE .NO ..:160' ORDINANCE AN ORDINANCE OF THE CITY OF TUKWILA, WASHINGTON, NO. 1599 REGULATING DEVELOPMENT ON SENSITIVE AREAS AND BUFFERS; AMENDING ORDINANCE NOS 1247 AND 1289 AND TUKWILA MUNICIPAL CODE (TMC) TITLE 18; AMENDING ORDINANCE NOS. 1331 AND 1334 AND TMC TITLE 21.04; AMENDING ORDINANCE NO 1014 AND TMC TITLE 17; AND PROVIDING FOR AN EFFECTIVE DATE. ORDINANCE AN ORDINANCE OF THE CITY OF TUKWILA, WASHINGTON AMENDING NO. 1608 CERTAIN PROVISIONS OF ORDINANCE NO. 1599 RELATING TO SENSITIVE AREAS AND SEPA REQUIREMENTS, AND AMENDING TUKWILA MUNICIPAL CODE CHAPTERS 18 AND 21. SECTION 1. FINDINGS. The City Council finds as follows: 1. The environmental features identified as sensitive areas which require regulation by Tukwila are: coal mine hazard areas, areas of potential geologic instability, watercourses, wetlands and important geologic and archaeologic resources. 2. The Sensitive Areas Ordinance will implement goals and policies of the 1978 Tukwila Comprehensive Land Use Policy Plan and the Puget Sound Water Quality Management Plan. 3. The 1990 Washington State Legislature approved ESHB 2929 (1990 Wash. Laws 17) (RCW 36.70A) which mandates that certain counties, and cities within those counties, address the protection of critical areas. Critical areas as defined in ESHB 2929 correlate generally with the Tukwila definitions and categories of sensitive areas. This Sensitive Areas Ordinance is designed to meet the challenges and satisfy the requirements of this act with regard to all critical areas except aquifer recharge areas and flood plains which will require a different regulatory approach or' are regulated in an existing title of the Tukwila Municipal Code. 4. Regulation of the use of sensitive areas benefits property owners by preventing and avoiding activities which would have adverse impacts on property. 5. Development in wetlands, watercourses and erosion hazard areas results in: 1) increased soil erosion and sedimentation of downstream water bodies, including navigable channels; 2) increased shoreline erosion; 3) degraded water quality due to increased turbidity and loss of pollutant removal processes; 4) elimination or degradation of wildlife and fisheries habitat; 5) loss of fishery resources from water quality degradation, increased peak flow rates, decreased summer low flows, and changes in the stream flow regimen; 6) loss of stormwater retention capacity and slow - release detention resulting in flooding, degraded water quality, and changes in the stream flow regimen of watersheds. 6. Development in areas of geologic instability present a danger to the development on the site as well as neighboring sites and natural resources, and require special design, construction and site development measures to minimize risks from these hazards. 7. Tukwila is in an earthquake prone region subject to ground shaking, subsidence, landslide and liquefaction. Special building design and construction measures are necessary to minimize risk from this hazard. 8. In their natural state, wetlands and watercourses provide many valuable social and ecological services, which are critical and cannot adequately be replicated or replaced, including: a. controlling flooding and stormwater runoff by storing or regulating natural flows; b. protecting water resources by filtering out water pollutants, processing biological and chemical oxygen demand, recycling and storing nutrients, and serving as settling basins for naturally occurring sedimentation; c. providing areas for groundwater recharge; d. providing habitat areas for many species of fish, wildlife, and vegetation, many of which are dependent on these water resources for their survival; e. providing open space and visual relief from intense development in urbanized area; f. providing recreation opportunities; and g. serving as areas for scientific study and natural resource education. krr&btMe,.cr °P•. Draft Codified SAO — October 18, 1991 Page 2 9. Tukwila in enacting this ordinance has relied on extensive scientific- documentation and testimony concerning these sensitive areas and the appropriate methods and mechanisms for their protection. This documentation is available in the Department of Community Development in file 89 -2 -CA: Sensitive Areas Ordinance. SECTION 2. ORDINANCE NO. 1247 AND TMC 18.06 AMENDED. TMC 18.06 is hereby amended to add the following sections: 18.06.045 Applicant "Applicant" means any person or business entity which applies for a development proposal, permit or approval subject to review under the Sensitive Areas Chapter. 18.06.049 Areas of Potential Geologic Instability. "Area of Potential Geologic Instability" means those areas subject to potential landslides and /or potential seismic instabilities. 18.06.175 Compensatory Mitigation "Compensatory mitigation" means replacing project - induced wetland and buffer losses or impacts, and includes, but is not limited to, the following: 1. Restoration — Actions performed to reestablish wetland and its buffer functional char- acteristics and processes which have been lost by alterations, activities, or catastrophic events within an area which no longer meets the definition of a wetland. 2. Creation — Actions performed to intentionally establish a wetland and its buffer at a site where it did not formerly exist. 3. Enhancement — Actions performed to improve the condition of an existing degraded wetland or its buffer so that the functions they provide are of a higher quality. 18.06.176 Constructed Wetlands or Watercourses. "Constructed wetland" or "constructed watercourses" means those wetlands or watercourses which an applicant can demonstrate were intentionally created from non - wetland or non - watercourse sites, including, but not limited to, irrigation and drainage ditches, grass -lined swales, canals, detention facilities, wastewater treatment facilities, farm ponds, and landscape amenities; and does not mean those wetlands and watercourses created through compensatory mitigation. 18.06.203 Density Transfer. "Density transfer" is a percentage number which represents a credit for housing units which are not allowed to be built in wetlands, watercourses or their buffers. The density transfer is used in a formula for determining the number of residential units allowed on the buildable portion of a lot containing wetlands, watercourses and their buffers. 18.06.255 Emergent Wetland. "Emergent wetland" means a regulated wetland with at least 30 percent of the surface area covered by erect, rooted, herbaceous vegetation as the uppermost vegetative strata. 18.06.262 Essential street, road, right-of-way or utility. "Essential street, road, right -of -way or utility" means a utility facility,utility system, street, road or right -of -way where no feasible alternative location exists based on an analysis of technology and system efficiency. 18.06.295 Forested wetland. "Forested wetland" means a regulated wetland with at least 20 percent of the surface area covered by trees greater than 20 feet in height. 18.06.322 Geologist. "Geologist" means a person who has earned a degree in geology from an accredited college or university, or a person who has equivalent educational training and has experience as a practicing geologist. 18.06.323 Geotechnical Engineer. "Geotechnical engineer" means a practicing, geotechnical /civil engineer licensed as a professional Civil Engineer with the State of Washington who has at least four years of professional employment as a geotechnical engineer with experience in landslide evaluation. 18.06.385 Impervious Surface. "Impervious Surface" means those hard surfaces which prevent or retard the entry of water into the soil in the manner that such water entered the soil under natural conditions pre- existent to developmt,jgrad' or „alters 'onny,t,e lend. Such surfaces include, but are not limited to, Draft Codified SAO — October 18, 1991 Page 3 rooftops, asphalt or concrete paving, driveways, parking lots, walkways, patio areas, storage areas, compacted surfaces, or other surfaces which similarly affect the natural infiltration or runoff patterns existing prior to development. 18.06.388 Isolated Wetlands. "Isolated wetlands" means those wetlands which: 1. are outside of and not contiguous to any 100 -year floodplain of a lake, river, or stream that is designated as a shoreline according to the City's Shoreline Master plan; and 2. have no contiguous hydric soil and hydrophytic vegetation between the wetland and any surface water. 18.06.455 Lot Coverage. "Lot Coverage" means the surface of the subject property covered with impervious surface, other than outdoor pools. 18.06.585 Ordinary High Water Mark "Ordinary high water mark" (OHWM) means the mark that will be found by examining the bed and banks of a stream and ascertaining where the presence and action of waters are so common and usual as to distinctly mark the soil from that of the abutting upland, in respect to vegetation. *In any area where the OHWM cannot be determined, the channel bank shall be substituted. In braided channels, the OHWM or substitute shall be measured so as to include the entire stream. 18.06.605 Performance Standards. Specific criteria for fulfilling environmental goals, and for beginning remedial action, mitigation, or contingency measures, which may include water quality standards, or other hydrological, geological, or ecological criteria. 18.06.625 Reach. "Reach" is a segment of a watercourse with uniform characteristics. 18.06.655 Regulated Activities "Regulated Activities" means any of the following activities which are directly undertaken or originate in a regulated wetland or watercourse or their buffers: 1. The removal, excavation, grading, or dredging of soil, sand, gravel, minerals, organic matter, or material of any kind; 2. The dumping, discharging, or filling with any material; 3. The draining, flooding, or disturbing of the water level or water table; 4. The driving of pilings; 5. The placing of obstructions; 6. The construction, reconstruction, demolition, or expansion of any structure; 7. The destruction or alteration of wetlands, watercourses or their buffers through clearing, harvesting, shading, intentional burning, or planting of vegetation that would alter the character of a regulated wetland, watercourse or buffer, provided that these activities are not part of a forest practice governed under Chapter 76.09 RCW and its rules; or 8. Activities that result in a significant change to the water sources of wetlands or watercourses. These alterations include a significant change in water temperature; physical or chemical characteristics, including quantity; and the introduction of pollutants. 18.06.656 Regulated Wetlands. "Regulated wetlands" means ponds or lakes thirty acres or less and those lands subject to the "wetland" definition contained in this chapter. Isolated wetlands that are 400 square feet or smaller in area may not require compensatory mitigation. 18.06.692 "Scrub -Shrub Wetland. "Scrub -shrub wetland" means a regulated wetland with at least 30 percent of its surface area covered by woody vegetation less than 20 feet in height as the uppermost strata. 18.06.695 Sensitive Areas. "Sensitive Areas" means wetlands; watercourses; areas of potential geologic instability other than Class 1 areas; abandoned coal mine areas; and important geological or archaeological sites. 18.06.697 Sensitive Area Buffer. "Sensitive Area Buffer" means the area contiguous to a sensitive area that is required for the continued maintenance, function and structural stability of the sensitive area. Draft Codified SAO — October 18, 1991 Page 4 18.06.698 Sensitive Area Tract. "Sensitive Area Tract" means a tract which is created to protect the sensitive area and its buffer, whose maintenance is assured, and which is recorded on all documents of title of record for all affected lots and subsequent owners. 18.06.762 Significant Tree. "Significant tree" is any woody vegetation, in any sensitive area or buffer, that is essential to or contributes to the function of a sensitive area and its buffers. 18.06.915 Utilities. "Utilities" are all lines and facilities related to the provision, distribution, collection, transmission or disposal of water, storm and sanitary sewage, oil, gas, power, information, telecommunication and telephone cable, or refuse, and includes facilities for the generation of electricity. 18.06.935 Watercourse. "Watercourse" means a course or route formed by nature or modified by man, generally consisting of a channel with a bed and banks or sides substantially throughout its length along which surface water flows naturally other than the Green /Duwamish River. The channel or bed need not contain water year- round. Watercourses do not include irrigation ditches, storm water runoff channels or devices, or other entirely artificial watercourses unless they are used by salmonids or to convey or pass through stream flows naturally occurring prior to construction of such devices. 18.06.938 Wetlands. "Wetlands" means those areas that are inundated or saturated by ground or surface water at a frequency and duration sufficient to support and that under normal circumstances do support a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include bogs, swamps, marshes, ponds, lakes and similar areas. Constructed wetlands are not considered wetlands for the purpose of this Chapter. However, those artificial wetlands intentionally created from non - wetland areas to mitigate conversion of wetlands as permitted by the City shall be considered wetlands. 18.06.939 Wetland Edge. "Wetland edge" means the boundary of a wetland as delineated based on the methodology used in the "Federal Manual for Identifying and Delineating Jurisdictional Wetlands" (1989), as revised or updated. SECTION 3. ORDINANCE NO. 1247 AND TMC 18 AMENDED. A new chapter 18.45 entitled "Sensitive Areas Overlay Zone" is hereby added to the Tukwila Municipal Code (TMC) to read as follows: 18.45.010 Purpose . A. The purpose of the Sensitive Areas Overlay Zone is to establish special standards for the use and development of lands based on the existence of natural conditions thereon in order to protect environmentally sensitive areas, including the natural character of Tukwila's wooded hillsides. B. Standards are hereby established to meet the following goals of protecting environmentally sensitive areas: 1. Minimize developmental impacts on the natural functions and values of these areas. 2. Protect quantity and quality of water resources. 3. Minimize turbidity and pollution of wetlands and fish - bearing waters and maintain wildlife habitat. 4. Prevent erosion and the loss of slope and soil stability caused by the removal of trees, shrubs, and root systems of vegetative cover. 5. Protect the public against avoidable losses, public emergency rescue and relief operations cost, and subsidy cost of public mitigation from landslide, subsidence, erosion, and flooding. 6. Protect the community's aesthetic resources and distinctive features of natural lands and wooded hillsides. 7. Prevent unlawful disturbance of archaeologic or geologic sites with historic or prehistoric artifacts. 8. Balance the private rights of individual property owners with the preservation of envi- ronmentally sensitive areas. 9. Prevent the loss of wetland and watercourse function and acreage, and strive for a gain over present conditions. Draft Codified SAO — October 18, 1991 Page 5 18.45.020 Sensitive Area Designation, Rating methodologies, Classifications and Applicability. A. This chapter applies to any use or development proposed on any legal lot of record, any portion of which is a sensitive area or a sensitive area buffer as defined in TMC Sections 18.06.695 and 18.06.697, and specifically including one or more of the following and their buffers: 1. Abandoned coal mines 2. Areas of Potential Geologic Instability - Class 2,3,4 and seismic instability areas (TMC 18.06.049 and 18.45.020(E)) 3. Wetlands (TMC 18.06.938) 4. Watercourses (TMC 18.06.935) 5. Areas that contain archaeological remnants of value to the archaeological research community, which includes but is not limited to colleges, universities or societies of professional archaeologists, or which is designated as important to save as a record of the area's past by the State Office of Archaeology and Historic Preservation. B. Sensitive Areas Maps and Inventories. 1. The distribution of many sensitive areas in Tukwila is displayed on the Sensitive Areas Maps, dated 1990, and on file with the DCD. 2. Studies, preliminary inventories and ratings of potential sensitive areas are on file with the DCD in the Sensitive Areas Notebook dated May 1990. 3. The maps and preliminary inventories and ratings are hereby adopted by reference. The actual presence or absence of sensitive areas as defined by or otherwise referred to in this chapter and as determined by the City of Tukwila will govern. The actual ratings and buffers for any sensitive area will be determined by the City of Tukwila using the methodologies and procedures provided in this chapter for each type of sensitive area. 4. All revisions, updates and reprintings of sensitive areas maps, inventories, ratings and buffers shall conform to this ordinance. C. Wetlands: For the purposes of this Chapter, wetlands are defined in TMC 18.06.938. A wetland boundary is the line delineating the outer edge of a wetland established by using the Federal Manual for Identifying and Delineating Jurisdictional Wetlands dated January 10, 1989, as revised or updated, and jointly published by the U.S. Environmental Protection Agency, the U.S. Fish and Wildlife Service, the U.S. Army Corps of Engineers, and the U.S. Soil Conservation Service. Wetland types and rating criteria are listed below. 1. Type 1 Wetlands. Those wetlands which meet any of the following criteria: a. The presence of species listed by the federal government or state as endangered, or threatened or the presence of critical or outstanding actual habitat for those species; b. Wetlands having 40% to 60% permanent open water in dispersed patches with two or more classes of vegetation; c. Wetlands equal to or greater than five acres in size and having three or more wetland classes, one of which may be substituted by permanent or open water; or 2. Type 2 Wetlands. Those wetlands which meet any of the following criteria: a. Wetlands greater than one acre in size; b. Wetlands equal to or less than one acre in size and having three classes; c. Wetlands equal to or less than one acre that have a forested wetland at least 20 percent coverage of total surface area; or d. The presence of heron rookeries or raptor nesting trees. e. The presence of native plant associations of infrequent occurrence. 3. Type 3 Wetlands. Those wetlands which are equal to or less than one acre in size and that have two or fewer wetland classes. For the purposes of this section, the U.S. Fish and Wildlife Service's Classification of Wetlands and Deepwater Habitats of the United States FWS /OBS -79/31 (Cowardin et al, 1979) contains the descriptions of wetland classes and subclasses. D. Watercourses: For the purposes of this Chapter, watercourses are defined in TMC 18.06.935. The City of Tukwila's Watercourse Study (1990) includes the methodology and criteria that will be used for determining watercourse ratings. or more wetland class comprised of Draft Codified SAO — October 18, 1991 Page 6 Watercourse ratings are based on the existing habitat functions: Each segment or reach of a watercourse is rated individually. The rating system will score a reach point total for each side of the watercourse. Watercourse types, rating scores, and rating criteria are described below. 1. Watercourse Types and Rating Scores: a. Type 1 Watercourse - 21 to 33 Points b. Type 2 Watercourse - 11 to 20 Points c. Type 3 Watercourse - -3 to 10 Points 2. Watercourse Rating Criteria a. Instream Features (1) Width of Watercourse - A measure of the average width of the channel at the ordinary high water mark. (2) Channel Capacity - Quantifies the ability of the channel to convey high flows without flooding. (3) Channel Stability - Measures the stability of the channel by evaluating evidence of bank failure, scour, and downcutting. (4) Fish Use and Fish Habitat - Anadromous and resident salmonid need protection measures if present. Rating depends on the number of different types of habitat present. b. Corridor Quality (1) Width of Unmaintained Vegetation - A• measure of the width of unmaintained vegetation from the Ordinary High Water Mark. (2) Vegetation Diversity - Quantifies the elements of terrestrial habitat associated with the watercourse corridor. (3) Corridor Barrier Function - Provides some measure of effectiveness of the buffer to limit intrusion and disturbance. (4) Surrounding Land Use - Evaluation of the land use immediately outside the of the vegetated corridor. E. Areas of Potential Geologic Instability. Areas of potential geologic instability are defined in TMC 18.06.049, and are classified as follows: 1. Class 1 areas, where landslide potential is low, which slope less than 15 percent. 2. Class 2 areas, where landslide potential is moderate, which slope between 15 and 40 per- cent and which are underlain by relatively permeable soils. 3. Class 3 areas, where landslide potential is high, which include areas sloping between 15 and 40 percent and which are underlain by relatively impermeable soils or by bedrock, and which also include all areas sloping more steeply than 40 percent. 4. Class 4 areas, where landslide potential is very high, which include sloping areas with mappable zones of ground water seepage, and which also include existing mappable landslide deposits regardless of slope. 5. Areas of potential seismic instability, with soft soils, loose sand and a shallow ground water table. F. Sensitive Areas Special Studies 1. Required. An applicant for a development proposal that includes sensitive areas, shall submit those studies as required by the City to adequately identify and evaluate the sensitive area and its buffers. 2. Waiver. If there is agreement between the Director of DCD and the applicant concerning the sensitive area classification and type, the Director of DCD may waive the requirement for sensitive area studies. There must be substantial evidence that the sensitive areas classification is correct, that there will be no detrimental impact to the sensitive areas or buffers, and that the goals, purposes, objectives and requirements of this ordinance will be followed. 3. Review of studies. The Department of Community Development will review the information submitted in the sensitive area studies to verify the information, confirm the nature and type of the sensitive area, and ensure the study is consistent with this Chapter. G. When this chapter imposes greater restrictions or higher standards upon the development or use of land than other laws, ordinances or restrictive covenants, the provisions of this chapter shall prevail. H. All other relevant standards of the TMC must also be met. Draft Codified SAO — October 18, 1991 Page 7 18.45.030 Interpretation The provisions of this chapter shall be held to be minimum requirements in their interpretation and application and shall be liberally construed to serve the purposes of this chapter. 18.45.040 Sensitive Area Buffers A. General. 1. Any land alteration must be located out of the buffer areas as required by this section. Buffers in general are intended to: a. Minimize long -term impacts of development on properties containing Sensitive Areas; b. Protect Sensitive Areas from adverse impacts during development; c. Preserve the edge of the Sensitive Area for its critical habitat value; and d. Prevent loading of potentially unstable slope formations. Land alteration is permitted for public access, supplemental planting and approved land uses of 18.45.080. An undisturbed sensitive area or buffer may substitute for the yard setback and landscape requirements of TMC 18.50 and 18.52. 2. Wetland and Watercourse Buffers are intended to: a. Provide shading to maintain stable water temperatures and vegetative cover for additional wildlife habitat; b. Provide input of organic debris, and uptake of nutrients; c. Provide an area to stabilize banks, to absorb overflow during high water events, and to allow for slight variation of aquatic system boundaries over time due to hydrologic or climatic effects; d. Reduce erosion and increased surface water runoff; e. Reduce loss of or damage to property; f. Intercept fine sediments from surface water runoff and serve to minimize water quality impacts; g. Preserve the edge for its habitat value; and h. Protect the sensitive area from human and domestic animal disturbance. 3. Buffers for Areas of Potential Geologic Instability are intended to: a. Protect slope stability; b. Provide erosion control and attenuation of precipitation surface and storm water runoff; c. Reduce loss of or damage to property; and d. Preserve the natural character of wooded hillsides where they exist. B. Special Buffer Studies. Applicants for a use or development on a legal lot of record within a sensitive area maximum buffer shall be required to conduct a sensitive area study to provide a buffer analysis for the sensitive area. This study may be waived by the DCD Director pursuant to 18.45.020 F.2. C. Ratings and Buffer Width. Ratings and appropriate buffers for wetlands and watercourses are listed below. 1. For wetlands: a. Type 1 - 100 -foot wide buffer b. Type 2 - 50 -foot wide buffer c. Type 3 - 25 -foot wide buffer 2. For watercourses, the buffer shall be as follows: a. Type 1 - 70 -foot wide buffer b. Type 2 - 35 -foot wide buffer c. Type 3 - 15 -foot wide buffer 3. Setbacks: a. All commercial and industrial developments shall be set back 15 feet and all residential development shall be set back 10 feet. This setback shall he measured from the foundation to the buffer's edge. Draft Codified SAO — October 18, 1991 Page 8 b. The Director of DCD may waive setback requirements-when a site plan demonstrates there will be no impacts to the buffer zone. 4. Variation of Standard of Creation of Variable Width Wetland/Watercourse Buffers: a. The DCD Director may reduce the standard wetland /watercourse buffers on a case -by- case basis, provided the buffer does not contain slopes 15% or greater. The approved buffer width shall not result in greater than a 50 percent reduction in width, and the reduced buffer shall not be less than 15 feet for wetlands and 10 feet for watercourses. Any buffer reduction proposal must demonstrate to the satisfaction of the DCD Director that it will not result in direct or indirect, short -term or long -term adverse impacts to wetlands or watercourses, and that: (1) The buffer is vegetated and includes an enhancement plan as may be required to improve the buffer function and value; or (2) If there is no significant vegetation in the buffer, a buffer may be reduced only if an enhancement plan is provided. The plan must include using a variety of native vegetation that improves the functional attributes of the buffer and provides additional protection for the wetland or watercourse functions and values. b. Buffers for all types of wetlands and watercourses will be increased when they are determined to be particularly sensitive to disturbance or the proposed development will create unusually adverse impacts. Any increase in the width of the buffer shall be required only after completion of a wetland or watercourse study by a qualified wetlands specialist or expert which documents the basis for such increased width. An increase in buffer width may be appropriate when (1) The development proposal has the demonstrated potential for significant adverse impacts upon the wetland or watercourse which can be mitigated by an increased buffer width; or (2) The area serves as habitat for endangered, threatened, sensitive or monitor species listed by the federal government or the State of Washington. c. Every reasonable effort shall be made to maintain the existing viable plant life in the buffers. Vegetation may be removed from the buffer as part of an enhancement plan approved by the Director of DCD. Enhancements will ensure that slope stability and wetland and watercourse quality will be maintained or improved. Any disturbance of the buffers for wetlands or watercourses shall be replanted with a diverse plant community of native northwest species that are appropriate for the specific site as determined by the DCD Director. If the vegetation must be removed, or because of the alterations of the landscape the vegetation becomes damaged or dies, then the applicant for permit must replace existing vegetation along wetlands and watercourses with comparable specimens, approved by the DCD Director, which will reproduce the existing buffer value within 5 years. d. The DCD Director shall require subsequent corrective actions and long -term monitoring of the project if adverse impacts' to regulated wetlands, watercourses or their buffers are identified. D. Areas of Potential Geologic Instability. 1. Each development proposal for a legal lot of record containing an area of potential geologic instability shall be subject to a geotechnical report pursuant to the requirements of Sections 18.45.060 and 18.45.080(6). The geotechnical report shall analyze and make recommendations on the need for and width of any buffers necessary to achieve the goals and requirements of this ordinance. Development proposals shall then include the buffer distances as defined within the geotechnical report. 2. Buffers may be increased by the DCD Director when an area is determined to be particularly sensitive to the disturbance created by a development. Such a decision will be based on a City review of the report as prepared by a qualified geotechnical consultant and by a site visit. 18.45.060 Procedures General: When an applicant submits an application for any building permit, subdivision, short subdivision or any other land use review which approves a use, development or future construction, the location of any sensitive areas and buffers on the site shall be indicated on the plans submitted. When a sensitive area is identified, the following procedures apply. The DCD Director may waive item numbers 1, 2, 4 and 5 of the following if the size and complexity of the project does not warrant that step in the procedures and the Director grants a waiver pursuant to TMC 18.45.020.F.2. Draft Codified SAO — October 18, 1991 Page 9 1. Sensitive Areas Study and Geotechnical Report: The applicant shall submit the relevant study as required in TMC 21.04.140 and this chapter. It is intended that sensitive areas studies and information be utilized by applicants in preparation of their proposals and therefore shall be undertaken early in the design stages of a project. 2. Any new residential subdivision, residential short subdivision, residential boundary line adjustment, or multiple family residential proposal which includes a sensitive area or its buffer on the site shall apply for a Planned Residential Development permit and meet the requirements of Chapter 18.46 of the Tukwila Zoning Code. 3. Denial of Use or Development: A use or development will be denied if it is determined by the DCD Director that the applicant cannot ensure that potential dangers and costs to future inhabitants of the development, adjacent and local properties, and Tukwila are minimized and mitigated to an acceptable level. 4. Pre - development Conference: The applicant, specialist(s) of record, contractor, and Department representatives will be required to attend pre- construction conferences prior to any work on the site. 5. Construction Monitoring: The specialist(s) of record shall be retained to monitor the site during construction. 6. The DCD Director may require the boundary between a sensitive area and its buffer and any development or use to be permanently identified with fencing, or with a wood or metal sign with treated wood, concrete or metal posts. Size will be determined at the time of permitting, and wording shall be as follows: "Protection of this natural area is in your care. Alteration or disturbance is prohibited pursuant to TMC 18.45. Please call the City of Tukwila for more information." 18.45.080 Uses and Standards A. General. The uses set forth in this entire section, including subsections A -H, may be located within a sensitive area or buffer, subject to the provisions of TMC 21.04 and of this section. 1. Maintenance and repair of existing uses and facilities provided no alteration or additional fill materials will be placed or heavy construction equipment used in the sensitive area or buffer. 2. Non - destructive education and research. 3. Passive recreation and open space. 4. Maintenance and repair of essential streets, roads, rights -of -way, or utilities. 5. Actions to remedy the effects of emergencies that threaten the public health, safety or welfare. B. Permitted Uses Subject to Administrative Review. The following uses may be permitted only after administrative review and approval by the DCD Director. 1. Maintenance and repair of existing uses and facilities where alteration or additional fill materials will be placed or heavy construction equipment used. 2. Construction of new essential streets and roads, rights -of -way and utilities. 3. New surface water discharges to wetlands or watercourses or their buffers from detention facilities, pre - settlement ponds, or other surface water management structures may be allowed provided that the discharge meets the clean water standards of RCW 90.48 and WAC 173.200 and 173.201 as amended, and does not increase the rate of flow to the wetland or watercourse beyond the level of the existing rate. 4. Regional stormwater detention areas may be allowed if use results in no decrease in rating of resource and enhances existing values and functions. Design shall be subject to the standards of this section and other applicable City of Tukwila standards. 5. Enhancement or other mitigation including landscaping. C. Wetlands. 1. General: a. No use or development may occur in a Type 1 and Type 2 wetland or its buffer except as specifically allowed by TMC 18.45.080 (A, B and H). Any use or development allowed is subject to the standards of this section. b. Only isolated Type 3 wetlands can be altered or relocated , and then only with the permission of the DCD Director. A mitigation or enhancement plan must be Draft Codified SAO — October 18, 1991 Page 10 developed and must comply with the standards of compensatory mitigation required in this chapter. c. Mitigation plans shall be completed for any proposals for dredging, filling, alterations, and relocation of wetland habitat allowed in TMC 18.45.080 (A, B and H). 2. Compensatory Mitigation: a. The mitigation plan shall be developed as part of a sensitive area study by a specialist approved by the DCD Director. Wetland and /or buffer alteration or relocation may be allowed only when a mitigation plan clearly demonstrates that the changes would be an improvement of wetland and buffer quantitative and qualitative functions. The plan shall follow the performance standards of this chapter and show how water quality, wildlife and fish habitat, and general wetland quality would be improved. b. In order to achieve the City's goal of no- net -loss of wetland functions and acreage, alteration of wetlands will require the applicant to provide a restoration, enhancement or creation plan to compensate for the impacts to the wetland and will compensate at a ratio of 1.5:1. c. Mitigation Location. (1) On -site compensation shall be provided except where the applicant can demonstrate that: (a) the hydrology and ecosystem of the original wetland and those who benefit from the hydrology and ecosystem will not be damaged by the on -site loss; and (b) on -site compensation is not scientifically feasible due to problems with hydrology, soils, waves, or other factors, or (c) compensation is not practical due to potentially adverse impact from surrounding land uses; or (d) existing functional values at the site of the proposed restoration are significantly greater than lost wetland functional values; or (e) that established regional goals for flood storage, flood conveyance, habitat or other wetland functions have been established and strongly justify location of compensatory measures at another site. (2) Off -site compensation shall occur within the same watershed where the wet- land loss occurred. In selecting compensation sites, applicants shall pursue siting in the following order of preference: (a) upland sites which were formerly wetlands; (b) idled upland sites generally having bare ground or vegetative cover consisting primarily of exotic introduced species, weeds, or emergent vegetation; (c) other disturbed upland. d. Mitigation Standards: The scope and content of a mitigation plan shall be decided on a case -by -case basis. As the impacts to the sensitive area increase, the mitigation measures to offset these impacts will increase in number and complexity. The components of a complete wetlands mitigation plan are as follows: (1) Baseline information of quantitative data collection or a review and synthesis of existing data for both the project impact zone and the proposed mitigation site. (2) Environmental goals and objectives that describe the purposes of the mitigation measures. This should include a description of site selection criteria, identification of target evaluation species and resource functions. Performance standards of the specific criteria for fulfilling environmental goals, and for beginning remedial action or contingency measures. They may include water quality standards, species richness and diversity targets, habitat diversity indices, or other ecological, geological or hydrological criteria. (4) Detailed construction plan of the written specifications and descriptions of mitiga- tion techniques. This plan should include the proposed construction sequence and construction management, and be accompanied by detailed site diagrams and blueprints that are an integral requirement of any development proposal. Monitoring and /or evaluation program that outlines the approach for assessing a completed project. An outline shall be included that spells out how the monitor- (3) (3) (5) Draft Codified SAO — October 18, 1991 Page 11 ing data will be evaluated by agencies that are tracking the mitigation project's progress. (6) Contingency plan identifying potential courses of action, and any corrective mea- sures to be taken when monitoring or evaluation indicates project performance standards have not been met. (7) Performance security or other assurance devices as described in TMC 18.45.135. e. Mitigation Timing: Where feasible, compensatory mitigation projects shall be . completed prior to activities that will permanently disturb wetlands and immediately after activities that will temporarily disturb wetlands. Construction of compensatory projects shall be timed to reduce impacts to existing wildlife, flora and water quality, and shall be completed prior to use or occupancy of the activity or development. 3. Essential Utilities a. Essential utilities must be constructed to minimize or where possible avoid wetland disturbance; and b. All construction must be designed to protect the wetland and its buffer against erosion, uncontrolled drainage, restriction of groundwater movement, slides, pollution, habitat disturbance, any loss of flood carrying and storage capacity and excavation or fill detrimental to the environment; and c. Upon completion of installation of essential utilities, wetlands must be restored to pre - project configuration, replanted as required and provided with maintenance care until newly planted vegetation is established; and d. All crossings must be designed for shared facilities in order to minimize adverse impacts and reduce the number of crossings. 4. Essential Streets, Roads and Rights -of -Way a. Essential streets, roads and rights -of -way must be designed and maintained to prevent erosion and avoid restricting the natural movement of groundwater; and b. Essential streets, roads and rights -of -way must be located to conform to the topography so that minimum alteration of natural conditions is necessary. The number of crossings shall be limited to those necessary to provide essential access; and c. Essential streets, roads and rights -of -way must be constructed in a way which does not adversely affect the hydrologic quality of the wetland or interrelated stream habitat. Where feasible, crossings must allow for combination with other essential utilities; and d. Upon completion of construction, the area affected must be restored to an appropriate grade, replanted according to a plan approved by the Director of DCD and provided with care until newly planted vegetation is established. 5. Public Use and Access: a. Public access shall be limited to trails, boardwalks, covered or uncovered viewing or seating areas, and displays and must be located in areas which have the lowest sensitivity to human disturbance or alteration; and b. Public access must be specifically developed for interpretive, educational or research purposes by, or in cooperation with, the City or as part of the adopted Tukwila Parks and Open Space Plan; and c. No motorized vehicle is allowed within a wetland or its buffer except as required for necessary maintenance, agricultural management or security; and d. Any public access or interpretive displays developed in a wetland and its buffer must, to the extent possible, be connected with a park, recreation or open -space area; and e. Vegetative edges, structural barriers, signs or other measures must be provided wherever necessary to protect Sensitive Areas by limiting access to designated public use or interpretive areas; and f. Access trails must incorporate design features and materials which protect water quality and allow adequate surface and ground water movement; and Must be located where they do not disturb nesting, breeding, and rearing areas and buffer areas, and must be designed so that sensitive plant and critical wildlife species are protected. 6. Dredging, digging or filling: a. Dredging, digging or filling within a wetland and its buffer may occur only with the permission of the DCD Director and only for the following purposes: (1) Uses permitted by Section 18.45.080A, B and H; or (2) Maintenance of an existing wetland; or g. Draft Codified SAO — October 18, 1991 Page 12 (3) Enhancement or restoration of habitat in conformance with an approved mitigation plan identified in a sensitive area study; or (4) Natural system interpretation, education or research when undertaken by, or in cooperation with, the City of Tukwila; or (5) Flood control or water quality enhancement by the City of Tukwila; or (6) Maintenance of existing water quality controls, for normal maintenance needs and for any diversion, rerouting, piping, or other alteration permitted by this chapter. b. Any dredging, digging or filling shall be performed in a manner which will minimize sedimentation in the water. Every effort will be made to perform such work at the time of year when the impact can be lessened. c. Upon completion of construction, the area affected must be restored to an appropriate grade, replanted according to a plan approved by the Director of DCD and provided with care until newly planted vegetation is established. D. Watercourses. 1. General: a. No use or development may occur in a watercourse or its buffer except as specifically allowed by TMC 18.45 080. Any use or development allowed is subject to the standards of this Section. b. Diverting or rerouting may only occur with the permission of the DCD Director and an approved mitigation plan. c. Any watercourse which has critical wildlife habitat or is necessary for the life cycle or spawning of salmonids, shall not be rerouted unless it can be shown that the habitat will be improved for the benefit of the species. d. Any watercourse which has no critical wildlife habitat may be rerouted if the waters flowing from the new configuration can be shown to do so in a manner that does not in any way adversely affect the habitat of a downstream salmonid bearing water. 2. Mitigation: a. Mitigation plans shall be completed for any proposals of dredging, filling, diverting, and rerouting of watercourses. b. The mitigation plan shall be developed as part of a sensitive area study by a specialist approved by the DCD Director. The plan must show how water quality, treatment, erosion control, pollution reduction, wildlife and fish habitat, and general watercourse quality would be maintained or improved. All such plans must be approved by the DCD Director. c. Mitigation Standards. The scope and content of a mitigation plan shall be decided on a case -by -case basis. As the impacts to the sensitive area increase, the mitigation measures to offset these impacts will increase in number and complexity. The components of a complete mitigation plan are as follows: (1) Baseline information of quantitative data collection or a review and synthesis of existing data for both the project impact zone and the proposed mitigation site. (2) Environmental goals and objectives that describe the purposes of the mitigation measures. This should include a description of site selection criteria, identification of target evaluation species and resource functions. Performance standards of the specific criteria for fulfilling environmental goals, and for beginning remedial action or contingency measures. They may include water quality standards, species richness and diversity targets, habitat diversity indices, or other ecological, geological or hydrological criteria. The following shall be considered the minimum performance standards for approved stream alter- ations: (a) Maintenance or improvement of stream channel dimensions, including the components of depth, width, length and gradient of the original location; (b) Bank and buffer configuration should be restored to an equal or enhanced state of the original stream; (c) The channel, bank and buffer areas shall be replanted with native vegetation which replicates or improves the original in species, sizes and densities. (d) The stream channel bed and the biofiltration systems shall be equivalent to or better than in the original stream. (3) Draft Codified SAO — October 18, 1991 Page 13 (e) The original fish and wildlife habitat shall be-maintained or enhanced. (f) Relocation of a watercourse shall not result in the new sensitive area or buffer extending beyond the development site and onto adjacent property without the agreement of the affected property owners. (g) A watercourse may be rerouted. (4) Detailed construction plan of the written specifications and descriptions of mitiga- tion techniques. This plan should include the proposed construction sequence and construction management, and be accompanied by detailed site diagrams and blueprints that are an integral requirement of any development proposal. Monitoring and /or evaluation program that outlines the approach for assessing a completed project. An outline shall be included that spells out how the monitor- ing data will be evaluated by agencies that are tracking the mitigation project's progress. (6) Contingency plan identifying potential courses of action, and any corrective mea- sures to be taken when monitoring or evaluation indicates project performance standards have not been met. Performance security or other assurance devices as described in TMC 18.45.135. (5) (7) d. Mitigation Timing: DCD approved plans must have the mitigation construction completed before the existing watercourse can be modified. 3. Essential Utilities a. Essential utilities must be constructed to minimize, or where possible avoid, disturbance of the watercourse and its buffer; and b. All construction must be designed to protect the watercourse and its buffer against erosion, uncontrolled drainage, restriction of groundwater movement, slides, pollution, habitat disturbance, any loss of flood carrying and storage capacity and excavation or fill detrimental to the environment; and c. Upon completion of installation of essential utilities, watercourses and their buffers must be restored to pre - project configuration, replanted as required and provided with maintenance care until newly planted vegetation is established; and d. All crossings must be designed for shared facilities in order to minimize adverse impacts and reduce the number of crossings. 4. Essential Streets, Roads and Rights -of Way a. Essential streets, roads and rights -of -way must be designed and maintained to prevent erosion and avoid restricting the natural movement of groundwater; and b. Essential streets, roads and rights -of -way must be located to conform to the topography so that minimum alteration of natural conditions is necessary. The number of crossings shall be limited to those necessary to provide essential access; and c. Essential streets, roads and rights -of -way must be constructed in a way which does not adversely affect the hydrologic quality of the watercourse and its buffer. Where feasible, crossings must allow for combination with other essential utilities; and d. Upon completion of construction, the area affected must be restored to an appropriate grade, replanted according to a plan approved by the Director of DCD and provided with care until newly planted vegetation is established. 5. Public Use and Access: a. Public access shall be limited to trails, boardwalks, covered or uncovered viewing and seating areas, and displays and must be located in areas which have the lowest sensitivity to human disturbance or alteration; and b. Public access must be specifically developed for interpretive, educational or research purposes by, or in cooperation with, the City or as part of the adopted Tukwila Parks and Open Space Plan; and c. No motorized vehicle is allowed within a watercourse or its buffer except as required for necessary maintenance, agricultural management or security; and d. Any public access or interpretive displays developed along a watercourse and its buffer must, to the extent possible, be connected with a park, recreation or open -space area; and e. Vegetative edges, structural barriers, signs or other measures must be provided wherever necessary to protect watercourses and their buffers by limiting access to designated public use or interpretive areas; and Draft Codified SAO — October 18, 1991 Page 14 f. Access trails must incorporate design features and materials which protect water quality and allow adequate surface and ground water movement; and Must be located where they do not disturb nesting, breeding and rearing areas and must be designed so that sensitive plant and critical wildlife species are protected. 6. Piping: a. Piping of any watercourse should be avoided. Piping may be allowed in any watercourse if it is necessary for access purposes. Piping may be allowed in Type III watercourses if the applicant complies with the conditions of this section, including: (1) excess capacity to meet needs of the system during a 100 -year flood event, and (2) flow restrictors, and water quality and existing habitat enhancement procedures. b. No process that requires maintenance on a regular basis will be acceptable unless this maintenance process is part of the regular and normal facilities' maintenance process or unless the applicant can show funding for this maintenance is ensured; and c. Piping in a watercourse sensitive area shall be limited and shall require approval of the DCD Director. Piping projects shall be performed pursuant to the following applicable standards: (1) The conveyance system shall be designed to comply with the standards in current use .and recommended by the Department of Public Works. (2) Where allowed, piping shall be limited to the shortest length possible as determined by the Director of DCD to allow access onto a property. Where water is piped for an access point, those driveways or entrances shall be consolidated to serve multiple properties where possible, and to minimize the length of piping. (4) When required by the Director of DCD, watercourses under drivable surfaces shall be contained in an arch culvert using oversize or superspan culverts for rebuilding of a stream bed. These shall be provided with check dams to reduce flows, and shall be replanted and enhanced according to a plan approved by the Director of DCD. When necessary to provide for fish passage, fish ladders shall be one foot vertical rise to ten foot horizontal distance, or as approved by the Washington State Department of Fisheries. (6) Stormwater runoff shall be detained and infiltrated to preserve the watercourse channel's dominant discharge. All construction shall be designed to have the least adverse impact on the watercourse, buffer and surrounding environment. (8) Piping shall be constructed during periods of low flow, or as specified by the State Department of Fisheries. (9) Water quality must be as good or better for any water exiting the pipe as for the water entering the pipe, and flow must be comparable. 7. Dredging, digging or filling a. Dredging, digging or filling within a watercourse or its buffer may occur only with the permission of the DCD Director and only for the following purposes: (1) Uses permitted by Section 18.45.080(1), (2) and (3); or (2) Maintenance of an existing watercourse; or (3) Enhancement or restoration of habitat in conformance with an approved mitigation plan identified in a sensitive area study; or (4) Natural system interpretation, education or research when undertaken by, or in cooperation with, the City of Tukwila; or (5) Flood control or water quality enhancement by the City of Tukwila; or (6) Maintenance of existing water quality controls, for normal maintenance needs and for any diversion, rerouting, piping, or other alteration permitted by this chapter. b. Any dredging, digging or filling shall be performed in a manner which will minimize sedimentation in the water. Every effort will be made to perform such work at the time of year when the impact can be lessened. c. Upon completion of construction, the area affected must be restored to an appropriate grade, replanted according to a plan approved by the Director of DCD and provided with care until newly planted vegetation is established. g. (3) (5) (7) Draft Codified SAO — October 18, 1991 Page 15 E. Areas of Potential Geologic Instability. 1. General: The uses permitted in the underlying zoning district may be undertaken on sites which contain areas of potential geologic instability subject to the standards of this section and the requirements of a geotechnical study. 2. Exemptions: Any temporary slope which has been created through legal grading activities may be regraded without application of this chapter under an approved permit. 3. Alterations: a. Prior to permitting alteration of an area of potential geologic instability, the applicant must demonstrate one of the following: (1) There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no significant risk to the proposed development or surrounding properties; or (2) The area of potential geologic instability can be modified or the project can be designed so that any potential impact to. the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability. b. Where any portion of an area of potential geologic instability is cleared for development, a landscaping plan for the site shall include tree re- planting with an equal mix of evergreen and deciduous trees, preferably native, and approved by the Director of DCD. Replacement vegetation shall be sufficient to provide erosion and stabilization protection. 4. Geotechnical Report: a. The applicant shall submit a Geotechnical Report appropriate to both the site conditions and the proposed development. A geotechnical investigation shall be required for development in Class 2, Class 3, Class 4 areas, and any areas identified as seismic or coal mine hazard areas unless waived pursuant to TMC 18.45.020.F.2. b. Geotechnical reports for Class 2 areas shall include at a minimum a site evaluation review of available information regarding the site and a surface reconnaissance of the site and adjacent areas. Subsurface exploration of site conditions is at the discretion of the geotechnical consultant. c. Geotechnical reports for Class 3, Class 4 and Coal Mine Hazard areas shall include a site evaluation review of available information about the site, a surface reconnaissance of the site and adjacent areas, and a subsurface exploration of soils and hydrology conditions. Detailed slope stability analysis shall be done if the geotechnical consultant recommends it in Class 3 or Coal Mine Hazard areas, and must be done in all Class 4 areas. d. Seismic hazard areas shall include an evaluation of site response and liquefaction potential for the proposed development area. For one or two story single family dwellings this evaluation may be based on the performance of similar structures under similar foundation conditions. For proposed developments including occupied structures other than one and two story single family dwellings, the evaluation shall include sufficient subsurface exploration to provide a site coefficient (S) for use in the static lateral force procedure described in the Uniform Building Code. e. Applicants shall retain a Geotechnical Engineer to prepare the reports and evaluations required in this Section. The Geotechnical Report and completed Site Evaluation checklist shall be prepared in accordance with the generally accepted geotechnical practices, under the supervision of and signed and stamped by the Geotechnical Engineer. The report shall be prepared in consultation with the appropriate City department. Where appropriate, a Geologist must be included as part of the geotechnical consulting team. The report shall make specific recommendations concerning development of the site. f. The geotechnical engineers required under this section must meet the qualification standards approved by the Director of DCD. Applicants shall provide a list of qualifications of the firm or individuals who will be doing the technical studies, and those shall be approved by the Director of DCD. If the engineers' credentials are not sufficient, the City may require applicants to use a different engineer or firm which does meet the City's standards. The opinions and recommendations contained in the report shall be supported by field observations and, where appropriate or applicable, by literature review, conducted by the Geotechnical Engineer which shall include appropriate explorations, such as borings or test pits, and an analysis of soil characteristics conducted by or under the supervision of the engineer in accordance with standards of the American Society of Testing and Materials or other applicable standards. If the evaluation involved geologic g. Draft Codified SAO - October 18, 1991 Page 16 evaluations or interpretations, the report shall be reviewed and approved by a Geologist. h. An independent review of geotechnical reports will be required per TMC 21.04.140. 5. Disclosures, Declarations and Covenants. a. It shall be the responsibility of the applicant to submit, consistent with the findings of the geotechnical report, structural plans which were prepared and stamped by a Structural Engineer. The plans and specifications shall be accompanied by a letter from the Geotechnical Engineer who prepared the Geotechnical Report stating that in his /her judgment, the plans and specifications conform to the recommendations in the Geotechnical Report and that the risk of damage to the proposed development site, from soil instability will be minimal subject to the conditions set forth in the report; and the proposed development will not increase the potential for soil movement. b. Further recommendations signed and sealed by the Geotechnical Engineer shall be provided should there be additions or exceptions to the original recommendations based on the plans, site conditions, or other supporting data. If the Geotechnical Engineer who reviews the plans and specifications is not the same engineer who prepared the geotechnical Report, the new engineer shall, in a letter to the City accompanying the plans and specifications, express his or her agreement or disagreement with the recommendations in the Geotechnical Report and state that the plans and specifications conform to his or her recommendations. c. The Architect or Structural Engineer shall submit to the City, with the plans and specifications, a letter of notation on the design drawings at the time of permit application stating that he or she has reviewed the Geotechnical Report, understands its recommendations, has explained or has had explained to the owner the risks of loss due to slides on the site, and has incorporated into the design the recommendations of the Report and established measures to reduce the potential risk of injury or damage that might be caused by any earth movement predicted in the Report. d. The applicant, or the owner of the site if the applicant is not the owner, shall submit a letter to the City, with the plans and specifications, stating that he or she understands and accepts the risk of developing in an area with potential unstable soils and that he or she will advise, in writing, any prospective purchasers of the site, or any prospective purchasers of structures or portions of a structure on the site, of the slide potential of the area. e. The owner shall execute a covenant, running with the land, on a form provided by the City. The City will file the completed covenant with the King County Department of Records and Elections at the expense of the applicant or owner. A copy of the recorded covenant will be forwarded to the owner. This covenant shall include: (1) The legal description of the Property; (2) A statement explaining that the site is in an area of potential instability; of the risks associated with development on the site, of any conditions or prohibitions on development, and of any features in this design which will require maintenance or modification to address anticipated soil changes; A statement waiving any claims the owner or his /her successors or assigns may have against the City of Tukwila for any loss, or damage to people or property either on or off the site resulting from soil movement arising out of the issuance of any permit(s) authorizing development on the site; (4) The date of issuance and number of the permit authorizing the development. 6. Assurance Devices: Whenever the City determines that the public interest would not be served by the issuance of a permit in an area of potential geologic instability without assurance of a means of providing for restoration of areas disturbed by and repair of property damage caused by slides arising out of or occurring during construction, the DCD Director may require assurance devices pursuant to Section 18.45.135. 7. Construction Monitoring: a. The applicant shall retain a Geotechnical Engineer to monitor the site during construction. The applicant shall preferably retain the Geotechnical Engineer who prepared the final Geotechnical recommendations and reviewed the plans and specifications. If a different consultant is retained by the owner, the new Geotechnical Engineer shall submit a letter to the City stating whether or not he /she agrees with the opinions and recommendations of the original Geotechnical Engineer. Further recommendations, signed and sealed by the Geotechnical Engineer, and supporting data shall be provided should there be exceptions to the original recommendations. b. The Geotechnical Engineer shall monitor, during construction, compliance with the recommendations in the Geotechnical Report, particularly site excavation, shoring, soil (3) Draft Codified SAO — October 18, 1991 Page 17 support for foundations including piles, subdrainage installations, soil compaction and any other geotechnical aspects of the construction. Unless otherwise approved by the City, the specific recommendations contained in the soils report must be implemented by the owner. The Geotechnical Engineer shall make written, dated monitoring reports on the progress of the construction to the City at such timely intervals as shall be specified. Omissions or deviations from the approved plans and specifications shall be immediately reported to the City. The final construction monitoring report shall contain a statement from the Geotechnical Engineer that, based upon his or her professional opinion, site observations and testing during the monitoring of the construction, the completed development substantially complies with the recommendations in the Geotechnical Report and with all geotechnical- related permit requirements. Occupancy of the project will not be approved until the report has been reviewed and accepted by the DCD Director. 8. Conditioning and Denial of Use or Developments: a. Substantial weight shall be given to ensuring continued slope stability and the resulting public health, safety and welfare in determining whether a development should be allowed. b. The City may impose conditions that address site work problems; which could include but are not limited to limiting all excavation and drainage installation to the dryer season, or sequencing activities such as installing erosion control and drainage systems well in advance of construction. A permit will be denied if it is determined by the DCD Director that the development will increase the potential of soil movement that results in an unacceptable risk of damage to the proposed development, its site, or adjacent properties. F. Abandoned Mine Areas. 1. Development of a legal lot of record containing an abandoned coal mine area may be permitted when a geotechnical report shows that significant risks associated with the abandoned mine workings can be eliminated or mitigated so that the site is safe. Approval shall be obtained from the DCD Director before any building or land altering permit processes begin. 2 Any building setback or land alteration shall be based on the geotechnical report. 3. The City may impose conditions that address site work problems, which could include but are not limited to limiting all excavation and drainage installation to the dryer season, or sequencing activities such as installing drainage systems or erosion controls well in advance of construction. A permit will be denied if it is determined that the development will increase the potential of soil movement or result in an unacceptable risk of damage to the proposed development or adjacent properties. G. Areas of important geological or archaeological evidence. 1. Development on a legal lot of record determined to have historic or prehistoric geological or archaeological evidence, shall be prohibited until that evidence has been studied or researched for any valuable information about our history. Removal or salvage of the evidence shall be done in accordance with RCW 27.53, and shall be performed in a timely manner. 2. Once the geologic or archaeological evidence or articles have been studied or researched, or the importance of the site is declared to be marginal or not of use to the scientific community, development shall be allowed on the site. Development shall not begin on such a site until the DCD Director gives approval. H. Permitted Uses Subject to Exception Approval. Other uses may be permitted upon receiving a reasonable use exception pursuant to TMC 18.45.115. A use permitted through a reasonable use exception shall conform to the procedures of this .chapter and be consistent with the underlying zoning. 18.45.090 Sensitive Areas Tracts A. In development proposals for planned residential or mixed area use developments, short subdivisions or subdivisions, boundary line adjustments and binding site plans, applicants shall create sensitive areas tracts, in lieu of an open space tract, per the standards of TMC 18.46.080. B. Applicants proposing development, involving uses other than those listed in TMC 18.45.090A, on parcels with sensitive areas or their buffers, may elect to establish a sensitive area tract which shall be: 1. If under one ownership, owned and maintained by the ownership, which assures the protection of the tract; or 2. Held in common ownership by multiple owners who shall collectively be responsible for maintenance of the tract; or Draft Codified SAO — October 18, 1991 Page 18 3. Dedicated for public use if acceptable to the City or other appropriate public agency. 18.45.115 Exceptions A. General. With the approval of the Director of DCD, isolated wetlands that are 400 square feet or smaller in area, and which are low value according to, the rating methodology used in the City's Water Resource Rating and Buffer Study, may not require the compensatory mitigation standards of this chapter. B. Piping. Piping will be allowed in Type 1 and Type 2 watercourses only where relocation or alteration of a watercourse is denied and would result in denial of all reasonable use. C. Reasonable Use Exceptions. 1. If application of this Chapter would deny all reasonable use of the property containing wetlands, watercourses or their buffers, the property owner or the proponent of a development proposal may apply for a reasonable use exception. 2. The application for a reasonable use exception shall be in a format specified by and filed with the Department of Community Development. Requirements may include an environmental impact statement pursuant to Washington Administrative Code 197 -11 -400. Reasonable use exceptions shall be decided by the Planning Commission following a public hearing noticed as specified in 18.92. 3. If the applicant demonstrates to the satisfaction of the Planning Commission that application of this Chapter would deny all reasonable use of the property, development may be allowed which is consistent with the general purposes of this ordinance and the public interest. 4. The Commission, in granting approval of the reasonable use exception, must determine that: a. No reasonable use with less impact on the sensitive area and its buffer is possible; b. There is no feasible on -site alternative to the proposed activities, including reduction in size or density, phasing of project implementation, change in timing activities, revision of road and lot layout, and /or related site planning activities that would allow a reasonable economic use with fewer adverse impacts to the sensitive area and its buffer; c. As a result of the proposed development there will be no increased or unreasonable threat of damage to off -site public or private property and no threat to the public health, safety or welfare on or off the development proposal site; d. Alterations permitted shall be the minimum necessary to allow for reasonable use of the property; e. The proposed development is compatible in design, scale and use with other development with similar site constraints in the immediate vicinity of the subject property; f. Disturbance of sensitive areas has been minimized by locating the necessary alterations in the buffers to the greatest extent possible; The inability to derive reasonable use of the property is not the result of actions by the applicant in segregating or dividing the property and creating the undevelopable condition after the effective date of this Chapter; and h. Any approved alteration of a sensitive area under this section shall be subject to conditions as established by this chapter and will require mitigation under an approved mitigation plan. If a development is approved as a reasonable use, Board of Architectural process, review and standards shall be applied. g. 18.45.120 Variances A. The Board of Adjustment shall review requests pursuant to TMC 18.72 for variance from the standards of this chapter unless excepted by Section 18.45.115. B. If a variance is granted, it shall be the minimum necessary to accommodate the permitted uses of the underlying zoning district, proposed by the application and the scale of the use may be reduced as necessary to meet this requirement. 18.45.125 Appeals A. Any aggrieved party who objects to or disagrees with Department of Community Development decisions or conditions for development in a sensitive area, shall appeal to the Planning Commission. Any such appeal shall be made in writing within ten days of the interpretation, condition or decision being appealed, and shall set forth the basis for the appeal. Draft Codified SAO — October 18, 1991 Page 19 B. In considering appeals of decisions or conditions, the following shall be considered: 1. The intent and purposes of the sensitive areas ordinance; 2. Technical information and reports considered by the Department of Community Development; and 3. Findings of the DCD Director shall be given substantial weight. 18.45.130 Recording Required The property owner receiving approval of a use or development pursuant to this chapter shall record the City- approved site plan clearly delineating the wetland, watercourse, areas of potential geologic instability or abandoned mine and their buffer designated by section 18.45.020 and 18.45.040 with the King County Division of Records and Election. The face of the site plan must include a statement that the provision of this chapter, as now or hereafter amended, control use and development of the subject property and provide for any responsibility of the property owner for the maintenance or correction of any latent defects or deficiencies. 18.45.135 Assurance Device A. In appropriate circumstances, the Department of Community Development Director may require a letter of credit or other security device acceptable to the City, to guarantee performance and maintenance requirements of this Chapter. All assurances shall be on a form approved by the City Attorney. B. When alteration of a sensitive area is approved, the Director of the Department of Community Development may require an assurance device, on a form approved by the City Attorney, to cover the monitoring costs and correction of possible deficiencies. Monitoring of alterations may be required for up to five (5) years. C. Release of the security does not absolve the property owner of responsibility for maintenance or correcting latent defects or deficiencies. 18.45.140 Assessment Relief A. Fair Market Value. The King County Assessor shall consider sensitive area regulations in determining the fair market value of land under RCW 84.34. B. Current Use Assessment. Established sensitive area tracts, as defined in TMC 18.06.698 and provided for in TMC 18.45.090, shall be classified as open space and owners thereof may qualify for current use taxation under RCW 18.34; provided such landowners have not received density credits, or setback or lot size adjustments as provided in TMC Chapter 18.46. C. Special Assessments. Landowners who qualify under Section 18.45 140B shall also be exempted from special assessments on the sensitive area tract to defray the cost of municipal improvements such as sanitary sewers, storm sewers and water mains. SECTION 4. ORDINANCE NOS. 1247 AND 1249 AND TMC 18.46 AMENDED. TMC 18.46 is hereby amended as follows: 1. TMC 18.46.010 is hereby amended to read in its entirety as follows: 18.46.010 Purpose. It is the purpose of this chapter to encourage imaginative site and building design and to create open space in residential developments by permitting greater flexibility in zoning requirements than is permitted by other sections of this title. Furthermore, it is the purpose of this chapter to: 1. Promote the retention of significant features of the natural environment, including topography, vegetation, watercourses, wetlands, and views; 2. Encourage a variety or mixture of housing types; 3. Encourage maximum efficiency in the layout of streets, utility networks, and other public improvements; and 4. Create and /or preserve usable open space for the enjoyment of the occupants and the general public. 2. TMC 18.46.030 is hereby amended to read in its entirety as follows: 18.46.030 Permitted uses. The following uses are allowed in planned residential development: 1. In R -1 districts, dwellings may be permitted which are harmonious with the surrounding residential character and built environment. Single unit detached dwellings will be preferable to attached units; however, attached two -unit dwellings will be considered if Draft Codified SAO — October 18, 1991 Page 20 necessary to accommodate interior zero lot lines for projects with sensitive areas and /or sensitive area buffers. 2. In R -2, R -3, R -4, and RMH districts, residential developments of all types regardless of the type of building in which such residence is located, such as single - family residences, duplexes, triplexes, fourplexes, rowhouses, townhouses or apartments; provided, that all residences are intended for permanent occupancy by their owners or tenants. Hotels, motels, and travel trailers and mobile homes and trailer parks are excluded; 3. Accessory uses specifically designed to meet the needs of the residents of the PRD such as garages and recreation facilities of a noncommercial nature; 4. In planned residential developments of ten acres or more, commercial uses may be permitted. Commercial uses shall be limited to those which are of a neighborhood convenience nature such as beauty or barber shops, drugstores, grocery stores and self - service laundries. 3. TMC 18.46.040 is hereby amended to read in its entirety as follows: 18.46.040 Site acreage minimum. The minimum site for a planned residential development shall be one acre, except sites con- taining sensitive areas and their buffers. 4. TMC 18.46.050 is hereby repealed. 5. TMC 18.46.060 is hereby amended to read in its entirety as follows: 18.46.060 Relationship of this chapter to other sections and other ordinances. A. 1. Lot Size. The minimum lot size provisions of other sections of this code are waived within the planned residential development. The number of dwelling units per net acre permitted in the underlying zone shall serve as the criteria to determine basic PRD density. 2. Building Height. Building heights may be modified within a PRD when it assists in maintaining natural resources, significant vegetation, and enhances views within the site without interfering with the views of adjoining property. For increases in building height, there shall be a commensurate decrease in impervious surface. 3. Setbacks. Yard requirements of TMC. 18.50 shall be waived within the PRD; however, setbacks and design of the perimeter of the PRD shall be comparable to or compatible with the bulk and streetscape of the existing development of adjacent properties or the type of development which may be permitted. B. Off - Street Parking. Off- street parking shall be provided in a PRD in the same ratio for types of buildings and uses as required in Chapter 18.56. However, for multiple family zoned sites with sensitive areas, a minimum of 2 parking stalls per unit will be allowed, with a 50% compact stalls allowance, and parking stalls in front of carports or garages will be allowed if the design does not affect circulation. C. Platting Requirements. The standards of the subdivision code for residential subdivisions shall apply to planned residential developments if such standards are not in conflict with the provisions of this chapter. Upon final approval of the PRD, filing of the PRD shall be in accordance with procedures of the subdivision code if any lots are to be transferred. D. Impervious Surface. The maximum amount of impervious surface calculated for the total development allowed on sensitive areas sites will be 50% for each single family development and each multi - family development. E. Recreation Space Requirements. Sensitive areas and stands of significant trees may be counted as area required to meet the recreation space minimums if usable passive recreation opportunities within these areas are demonstrated. Opportunities could include connection and continuation of area -wide trail systems, wildlife or scenic viewing opportunities, or picnic areas. F. Landscape and Site Treatment for Sites with Class 2, Class 3 and Class 4 Geologic Hazard Areas. 1. Down slope and side yard buffers - Elevations and off -site perspectives shall show minimum landscape coverage of 25% of the structures at time of project completion with anticipated 40% coverage within 15 years. This standard may supplement or be in lieu of the applicable landscape yard requirement. Draft Codified SAO — October 18, 1991 Page 21 2. Any road or access drive which cuts approximately perpendicular to a slope to the ridgeline of a hill shall have minimum five foot planted medians. The tree shall be a species that provides a branch pattern sufficient to provide at maturity, 50 % coverage of the pavement area. Roads or drives which require retaining walls parallel to the topographic line shall plant roadside buffers of northwest native plant species. G. The Board of Architectural Review shall review guidelines for single- and multi - family developments. The design and review of the PRD shall also utilize the guidelines of TMC 18.60.050. H. For single - family developments, the site plans shall include placement and footprint of the residences, driveways and roads. 6. TMC 18.46.070 is hereby amended to read in its entirety as follows: 18.46.070 Density standards. A. The basic density shall be the same as permitted by the underlying zone district. The dwelling units per net acre for the residential zones are as provided in Chapter 18.50. B. Single family. 1. In R -1 single - family residential districts, the planning commission may recommend, and the City Council may authorize, a minimum lot size not less than the yard requirements of the R -1 -7.2 district following findings that the amenities or design features listed in subsections a. through d. of 18.46.070B.2. below are substantially provided. 2. In R -1 single - family residential districts on sites containing sensitive areas or their buffers, the planning commission may recommend, and the City Council may authorize, a minimum lot size less than the yard requirements of the R -1 -7.2 district following findings that the amenities or design features listed below in subsections a. through d. are substantially provided: a. At least fifteen percent of the natural vegetation is retained (in cases where significant stands exist); and b. Advantage or enhancement is taken of unusual or significant site features such as views, watercourses, or other natural characteristics; and c. Separation of auto and pedestrian movement, especially in or near areas of recreation; and d. Development aspects of the PRD complement the land use policies of the compre- hensive plan. C. In multiple - family residential districts, the planning commission may recommend, and the City Council may authorize, a dwelling unit density not more than twenty percent greater than permitted by the underlying zones or an increase equal to the allowable density credits as set forth in 18.46.070(D), if the site contains sensitive areas or buffers following findings that the amenities or design features listed below are substantially provided: 1. A variety of housing types are offered; 2. At least fifteen percent of the natural vegetation is retained (in cases where significant stands exist); 3. Advantage is taken or enhancement is achieved of unusual or significant site features such as views, watercourses, wetlands or other natural characteristics; 4. Separation of auto and pedestrian movement, especially in or near areas of recreation; and 5. Developmental aspects of the PRD complement the land use policies of the comprehensive plan. D. Density Transfer 1. Density transfers are intended to provide for the protection of wetlands, watercourses, and associated buffers while allowing development which is consistent with existing zoning to the greatest extent possible. 2. Density transfers are the percentage credits to be used in calculating the number of dwelling units for a residential site containing undevelopable sensitive areas or buffers. The calculation of the maximum units per buildable acre of a site with protected areas shall be equal to: (DU = Dwelling Units) [(DU /acre)(Buildable Acres)] + [(DU /acre)(Sensitive Areas and Buffer)(Density Transfer)] Draft Codified SAO — October 18, 1991 - Page 22 3. Density transfer credits shall be determined from the table below: PERCENTAGE OF SITE IN DENSITY SENSITIVE AREAS & BUFFER TRANSFER 1 -10 30% 11 - 20 27% 21 - 30 24% 31 -40 21% 41 - 50 18% 51 - 60 15% 61 - 70 12% 71 -80 9% 81 -90 6% 91 -100 3% 4. The density transfer can only be used within the development proposal site. Any such modifications shall be reviewed and approved through the site development process in TMC 18.60. 5. Development of the transferred density shall be confined to buildable areas of the site, and shall not intrude on sensitive areas or their buffers. 7. TMC 18.46.080 is hereby amended to read in its entirety as follows: 18.46.080 Open space. A. Each planned residential development shall provide not less than twenty percent of the gross site area for common open space which shall: 1. Provide either passive or active recreation concentrated in large usable areas; and 2. Network with the trail and open space system of the city and provide a connection and extension, if feasible; 3. Be under one ownership, owned and maintained by the ownership; or be held in common ownership by all of the owners of the development by means of a home owners or similar association. Such association shall be responsible for maintenance of the common open space; or be dedicated for public use, if acceptable to the city or other appropriate public agency. B. Planned residential developments shall set aside sensitive areas and their buffers in a sensitive areas tract as required by Chapter 18.45.090, and will be exempted from other open space requirements of this section. 8. TMC 18.46.090 is hereby amended to read in its entirety as follows: 18.46.090 Relationship to adjacent areas. A. The design and layout of a planned residential development shall take into account the integration and compatibility of the site to the surrounding areas. The perimeter of the PRD shall be so designed as to minimize any undesirable impact of the PRD on adjacent properties. B. Setbacks from the property lines of the PRD shall be comparable to or compatible with, those of the existing development of adjacent properties or, if adjacent properties are undeveloped , the type of development which may be permitted. 9. TMC 18.46.110 is hereby amended to read in its entirety as follows: 18.46.110 Application procedure required for PRD approval. A. Filing of Application. Application for approval of the PRD shall be made on forms prescribed by the Department of Community Development and shall be accompanied by a filing fee as required in Chapter 18.88 and by the following: 1. Justification for the density bonus, if requested by the applicant, 2. Program for development including staging or timing of development, 3. Proposed ownership pattern upon completion of the project, 4. Basic content of any restrictive covenants, Draft Codified SAO — October 18, 1991 Page 23 5. Provisions to assure permanence and maintenance . of common open space through a home owners association, or similar association, condominium development or other means acceptable to the city; 6. An application for rezone may be submitted with the PRD application if rezoning is necessary for proposed density. Fees for rezone request shall be in addition to those of the PRD application; 7. An application for preliminary plat may be submitted with the PRD application, if necessary. Fees for the subdivision shall be in addition to those of the PRD application. 8. Graphic images of development in any sensitive area or buffer, including photo- montage or computer - generated perspectives in a standardized format required by the Director of the Department of Community Development. 9. Every reasonable effort shall be made to preserve existing trees and vegetation and integrate them into the subdivision's design by preparing a tree inventory of the significant vegetation on site as part of the preliminary plat application. A tree and vegetation retention/removal plan shall be part of any preliminary plat application. Such tree and vegetation retention/removal plan shall assure the preservation of significant trees and vegetation. B. Planning Commission Public Hearing. The planning commission shall hold at least one public hearing on the proposed PRD, and shall give notice thereof pursuant to Chapter 18.92 of this title. The public hearing shall not be held before completion of all necessary and appropriate review by city departments. This review shall be completed within a reasonable period of time. C. Planning Commission Recommendation. Following the public hearing, the planning commission shall make a report of its findings and recommendations with respect to the proposed PRD and the criteria of this chapter and forward the report to the City Council. D. City Council Public Hearing. 1. After receipt of the planning commission report, the City Council shall hold a public hearing on the proposed PRD as recommended by the planning commission. The City Council shall give approval, approval with modifications, or disapproval to the proposed PRD. 2. The PRD shall be an exception to the regulations of the underlying zoning district. The PRD shall constitute a limitation on the use and design of the site unless modified by ordinance. 10. TMC 18.46.112 is hereby amended to read in its entirety as follows: 18.46.112 Review criteria. The Planning Commission and City Council shall find that the proposed development plans meet all of the following criteria in their decision making. 1. Requirements of the subdivision code for the proposed development have been met, if appropriate; 2. Reasons for density bonuses meet the criteria as listed in Section 18.46.070; 3. Adverse environmental impacts have been mitigated; 4. Compliance of the proposed PRD to the provisions of this chapter and Chapter 18.45; 5. Time limitations, if any, for the entire development and specified stages have been documented in the application; 6. Development in accordance with the comprehensive land use policy plan and other relevant plans; 7. Compliance with the BAR review guidelines (TMC 18.60.050); and 8. Appropriate retention and preservation of existing trees and vegetation recommended by the Director of Community Development. 11. TMC 18.46.120 is hereby amended to read in its entirety as follows: 18.46.120 Application procedures for building permit. The following procedures are required for approval of construction for the proposed planned residential development: 1. Time Limitation. A complete application for the initial building permit shall be filed by the applicant within twelve months of the date on which the City Council approved the PRD. An extension of time for submitting an application may be requested in writing by the applicant, and an extension not exceeding six months may be granted by the Director of Draft Codified SAO — October 18, 1991 Page 24 DCD. If application for the initial building permit is not made within twelve months or within the time for which an extension has been granted, the plan shall be considered abandoned, and the development of the property shall be subject to the requirements and limitations of the underlying zone and the subdivision code. 2. Application. Application for building permit shall be made on forms prescribed by the DCD and shall be accompanied by a fee as prescribed by the building code. 3. Documentation Required. All schematic plans either presented or required in the approved PRD plans shall be included in the building permit application presented in finalized, detailed form. These plans shall include but are not limited to landscape, utility, open space, circulation, and site or subdivision plans. Final plats and public dedication documents must be approved by the City Council before the issuance of any building permits. 4. Sureties Required for Staging. If the PRD is to be developed in stages, sureties, or other security device as shall be approved by the City Attorney, shall be required for the complete PRD. The various stages or parts of the PRD shall provide the same proportion of open space and the same overall dwelling unit density as provided in the final plan. 5. Department of Community Development Action. The Department of Community Development shall determine whether the project plans submitted with the building permit are in compliance with and carry out the objectives of the approved PRD. Following approval of the DCD, the city clerk shall file a copy of the approved PRD plan with the official records of the city and the originals shall be recorded with the King County department of records and elections. After all approvals, the official zoning map shall be amended to reflect the PRD by adding the suffix "PRD" to the designation of the underlying zone. SECTION 5. ORDINANCE NO. 1247 AND TMC 18.70.050 AMENDED. TMC 18.70.050 is hereby amended to add subsections (6) and (7) as follows: (6) In wetlands, watercourses and their buffers, existing structures that do not meet the requirements of Chapter 18.45 may be remodeled, reconstructed or replaced, provided that: (a) the new construction does not further intrude into or adversely impact an undeveloped sensitive area or the required buffer; and (b) the new construction does not threaten the public health, safety or welfare, and (c) the structure otherwise meets the requirements of this chapter. In areas of potential geologic instability, coal mine hazard areas, and buffers, as defined in TMC 18.45, existing structures may be remodeled, reconstructed or replaced, provided that: (a) the new construction is subject to the geotechnical report requirements and standards of TMC 18.45.080E and F; and (b) the new construction does not threaten the public health, safety or welfare; and (c) the new construction does not increase the potential for soil or result in unacceptable risk or damage to existing or potential development or to neighboring properties; and (d) the structure otherwise meets the requirements of this chapter. (7) SECTION 6. ORDINANCE NO. 1014 AND TMC TITLE 17 AMENDED. 1. TMC 17.08.030 is hereby amended to read in its entirety as follows: 17.08.030 Principles of acceptability. The following principles shall determine the acceptability of boundary line adjustments and short subdivisions: 1. Create legal building sites with respect to zoning and health regulations; 2. Establish access to a public road for each segregated parcel; 3. If adjacent to another municipality or King County, take into consideration the subdivision standards of that jurisdiction, as well as the requirements of this code; 4. Make adequate provision for: drainage ways, streets, alleys, other public ways, water supplies and sanitary wastes, as deemed necessary. 5. Comply with Chapter 17.20, Design standards for the subdivision of land, and Chapters 17.24 and Chapter 17.28 Minimum standards for Residential and Commercial Subdivision Design, respectively. Draft Codified SAO — October 18 1991 Page 25 6. Actions by the applicant to get a boundary line adjustment or short subdivision shall not result in the inability to derive reasonable economic use of the property or create an undevelopable lot under Chapter 18.45.115 unless that lot is to be dedicated for exclusive use as open space or common tract. 2. TMC 1Z08.040 is hereby amended to read in its entirety as follows: 17.08.040 Application requirements. A. Application for a short subdivision shall be made with the Department of Community Development on forms prescribed by that office. Said application shall be accompanied by ten copies of the short subdivision plat. B. If the subject site is within the sensitive area overlay zone, administrative review and approval of a Planned Residential Development (PRD) shall be required for the proposed short subdivision and boundary line adjustment. The standards and criteria for PRD design and review are those listed in TMC 18.46. The procedure shall be those listed in 17.08.060. C. The short subdivision plat and boundary line adjustment shall conform to the following requirements: 1. Shall be a neat and accurate drawing by a land surveyor on reproducible material at a decimal scale. The plat map shall measure between eight and one -half inches by eleven inches and eight and one -half inches by fourteen inches. 2. Shall show how the proposed subdivision will be served by streets and utilities. 3. Each application shall be accompanied by a preliminary title report dated within thirty calendar days of the submission date. 3. TMC 17.08.060 is hereby amended to read in its entirety as follows: 17.08.060 Review Procedures. A. Referral to Other Departments. Upon receipt of an application for a short subdivision, the planning division of the Department of Community Development shall transmit one copy of the application to each member of the short subdivision committee, and one copy to any department or agency deemed necessary. The application shall be transmitted at least five working days prior to the short subdivision committee meeting. B. Notice to adjacent property owners. If a PRD is being processed as part of the short plat application per 17.08.040 (b), notice shall be mailed to all property owners within 300 feet, fifteen days prior to the Short Subdivision Committee meeting, notifying them of the pending proposal and requesting comments. C. Short Subdivision Committee. The short subdivision committee shall consist of the director of the office of community development, who shall be chairman; the public works director; and the fire chief; or their designated representatives. D. Short Subdivision Committee Meeting. A meeting of the short subdivision committee, attended by the applicant at his option, shall be held no later than twenty calendar days from receipt of a complete application. Said meeting shall be open to the public. 1. Quorum. All members of the short subdivision committee or their representative must be present in order for action to be taken. 2. Action. The short subdivision committee may approve, approve with modifications, or deny the application for a short subdivision. The decision of the short subdivision committee shall be made at the meeting and the applicant notified in writing of such decision within three working days. An additional meeting may be called if a decision is not reached at the first meeting. The second meeting .should be no later than seven days after the first meeting. An applicant may request to have an application on which the short subdivision committee has taken action reopened by the committee if it is found by the director of the office of community development and the applicant that new information has come to light that might affect the action taken by the short subdivision committee. 3. Appeal. The decision of the short subdivision committee shall.be final, unless an appeal by any aggrieved party is made to the planning commission within ten calendar days of the date of decision of the short subdivision committee. Said appeal shall be in writing to the planning commission and filed with the office of community development. The planning commission shall act on said appeal within forty calendar days unless an extension thereto is agreed to, in writing, by the applicant. The decision of the planning commission shall be final and conclusive unless, within ten calendar days, any aggrieved party files with the city clerk a written appeal addressed to the City Council. The City Council shall act on said appeal within twenty days of the date of appeal. Draft Codified SAO — October 18, 1991 Page 26 4. TMC 17.12.030(a) and (b) are hereby amended to read in their entirety as follows: 17.12.030 Preliminary plat procedures. (a) Application. (1) Application for a preliminary plat shall be filed with the planning division on forms prescribed by that office. A complete application must be filed at least thirty calendar days prior to the date of the planning commission meeting at which it is to be considered. (2) The application shall be accompanied by a fee of eight hundred dollars plus seventy -five dollars for each proposed lot. At least five copies of the preliminary plat shall be submitted, which shall be prepared by a land surveyor. (4) Fifteen copies of the preliminary plat, photographically reduced to eight and one -half inches by eleven inches or eight and one -half inches by fourteen inches, shall accompany the application. A list of the names and addresses of all owners of record of property within three hundred feet of the external boundaries of the proposed subdivision. (6) Environmental information shall be prepared and submitted in accordance with the guidelines established under the State Environmental Policy Act of 1971, as amended, Title 21 of this code, as amended, and the Sensitive Areas Overlay District, as amended in Title 18.45 of this code, as may be amended thereafter. Said information is a part of and must accompany the preliminary plat application. (b) Preliminary Plat Requirements. The following shall be part of the preliminary plat: (1) Vicinity Map. Adequate to readily identify the location of the plat in relation to its surrounding vicinity. (2) Preliminary Plat. The preliminary plat shall include all of the following: (A) The subdivision name and number, and the name and address of the land surveyor; (B) The date of preparation, the true north point, a graphic scale and legal description of the property proposed for subdivision. Plats shall be drawn to an appropriate engineering (decimal) scale; (C) All existing conditions shall be delineated. The location, width and names of all existing or prior platted streets or other public open spaces, permanent buildings and structures, and section and municipal corporation lines within or adjacent to the property proposed for subdivision shall be shown. In the case of a resubdivision, the lots, blocks, streets, alleys, easements and parks of the original subdivision being vacated, shall be shown by dotted lines in their proper position in relation to the new arrangement of the subdivision; the new subdivision being clearly depicted in solid lines so as to avoid ambiguity. Existing sewer and water lines, culverts, or other underground facilities within the property proposed for subdivision indicating pipe sizes, grades and exact location as obtained from public records shall be shown. Boundary lines of adjacent lands, whether subdivided or unsubdivided shall be indicated by dotted lines for a distance of three hundred feet from the external boundary of the property proposed for subdivision and shall include the existing land use classification. (D) A survey of existing trees and vegetation with a retention /removal plan which assures the preservation of significant trees and vegetation. (E) Existing contours (solid) and proposed contours (dotted) at intervals of five feet or less and referenced to the United States Coast and Geodetic Survey (USC & GS) datum. All contour lines shall be extended at least one hundred feet beyond the external boundaries of the property proposed for subdivision; (F) The names, locations, widths, and other dimensions of proposed streets, alleys, easements, parks and other open spaces, reservations, and utilities together with the purpose and any conditions or limitations of such reservations clearly indicated; (G) Clearly indicate the source of water supply, method of sewage disposal, and manner of surface runoff control; (H) Indicate the approximate dimensions of each lot and all lot and block numbers; (I) Indicate the acreage of land to be subdivided, the number of lots and the area of the smallest lot; (3) (5) Draft Codified SAO — October 18, 1991 Page 27 5. TMC 17.20.010 is hereby amended to read in its entirety as follows: 17.20.010 Purpose. It is the purpose of this chapter to provide for the protection of valuable, irreplaceable envi- ronmental amenities and to make urban development as compatible as possible with the ecological balance of the area. Goals are to preserve sensitive areas and their buffers, protect groundwater supply, prevent erosion, and to preserve trees, vegetation, and drainage patterns. These are beneficial to the city in lessening the costs of development to the city as a whole, to the subdivider in creating an attractive and quality environment, and in attainment of the goals and objectives of the Tukwila comprehensive land use policy plan. 6. TMC 17.20.020 is hereby amended to read in its entirety as follows: 17.20.020 Environmental considerations. A. Unsuitable Land. Land which meets the definition of a sensitive area or its buffer as defined in TMC 18 or is subject to the Flood Zone Control Ordinance as defined in TMC 16.52, shall be platted to reflect the standards and requirements of the Sensitive Areas Overlay zone, TMC 18.45, 18.46, the Planned Residential Development, and the Flood Zone Control Ordinance, TMC 16.52. B. Trees. Every reasonable effort shall be made to preserve existing trees and vegetation and integrate them into the subdivision's design by preparing a tree inventory of the significant vegetation on site as part of the preliminary plat application. 7. TMC 17.20.040 is hereby amended to read in its entirety as follows: 17.20.040 Grading. Prior to any grading within a proposed or approved subdivision, a land alteration permit shall be obtained in accordance with the provisions of Chapter 70 of the Uniform Building Code. 8. TMC 17.20.050 is hereby amended to read in its entirety as follows: 17.20.050 Streets. A. Extension. Proposed street systems shall extend existing streets at the same or greater width, unless otherwise approved by the department of public works and authorized by the City Council in approval of the plat. Where appropriate, streets shall be extended to the boundaries of the plat to insure access to neighboring properties. The City's goal is to have an integrated system of local streets rather than a system of cul -de -sacs. Grading of steep topography may be necessary to achieve this objective. However, in sensitive areas, the layout and construction of streets shall follow the standards and procedures of the sensitive areas overlay zone. B. Names. All proposed street names or numbers shall be subject to approval by the Department of community development. C. Intersections. Any intersection of public streets, whatever the classification, shall be at right angles as nearly as possible and not be offset insofar as practical. 9. TMC 17.24.020 is hereby amended to read in its entirety as follows: 17.24.020 Street layout. Street layout shall provide for the most advantageous development of the subdivision, adjoining areas, and the entire neighborhood. Evaluation of street layout shall take into consideration potential circulation solutions. While it is important to minimize the impact to the topography from creating an integrated road system, improved site development and circulation solutions shall not be sacrificed to minimize the amount of cut and fill requirements of the proposal. Where sensitive areas are impacted, the standards and procedures for rights-of-way in the sensitive areas overlay zone shall be followed. Draft Codified SAO — October 18, 1991 Page 28 10. TMC 17.24.040 is hereby amended to read in its entirety as follows: 17.24.040 Public rights -of -way. A. Right -of -way Width. Street widths may vary according to function, traffic generated and topography. The following minimum street widths for streets, as defined in the Tukwila comprehensive land use policy plan, shall apply unless otherwise approved by the department of public works and authorized by the City Council in approval of the preliminary plat or by the short subdivision committee in the case of short plats. TYPE OF STREET RIGHT -OF -WAY PAVEMENT Major Arterial 80 feet 60 feet Secondary Arterial 75 feet 48 feet Collector 60 feet 36 feet Local Street 50 feet * 30 feet Cul -De -Sac Roadway 40 feet 26 feet Turnaround 80 feet diameter 60 feet diameter Alley 20 feet 15 feet * Right -of -way may be reduced to 40 feet with five foot easements on each side if the subject roadway will be traversing or adjacent to wetlands and watercourses. B. Design. The design and alignment of all public streets shall conform to the following standards unless otherwise approved by the department of public works: 1. Street Intersection Offsets. Where street intersections must be offset, such offsets shall not measure less than two hundred fifty feet from centerline to centerline. 2. Cul -de -sacs. Cul -de -sacs shall not exceed a length of six hundred feet unless authorized in accordance with Section 17.32.010. 3. Street Grades. Street grades shall not exceed 15 percent. However, provided there are no vehicular access points, grades may be allowed up to 18 percent, for not more than 200 feet when: a. exceeding the grades would facilitate a through street and connection with the larger neighborhood; and b. the greater grade would minimize disturbance of sensitive slopes;and c. the Fire Marshal grants approval. 4. Tangents. Minimum tangents shall conform to department of public works standards. 5. Horizontal Curves. Minimum curve radii shall conform to department of public works standards. 6. Vertical Curves. Changes in grade shall conform to department of public works standards. C. Right -of -way Improvement. All right -of -way improvements shall conform to department of public works standards. D. Utilities. All utilities designed to serve the subdivision shall be placed underground and, if located within a sensitive area, shall be designed to meet the standards of the sensitive areas overlay zone. Those utilities to be located beneath paved surfaces shall be installed, including all service connections, as approved by the department of public works; such installation shall be completed and approved prior to application of any surface materials. 1. Sanitary Sewers: Unless septic tanks are specifically approved by the appropriate health agencies, sanitary sewers shall be provided at no cost to the city and designed in accordance with city standards. 2. Storm Drainage: An adequate drainage system shall be provided for the proper drainage of all surface water; the amount of runoff shall be determined by the rational method. Cross drains shall be provided to accommodate all water flow and shall be of sufficient length to permit full width roadway and required slopes. The diameter to be provided shall be determined by Manning's Equation, but in no case shall the inner diameter be less than twelve inches. 3. Water System: The water distribution system including the locations of fire hydrants shall be designed and installed in accordance with city standards. Draft Codified SAO — October 18 1991 Page 29 E. Public Use and Service Areas. 1. Due consideration shall be given by the subdivider to the allocation of adequately sized areas for public service or usage. 2. Easements. Easements may be required for the maintenance and operation of utilities as specified by the public works department. F. Blocks. 1. Length. Blocks should not be less than three hundred feet nor more than one thousand feet in length. Where circumstances warrant, the planning commission may require one or more public pathways of not less than six feet nor more than fifteen feet in width dedicated in the city to extend entirely across the width of the block at locations deemed necessary. 2. Width. Blocks shall be wide enough to allow two tiers of lots, except where abutting a major street or prevented by topographical conditions or size of the property, in which case the planning commission may approve a single tier. G. Lots. 1. Arrangement. Insofar as practical, side lot lines shall be at right angles to street lines or radial to curved street lines. Each lot must have access to a public street that is approved at the time of plat review; however rather than designing flag lots, access shall be accomplished with common drive easements. Location of yards shall reflect the prevailing pattern within the neighborhood. For example, if adjacent developed lots front on the street the subdivision's lots fronting the street should also establish front yards for those lots. 2. Minimum Size. The size, shape, and orientation of lots shall meet or exceed the minimum area and width requirements of the applicable zoning classification and shall be appropriate for the type of development and use contemplated. 3. Corner Lots. Corner lots may be required to be platted with additional width to allow for the additional side yard requirements. 4. Through Lots. Residential through lots are not encouraged and shall only be approved if there is a topographic or traffic safety concern preventing double tiered lots. Approved through lots shall be permitted access to only one street, unless otherwise approved by the department of public works and shall provide a. fifteen foot rear yard buffer of native vegetation. 5. Property Corners at Intersections. All lot corners at intersections of dedicated public rights -of- way shall have a minimum radius of twenty -five feet. H. Trees. Each lot within a new subdivision shall be landscaped with at least one tree in the front yard to create a uniform streetscape. Installation shall be required per 17.12.040(3) I. Other Improvements. 1. Monuments. a. Concrete permanent control monuments shall be established at each and every controlling corner of the subdivision. Interior monuments shall be located as determined by the department of public works. b. All other lot corners shall be marked with suitable metal or wood markers. 2. Street Signs. The subdivider shall be responsible for the initial cost of any street name or number signs, or street markings, including installation thereof, necessary in the subdivision as required by the department of public works. SECTION 7. ORDINANCE NOS. 1331 AND 1344 AND TMC 21.04 AMENDED. 1. TMC 21.04.040 is hereby amended to read in its entirety as follows: 21.04.040 Definitions -- Additional. In addition to those definitions contained within WAC 197 -11 -700 through 799, when used in this chapter the following terms shall have the following meanings, unless the content indicates otherwise: 1. "Department" means any division, subdivision or organizational unit of the city established by ordinance, rule or order. 2. "Early notice" means the city's response to an applicant stating whether it considers issuance of the determination of significance likely for the applicant's proposal. 3. "Environmentally sensitive area" — see 21.04.300. Draft Codified SAO — October 18, 1991 Page 30 4. "Notice of action" means the notice of the time for commencing an appeal of a SEPA determination that the city or the applicant may give following final city action upon an application for a permit or approval when said permit or approval does not have a time period set by statute or ordinance for commencing an appeal. 5. "Official notice" means the notice that the city shall give of the date and place for commencing an appeal of final city action upon an application for a permit or approval where said permit or approval has a time period set by statute or ordinance for commencing appeal. 6. "SEPA Rules" means WAC Chapter 197 -11 adopted by the Department of Ecology. 2. TMC 21.04.130 is hereby amended to read in its entirety as follows: 21.04.130 Threshold determination -- Review at conceptual stage. A. If the city's only action on a proposal is a decision on a building permit or other license that requires detailed project plans and specifications, the applicant may request in writing that the city conduct environmental review prior to submission of the detailed plans and specifications. B. In addition to the environmental documents an applicant may be required to submit the following information: 1. Conceptual site plans and building plans; 2. Other information as the responsible official may determine. 3. Environmentally sensitive areas studies as described in TMC 21.04.140 for sensitive areas. 3. TMC 21.04.140 is hereby amended to read in its entirety as follows: 21.04.140 Threshold determinations -- Environmental checklist. A. A completed environmental checklist shall be filed at the same time as an application for a permit, license, certificate or other approval not exempted by this chapter. The checklist shall be in the form of WAC 197 -11 -960 with the following additions: 1. If the site is an environmentally sensitive area, a sensitive area study that meets the requirements of the SEPA official may be required. The SEPA official may waive any study requirements determined to be unnecessary for review of a particular use or application. Environmentally sensitive area studies shall have three components: a site analysis, an impact analysis, and proposed mitigation measures. More or less detail may be required for each component depending on the size of the project, severity of potential impacts, and availability of information. Funding for a qualified professional, selected and retained by the City shall be paid for by the applicant, to review the geotechnical reports on Class 2 and 3 Landslide, Seismic and Coal Mine Hazard areas if the geotechnical report indicates Class 3 or 4 characteristics; and will be required in all Class 4 Landslide Hazard areas. 2. Identification of conflicts with the policies of the comprehensive land use policy plan and proposed measures to reduce the conflicts; 3. Description of the objectives of the proposal, the alternative means of accomplishing these objectives, comparison of the alternatives and indication of the preferred course of action. B. A checklist is not needed if the city and the applicant agree an EIS is required, SEPA compliance has been completed, or SEPA compliance has been initiated by another agency. C. The city shall use the environmental checklist to determine the lead agency and, if the city is the lead agency, for making the threshold determination. D. For private proposals, the applicant is required to complete the environmental checklist. The city may provide information as necessary. For city proposals, the department initiating the proposal shall complete the environmental checklist for that proposal. E. The city may decide to complete all or part of the environmental checklist for a private proposal, if either of the following occurs: 1. The city has technical information on a question or questions that is unavailable to the private applicant; or 2. The applicant has provided inaccurate information on previous proposals or on proposals currently under consideration. It : Draft Codified SAO — October 18 1991 Page 31 4. TMC 21.04.150 is hereby amended to read in its entirety as follows: 21.04.150 Threshold determinations -- Mitigated DNS. A. The responsible official may issue a determination of nonsignificance (DNS) based on conditions attached to the proposal by the responsible official or on changes to, or clarifications of, the proposal made by the applicant. B. An applicant may request in writing early notice of whether a Determination of Significance (DS) is likely. The request must: 1. Follow submission of a permit application and environmental checklist for a nonexempt proposal for which the department is lead agency; and 2. Precede the city's actual threshold determination for the proposal. C. The responsible official's written response to the request for early notice shall: 1. State whether the city currently considers issuance of a DS likely and, if so, indicate the general or specific areas of concern that are leading the city to consider a DS; and 2. State that the applicant may change or clarify the proposal to mitigate the indicated impacts, and may revise the environmental checklist and /or permit application as necessary to reflect the changes or clarifications. D. When an applicant submits a changed or clarified proposal, along with a revised environmental checklist, the city shall base its threshold determination on the changed or clarified proposal. 1. If the city indicated specific mitigation measures in its response to the request for early notice, and the applicant changed or clarified the proposal to include those specific mitigation measures, the city shall issue and circulate a determination of nonsignificance if the city determines that no additional information or mitigation measures are required. 2. If the city indicated areas of concern, but did not indicate specific mitigation measures that would allow it to issue a DNS, the city shall make the threshold determination, issuing a DNS or DS as appropriate. 3. The applicant's proposed mitigation measures, clarifications, changes or conditions must be in writing and must be specific. 4. Mitigation measures which justify issuance of a mitigated DNS may be incorporated in the DNS by reference to agency staff reports, studies or other documents. E. The city shall not act upon a proposal for which a mitigated DNS has been issued for fifteen days after the date of issuance. F. Mitigation measures incorporated in the mitigated DNS shall be deemed conditions of approval of the licensing decision and may be enforced in the same manner as any term or condition of the permit or enforced in any manner specifically prescribed by the city. Failure to comply with the designated mitigation measures shall be grounds for suspension and /or revocation of any license issued. G. If the city's tentative decision on a permit or approval does not include mitigation measures that were incorporated in a mitigation DNS for the proposal, the city should evaluate the threshold determination to assure consistency with WAC 197- 11- 340(3)(a) relating to the withdrawal of a DNS. H. The city's written response under subsection (c) of this section shall not be construed as a determination of significance. In addition, preliminary discussion of clarification or changes to a proposal, as opposed to a written request for early notice, shall not bind the city to consider the clarifications or changes in its threshold determination. 5. TMC 21.04.270 is hereby amended to read in its entirety as follows: 21.04.270 SEPA -- Policies. A. The policies and goals set forth in this chapter are supplementary to those in the existing authorization of the city. B. The city adopts by reference the policies in the following city codes, ordinances, resolutions and plans as now existing or as may be amended hereafter: 1. Zoning Code -TMC Chapter 18; 2. Shoreline Master Plan- Ordinance 898; 3. Comprehensive Land Use Policy Plan- Ordinance 1246; 4. Long Range Parks and Open Space Plan - Ordinance 1315; 5. Subdivision Ordinance -TMC Chapter 17.04; 6. Comprehensive Sewer Plan - Resolution 904; Draft Codified SAO — October 18, 1991 Page 32 7. Comprehensive Water Plan - Resolution 873; 9. Uniform Building Code -1982 Edition - Ordinance 1287; 9. Transportation Improvement Plan - Resolution 917; 10. Annexation Policy Plan - Resolution 626; 11. Sidewalk Ordinance- Ordinance 1233; 12. Standard Specifications for Municipal Construction - Ordinance 1250. 6. TMC 21.04.300 is hereby amended to read in its entirety as follows: 21.04.300 Environmentally Sensitive Areas. A. Environmentally sensitive areas designated on the zoning maps and /or as now defined in TMC 18.45.020, or as hereafter amended, designate the locations of environmentally sensitive areas within the City and are adopted by reference. In addition to those areas identified in WAC 197 -11- 908 and for purposes of this chapter, environmentally sensitive areas shall also include wooded hillsides, and the Green /Duwamish River and its shoreline zone as defined by the Tukwila Master Program. For each environmentally sensitive area, all categorical exemptions within WAC 197 -11 -800 are applicable. B. The City shall treat proposals located wholly or partially within an environmentally sensitive area no differently than other proposals under this chapter, making a threshold determination for all such proposals. The City shall not automatically require an EIS for a proposal merely because it is proposed for location in an environmentally sensitive area. C. Certain exemptions do not apply on lands covered by water, and this remains true regardless of whether or not lands covered by water are mapped. SECTION 8. SEVERABILITY. If any portion of this Ordinance, as now or hereafter amended, its application to any person or circumstances is held invalid or unconstitutional, such adjudication shall not affect the validity of the Ordinance as a whole, or any section, provision, or part thereof not adjudicated to be invalid or unconstitutional and its application to other persons or circumstances shall not be affected. SECTION 9. EFFECTIVE DATE. This Ordinance shall take effect five (5) days from the date of publication of the attached summary which is hereby approved. From Ordinance No. 1608: Section 6. Refund of SEPA Checklist Fees. The Mayor shall authorize the Finance Director to refund SEPA Checklist fees received, between the effective date of Ordinance No. 1599 and the effective date of this ordinance, from applicants now made exempt pursuant to Section 1 of this Ordinance. UNCIL A GEP DA SYNOPSIS ITEM NO. Initials - - - - - -- Meeting Date Prepared by Mayor's review Council revlew Jan. 10, 1994 • > T:EM >> IFOpJ CAS Number: Original Agenda Date: Agenda Item Title: Appeal of the Planning Commission decision File #93 -01 -APP: Hillcrest. Original Sponsor:, Council Admin. Timeline: January 10, 1994 Public Hearing. Sponsor's Summary: The City Council to consider the City's appeal of the Planning Commission's decision upholding an appeal by LeRoy Lowe for slope stability requirements in the Sensitive Areas Ordinance. Recommendations: Sponsor: Committee: Administration: The decision by the Planning Commission be reversed. Cost Impact (if known): Fund Source (if known): Meeting Date Action 1 -10 -94 . Meeting Date 1 -10 -94 Attachments Attachment A emoran um to ayor ec. 3 Attachment B Letter of Appeal Oct. 25, 1993 Attachment.0 Planning Commission Minutes Oct. 14, 1993 Attachment D Submittals at Public Hearing (cross section to be submitted at hearing). Attachment E Staff Report Oct. 5, 1993 TO: City of Tukwila John W. Rants, Mayor Department of Community Development Rick Beeler, Director MAYOR RANTS FROM: JOHN MCFARLAND ACTING DCD DIRECTOR DATE: DECEMBER 27, 1993 RE: APPEAL OF THE PLANNING COMMISSION DECISION FILE # 93 -01 -APP HILLCREST BACKGROUND The applicant, LeRoy Lowe, is requesting a Boundary Line Adjustment to replat property he controls into five single family lots for development. The applicant's site contains wetlands and steep slopes. On July 20, 1993, the applicant appealed staff decision regarding the need for further geotechnical evaluation of slope stability as recommended by the peer review consultant. On October 14, 1993, the Planning Commission upheld the appeal on a 3 to 2 vote. (Attachment C Planning Commission minutes). On October 25, 1993, the Director of Community Development appealed the Planning Commission decision. (Attachment B letter of appeal). ISSUES As noted in the Staff Report, the City Council needs to consider three items contained in TMC 18.45.125(b) Appeals, in making their decisions; 1) Intent/Purpose of the Sensitive Areas Ordinance. Has the applicant addressed the following purposes statements contained in the Sensitive Areas Ordinance: TMC 18.45.010 Purpose (1) "Minimize development impacts on the natural functions and values of these areas." (4) "Prevent erosion and the loss of slope and soil stability caused by the removal of trees, shrubs, and root systems of vegetative cover." 6300 Southcenter Boulevard Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665 Page 2 93 -01 -APP: Hillcrest (5) "Protect the public against avoidable losses, public emergency rescue and relief operations cost, and subsidy cost of public mitigation from landslide, subsidence, erosion and flooding." 2) Technical Information As reflected by the numerous reports contained in the attachments, both staff and the applicant have hired technical experts to review this site. The peer review consultant believes that both shallow and deep- seated stability analyses are necessary to evaluate the geologic stability of the proposed project to address the requirements of the Sensitive Areas Ordinance. As of this time, no deep- seated analysis has been performed for the project. 3) DCD Director's findings shall be given substantial weight The City retained Shannon & Wilson, Inc. geotechnical consultants, to provide peer review of technical material. The Sensitive Areas Ordinance requires the applicant to demonstrate one of the following: In accordance with TMC 18.45.080 (e)(3)(A) i) "There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no significant risk to the proposed development or surrounding properties"; or ii) "The area of potential geologic instability can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability." Using the information provided by the peer review geotechnical expert and City staff, the DCD Director concluded "detailed slope stability analysis" in TMC 18.45.080(e)(4)(c) can be interpreted as requiring both shallow and deep- seated slope stability investigations. RECOMMENDATION The Planning Commission's decision to lift the burden from the applicant is contrary to TMC 18.45. Based upon the information presented at the hearing on October 14, 1993, staff recommends the decision by the Planning Commission be reversed. SUMMARY OF PUBLIC NOTICE 1. Seattle Times, December 31, 1993 2. Site posted, December 28, 1993 3. Notice to property owners, December 30, 1993 4. Notice to participants at Planning Commission Public Hearing, December 30, 1993 City of Tukwila John W. Rants, Mayor Department of Community Development Rick Beeler, Director October 25, 1993 TO: City Council FROM: Rick Beeler, Director -DC RE: Hillcrest Development - 93 -01 -APP. i".:CT 25 41:-.%3 riTY O 71,:4:1 %1L . On behalf of the City of Tukwila I hereby appeal the October 14, 1993, decision of the Planning Commission upholding the appeal by Leroy Lowe of my decision to require deep- seated soil borings pursuant to the Sensitive Areas Ordinance (Chapter 18.45, Tukwila Municipal Code - "TMC "). The basis for this appeal is that substantial weight was not given my decision and that the Planning Commission substituted their judgment for the City's peer review geotechnical consultant. BACKGROUND Hillcrest is a reconfiguration (boundary line adjustment) of 7 existing single family zoned lots into 5 lots in order to accommodate the•wetlands and a Class 3 hillside on the property at the southwest corner of Slade Way and S. 160th St. This property is part of the Puget Western property on the east side of Slade Way, on which the Valley View Estates multifamily development failed to achieve a satisfactory stability coefficient to support development. The applicant, Leroy Lowe, submitted geotechnical reports which were subjected to peer review by the City's consultant, Shannon & Wilson, Inc. That review culminated in the July 28, 1993, finding of Shannon & Wilson that: "...the applicant has not adequately addressed the potential for landsliding on this property, nor has he demonstrated that the property meets the requirements set forth in the City of Tukwila Sensitive Areas Ordinance... ". On July 12, 1993, the applicant was notified that staff, on my behalf, agreed with Shannon & Wilson, thereby affirming Shannon & Wilson's May 5, 1993, conclusion that: "The slope stability analysis report for the proposed Hillcrest Development addresses the stability of the north - trending steep slope that runs through the center of the property rather than the stability of the property as a whole. In our opinion, the primary concern for stability of the property is related to the potential 6300 Southcenter Boulevard, Suite. #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665 1 Hillcrest Appeal, 10 -25 -93 Page 2 for deep seated failure that could affect much if not all of the property, rather than the smaller, more localized type of failures with are addressed in this report." On July 20, 1993, Mr. Lowe filed a timely appeal of staffs decision pursuant to TMC 18.45.125 to the Planning Commission. The Planning Commission heard the appeal on October 14, 1993, and upheld the appeal (3:2 vote). SUBSTANTIAL WEIGHT TMC 18.45.125(b)(3) requires the Planning Commission to give the decision of the DCD Director "substantial weight ". Attached is the staff report to the Commission, which contains the above referenced documents. Staffs request for the deep - seated soil borings was based upon the peer review recommendations of Shannon & Wilson. Those borings were found necessary to substantiate the applicant's undocumented claim that the hillside, and upslope and downslope, will be stable after construction of the development. Documentation has still not been provided to substantiate that the deep - seated stability concerns of staff and Shannon & Wilson are unfounded. The Planning Commission erred in not giving "substantial weight" to the staff decision. The applicant provided no documentation that overcame the "substantial weight" burden or proof. This is a geotechnical issue required to be resolved before construction can proceed on the steep hillside or wetland areas (relocated) (TMC 18.45.080(e)(4)(c). This is not a matter of a "flexible interpretation" of TMC 18.45. GEOTECHNICAL JUDGMENT The applicant's property continues to be owned by Puget Western and is the westerly extension, under Slade Way, of the "Valley View Estates" portion of the large Puget Western holding. A significant landslide occurred in the early 1960's that required installation of a substantial deep- seated drainage system to stabilize the undeveloped Puget Western property. That system is deteriorating and the Valley View Estates development proposal failed because the property could not be safely stabilized. In addition Slade Way displays signs of sloughing. Based on this information, the question became how much of the westerly portion of . Puget Western's property (applicant's ownership) possessed similar deep- seated slippage potential or difficulty in achieving a safe stability coefficient. Therefore, staff requested of the applicant geotechnical studies to demonstrate the existing and future stability of the hillside and upslope and downslope properties. Once that information was provided, peer review was conducted, a normal practice in Class 3 hillside conditions. That peer review pointed out deficiencies in the prior geotechnical studies as stated above. 7 Hillcrest Appeal, 10 -25 -93 Pa •e 3 St= : and Planning Commissioners lack geotechnical engineering qualifications to judge g. stechnical reports. That is why peer reviews are conducted to give staff a second o • inion or added insurance that public life and safety are protected. e Planning Commission erred in offering their opinion that this specific Class 3 hillside di . not need deep - seated testing to document hillside stability. No conclusive d cumentation, only opinion, was provided to demonstrate what would be found in that te. ' g. Without the testing in this instance, no firm conclusions can be drawn regarding th stability of the hillside. Not to require the testing incurs an undue risk to the future re idents of I3illcrest, surrounding property owners, and the City in general. C 1 NCLUSION s appeal is unusual in that staff rarely contests a decision of the Planning Commission w s o works hard to make reasoned decisions. Normally those decisions do not have public and safety ramifications for residents, but in the specific situation'of Hillcrest, peer ew by an expert clearly indicated that danger exists unless documented otherwise. It is e applicant's responsibility to demonstrate that life and safety are not endangered. The P arming Commission's decision to lift that burden from the applicant, in this specific ce, is contrary to TMC 18.45 and their expertise. Therefore, the decision must be r ;versed. City of Tukwila John W. Rants, Mayor Department of Community Development Rick Beeler, Director PLANNING COMMISSION MINUTES OCTOBER 14, 1993 Mr. Meryhew called the meeting to order at 8:00 p.m. Members present were Messrs. Meryhew, Knudson, Malina, Haggerton, and Mrs. Craft. Mr. Flesher was excused and Mr. Clark was absent. Representing the staff were Jack Pace, Gary Schulz, Dan Clayton (consultant) and Sylvia Osby. In approving the minutes of September 23, 1993, the Commission addressed the second item on the agenda regarding Dujardin Development Co. L92 -0064, L92 -0065 and L92 -0014. Mr. Haggerton noted that on page 2, the last sentence of paragraph 6, he wanted the sentence changed to read, "Mr. Haggerton said that it doesn't seem staff has as many concerns with the project as he did ". In addition, Mr. Malina stated his rationale for his motion as written in the September 23, 1993 minutes. He said the reason for reducing the number of lots from 41 to 39, falls under the same direction of the minutes of the Vision Tukwila group. In their minutes they were also considering reducing the required lot size from 7200 sq. ft. to 6500 sq. ft. He added that he felt that a lot size of 3954 sq. ft. is too small. Following the minutes from the visioning process, he felt it was very important to increase the lot sizes and try to stay close to the 6500 sq. ft. size. Mr. Malina clarified the motion as follows: FOLLOWING THE DIRECTION OF THE VISION TUKWILA MINUTES, OF DECREASING THE 7200 SQ. FT. REQUIREMENT TO 6500 SQ. FT., HE FELT IT WAS IMPORTANT TO TRY TO STAY AS CLOSE TO THE VISIONING PROCESS AS POSSIBLE. He added that to him, it was inappropriate to have any lot in the City on a PRD that is less than 3900 sq. ft. MR. MALINA MOVED TO APPROVE THE MINUTES OF SEPTEMBER 23, 1993 AS AMENDED AND CLARIFIED. MR. KNUDSON SECONDED THE MOTION AND THE MOTION WAS UNANIMOUSLY APPROVED. 6300 Southcenter Boulevar4 Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665 4 Planning Commission Minutes October 14, 1993 L93- 0001 -APP, 90- 13 -BLA, 91 -3 -APRD: Hillcrest Appeal Page 2 Jack Pace presented the staff report. He stated that the Sensitive Areas Ordinance requires all appeals regarding the SAO to go before the Planning Commission. There are three factors that need to be addressed when appealing a staff decision. The Commission needs to look at the intent of the sensitive areas ordinance, the technical information provided and when making their decision, they need to give substantial weight to the Director's decision. He continued by saying that this project is before the Commission because the proposed site is on Class 3 and Class 4 slopes which require a geotechnical study. The issue which needs to be resolved between the applicant and staff is the scope of the requirements in the SAO. He then entered into the record a copy of a letter submitted by the applicant after the packets were mailed out. LeRoy Lowe: Mr. Lowe introduced George Lamb, Neil Twelker, Dick. Stuth, A.J. Breadberg, and Dick Osborne. On May 21, 1990, the City Council and Mayor unanimously voted to lift the moratorium on this project because it was the kind of project the City wanted. George Lamb, 12016 115 Ave. NE, Kirkland: He stated that his company, Cascade Geotechnical, has prepared a number of reports on this property. Their initial study of the site determined that it could be safely developed, and they performed additional studies regarding the permitting situation. They also performed a slope stability study. He said they concluded that the property is stable now and the proposed improvement will improve both the local and regional stability. The problem which occurred on Slade Way in 1961 was stabilized by a DOT drainage program. Over the past 30 years, the drainage has become clogged and the failure of that drainage caused some reactivation on the slide, which is moving to the northeast. Left unattended, with no improvement, one would expect that this movement would impact Slade Way. It is speculation to think how this movement would impinge beyond Slade Way. He said they have repeatedly analyzed the stability of the slope itself and repeatedly been advised that what they should be addressing is the stability of the property beyond Slade Way which doesn't belong to Mr. Lowe. The site is now stable, the proposed development will improve that stability both on site and down hill. To impact the Hillcrest development, Slade Way would have to fail completely. Neil Twelker, 5645 42 Ave. W., Seattle: The critical question that has arisen is, do we need more investigative effort. A great deal can be learned from the City's consultant, Shannon & Wilson. Their report indicates that the slope below Slade Way has been the subject of intensive geotechnical investigations since 1960. It also indicates that remedial measures to stabilize the hillside were accomplished by WSDOT. These measures consisted of regrading, surface drainage, and installing a system 5 Planning Commission Minutes October 14, 1993 of vertical and horizontal drains. These measures were effective in lowering the artisan pressures and stabilizing the slide at that time. There is no dearth of information on this slide, there is no dearth of knowledge or agreement as to what could be done to correct it. Mr. Twelker said that they have put in borings within 100 -200 feet of the Hillcrest property. Dick Stuth, 13232 138 Ave SE, Renton: Mr. Stuth said he is not a geotechnical engineer and that his involvement deals with the utilities and the drainage of the site. He added that this is nothing unusual, a system can be designed to accommodate any storm event that the City would require. Everything that can be done in the way of storm drainage will only enhance the site and the stability of Slade Way. When the City hires a consultant, they hire them with the hidden agenda of how to say "no" in a technical way. Page 3 Anthony Breadberg, 5800 Soundview Dr., Gig Harbor: He stated that he did the wetland study for the property. He said that the appellant is minimizing the impacts on the natural values and functions of these areas. There is an existing 5 lots and they are trying to re -align the lots to minimize the impacts. They can prevent erosion, the loss of slope and soil stability. They are planning on de- watering the slope to protect the slope for the Hillcrest subdivision. They need to get Hillcrest out of the way and get on to the real issue of protecting Slade Way which has nothing to do with Mr. Lowe's project. He added that several deep- seeded analyses have been completed in the area. He said that there were two professional engineers who are willing to put their reputations on the line to say that these houses will stay. He said that he found it difficult to believe that a recommendation could be made to not accept this information by very esteemed engineers. Dealing with the wetland issue can help the slope stability. The Planning Commission or the City of Tukwila has the option of dealing with the wetland issue for public safety, these wetlands can be exempted from the Sensitive Area Ordinance. Mr. Haggerton asked how much time Mr. Breadberg has spent on this property. Mr. Breadberg said six to eight hours on the property. Mr. Knudson asked how long the consultant's guarantee or insurance is good for. Mr. Breadberg said that Cascade Geotechnical will be around for a long time and they will have liability insurance. Mr. Twelker said that as a practical matter, as long as one is alive, they are liable for any defects designed into a project or anything they have sealed. There is a new law that says that if one designs a building and they find a defect, but they do not file a litigation against the architect within the first six years, then they do not have a right of remedy. But if they do not know about the defect, they have the right of remedy forever. Planning Commission Minutes October 14, 1993 Mr. Lowe said that as a practical matter, one is on the hook forever for something that they put their seal of approval on. • Page 4 Dick Osborne, 21805 NE 1st, Redmond: He stated that he is a licensed real estate agent. He added that the stability on the slope has an enormous impact on the adjacent property owners and all of the property owners in the area who use Slade Way. Washington is a full- disclosure State and a seller of a home must disclose any and all defects that have to do with that property that might influence the decision of a potential buyer. If Mr. Lowe is not allowed to participate in the stabilization of Slade Way, the instability of Slade Way will be a serious factor in the marketing of anyone's property who uses Slade Way as access. Also, anything affecting the use of the property has to be disclosed as well. Mr. Meryhew asked if any existing properties in the neighborhood have covenants that state that there is a potential for sliding. Mr. Osborne said that he is not specifically aware of what the covenants and the easements, etc. are in that particular area. He added that full- disclosure is retroactive. Mr. Lowe stated that to render a professional opinion on the prognosis of stability of a piece of property is one thing. To deliberately ignore a direct threat to a community by withholding a known remedy is another thing. One is a simple error of judgement, the other a willful derelict of duty. Jack Pace said the Planning Commission and Council had lengthy deliberations on the Sensitive Area Ordinance. This provides a framework for how staff is supposed to process applications, and the criteria for which decisions are made. There are two key parts; the alterations and what is needed regarding a geotechnical report. These are the key elements staff is using to make their decisions. The issue before the Commission is whether they are interpreting the ordinance correctly. Dan Clayton, Shannon & Wilson, 400 N. 34 St., Seattle: Mr. Clayton said that he had been retained by the City to provide an objective peer review of work that had been done as of September 1992 by Mr. Lowe's consultants. That consisted of two geotechnical reports, subsurface investigations of the property itself, and some recommendations for design and ultimate development of the property. In that review, he looked not only at the work that was provided by Mr. Lowe, but also at a number of other studies. The most significant of those involved the hillside below Slade Way and below the Hillcrest development, specifically, a number of studies that were done by various firms, including Shannon & Wilson, beginning in 1960 on the overall slide which occurred in 1960 and 1961. As a result of those various studies, they came to a number of conclusions. Specifically, according to the SAO, the property itself falls into a Class IV area 7 Planning Commission Minutes Page 5 October 14, 1993 of potential geologic instability, meaning landslides. The parameters of the SAO are slopes of 25 degrees or more; seepage on the property, evidence of previous landslides, evidence of landsliding in the near vicinity, all of which have been identified on the Hillcrest development. Specifically, the back -slope is over 25 degrees. A number of consultants have identified that slope as a landslide scarp. In reviewing the subsurface investigations that were performed on behalf of Mr. Lowe, they are inconclusive because there was no attempt to determine if there was a landslide or not. There is some evidence that is suggestive that the interpretation by these various consultants in the past, and Shannon & Wilson, is correct. The interpretation of a scarp being behind, and a slope passing underneath, qualifies the site as being potential landslide materials. There is no indication as to whether they are or are not landslide materials in the Cascade reports and the work that had been done for Mr. Lowe at that time. Down the hillside, there is plenty of evidence of landslide in the vicinity. The requirements of the SAO say that "...prior to permitting alteration of an area of potential geologic instability, the applicant must demonstrate one of the following: 1) there is no evidence of past instability or earth movement in the vicinity of the proposed developments; and quantitative analysis of slope stability analysis indicates no significant risks to the proposed development or surrounding properties. 2) the area of potential instability can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased and the increase in surface water discharge or sedimentation shall not decrease slope stability." As far as modifying it to decrease the potential impact, Mr. Clayton said that he would contend that one would have to understand what that potential impact is before you figure out how to modify it. With regard to decreasing the slope stability, he agrees that this development is not going to diminish the stability of the hillside. And with respect to shallow landsliding, the dewatering of the property should increase the stability. There are two issues: 1) there is the potential for a shallow landslide and; 2) there is the potential for a deep landslide, comparable to what has occurred down the slope. Mr. Twelker has outlined where Shannon & Wilson's postulated landslide is, and no evidence has been offered to argue that landslide is not there. With respect to the retreat of the bluff itself, there are some peats on the property which would argue that is probably not the case, it probably is a landslide scarp. It's up to a developer to determine whether it is or not, and that has not been done. The development will increase the stability with respect to the shallow landslides. What is done in the upper ten feet has very little to do with how the deep stability will behave. The evidence from down slope is that that stability is going to diminish as the dewatering system denigrates and if that's the case, the stability will decrease. Planning Commission Minutes October 14, 1993 Page 6 In conclusion Mr. Clayton said that he was hired to address the Sensitive Area Ordinance and whether or not the project has met the requirements of that ordinance and with regard to a Class IV landslide area, they have not been met. What is required is to look at .the deep- seeded slope stability of the property itself. Mr. Meryhew asked where in the SAO does it get into the deep seeded slope stability. Mr. Clayton said that he has provided an interpretation that there is a potential for deep - seeded sliding. Mr. Haggerton asked if there was any question in Mr. Clayton's mind that this is a Class IV wetland area. Mr. Clayton said that there was no question about that. Mr. Knudson asked if dewatering the site would decrease the potential for a large slide. Mr. Clayton said that the interpretation of the deep- seeded slide is that a large driving force behind that was hydrostatic up -lift on the soils. The reason why the well system was effective was because it lowered the water by 50 -60 feet, removing the up -lift pressure on the slip plain. For the up -lift pressure to exist, there has to be a confining layer between the surface and where the slip plain is, and its 30 -50 feet in the deep slide plain. Effectively, the water that would be removed, the improvements that would occur on the Hillcrest development, are going to happen in the upper sand, which is not a confined layer and it would be surface water off the hillside. That takes a little load off the hillside, but not a lot. He continued by saying that he doesn't see any evidence to indicate that there is any hydrologic continuity between that water and the high pressure water below. Removing the water from the upper part of the slide will help with the surface slides that are occurring on the property, but that's not the issue that he is concerned with. Mr. Malina asked Mr. Clayton to hi -light for the rest of the Commission what Professor Tubbs had to say with the intent of the SAO. Mr. Clayton explained that the question came up in a meeting with Mr. Lowe and his consultants as to what the intent of the second item in the SAO was about. Specifically, the disagreement was whether they were dealing with the potential impact to the project itself or whether they are talking about adjoining properties. Mr. Tubbs indicated that they did not write the words, but their intention was that the SAO require that there not be an impact to the property. Mr. Malin asked Mr. Clayton what his definition of quantitative analysis is. Mr. Clayton said that a quantitative slope stability analysis is one in which the various 9 Planning Commission Minutes Page 7 October 14, 1993 parameters of the slope and the ground water in the slope and the soil properties are all taken into consideration in a calculation in which a number is generated which is the risk value for slope failure. Mr. Malina asked how deep a deep soil study is. Mr. Clayton said in order to assess hydrostatic up -lift below a confining plain, it would be necessary to know what the water levels are. Based on the information they have, it's probably in the order of 50 -70 feet down. Mr. Malina asked if that wasn't already done. Mr. Clayton said that the slide mass that WSDOT was dealing with ended at Slade Way. The borings that were done for this development went 30 feet. Mr. Malina asked how he felt about Section E of 18.47 and part of 18.48. Is it appropriate to have that section within the SAO. Mr. Clayton said that the cities who ignore slope stability problems find themselves in a lot of trouble. It is very reasonable and provides a means for safeguarding public safety. • Mr. Malina asked Mr. Clayton if he was a developer, would he be compelled to comply with the SAO. Mr. Clayton said his recommendation from the out -set would have been to do a deep slope stability analysis. Mr. Malina asked if he was satisfied with Mr. Tubb's interpretation of the direction of the SAO. Mr. Clayton said it was consistent with his interpretation. Mr. Meryhew asked what Mr. Clayton thought the risk would be if they were to develop the site and what might the consequences be. Mr. Clayton said there are a couple of concerns: 1) how will the soil behave under normal static conditions and; 2) how will it behave under dynamic conditions, such as in the event of an earthquake. Mr. Meryhew asked if the deep- seeded analysis will prove conclusively that an earthquake will not create a major problem on the slope. Mr. Clayton responded by saying there is a subjective element to it. ID Planning Commission Minutes Page 8 October 14, 1993 Mr. Knudson asked where Mr. Clayton would like to see the deep- seeded sample taken. Mr. Clayton said it needs to be down below the interface on the property. He added that the drawings that he has seen would not address a deep- seeded failure. Mr. Malina asked if the study concluded that there is a problem, could that effect the removal of trees and vegetation. Mr. Clayton responded by saying that it would be at the surface level. At this point they are far from understanding the deep- seeded stability and that it's idle speculation to think what removing a couple of trees would do. He said that he didn't think that would have any impact on the deep- seeded stability. Mr. Malina asked if the additional packing of the ground due to the structures that are being built would change the table at all. Would they need to go below 50 -60 feet? . Mr. Clayton said that he didn't think it would affect that. Those things are surficial materials and problems. There is going to be some leakage, but it isn't significant compared to the magnitude of water that exists below that relatively impermeable layer. Mr. Malina asked where does WSDOT come in on all of it and where does the liability lie. Mr. Clayton said that at the time of the Valley View development, it was pointed out that a significant de- watering system needed to be installed in order for the development to occur, or WSDOT had to renew their system and get it working properly. Their response was no, that system is designed for construction of that road, it wasn't designed for residential development or other development on the hillside. Mr. Malina asked if there is a liability issue for a developer who may have to go through a deep- seeded study to approach WSDOT's wells. Does one have to get special permission. Mr. Clayton said that they have to get permission from WSDOT to go into their wells. Mr. Haggerton asked if there was any documentation regarding how the earthquake affected this property. Mr. Clayton said that he didn't think there was any indication that there was any sliding at that time. Mr. Meryhew said that there was indication that WSDOT may just need to do maintenance on the wells. He asked if that was a normal function that they are supposed to be performing periodically? 11 Planning Commission Minutes Page 9 October 14, 1993 Mr. Clayton said that he thought they do conduct some minimal maintenance. Mr. Meryhew asked if there was any way to put the pressure on WSDOT to take a look at them. Mr. Clayton said that he could not answer that. Mr. Meryhew asked if the freeway construction after 1960 and 1961 had any affect on this property. Mr. Clayton said that he did not know. There is drainage under Slade Way. There are some horizontal drains that were installed in 1961. Mrs. Craft (inaudible). Jack Pace said that it was because of the history with this site that they decided to have a peer review. Mr. Meryhew asked what WSDOT's responsibilities and obligations are. Mr. Pace said that the issues being dealt with tonight are the applicant's specific request. The City is not asking him to take care of a City problem or WSDOT problem. They are focusing on the stability of those houses on that site. Mr. Twelker said that with regard to the proposed treatment to accommodate for possible loss of grade of the foundations, they would probably stretch that to three feet. Beyond that, things would get tough. He said that he agrees that the water table in the vicinity of the Hillcrest development is not really connected to the water table that causes the problem. The amount of water that escapes from the Hillcrest property is a combination of surface and sub - surface water. Some of the sub - surface water is derived from local infiltration from meteoric water, some comes from off the property. Mr. Twelker read from page 10 of the Shannon & Wilson report, "With regard to the potential for deep - seeded sliding, it is our opinion that the most productive approach the city can take to improve hillside stability, would be to encourage WSDOT to rehabilitate their hillside drainage system. If measures are not taken to improve the system it may also be appropriate for the City to develop an on -going ground water monitoring program to assess the effectiveness of the drainage system ". Mr. Twelker said that he agrees with Mr. Clayton completely there. Mr. Lamb said that he agrees with Mr. Clayton that WSDOT should fix the mess they left or have the city fix it themselves. The site is now stable and the proposed improvement will make it more stable, the downhill problem was stabilized by these drains and they have failed over the years. Left unattended, it will impact Slade Way long before it gets anywhere near these other properties and there's no chance in the world that the highway department /2 Planning Commission Minutes Page 10 October 14, 1993 and the City of Tukwila are going to watch a major access street close. David Morgan, 5190 S. 166: Mr. Morgan said that on his property, there is not any sand. Four feet down there is blue clay which slides like water during an earthquake. On the Hillcrest property there is a drain field which has water running about 90% of the time, while his property is dry. Mr. Morgan said that he has put in a drain field around his property which goes dow five feet and covers almost 400 feet. He said that when he walks off his property, there is water. Where is the water going to go when those houses are built; there is already a twenty -year drain field there that can't handle it. WSDOT is not going to protect the hillside. When will the traffic, water run -off, water retention, sewer, water availability, and sidewalk issues be addressed? Mr. Meryhew asked where Mr. Morgan lived in relation to the proposed site. Mr. Morgan stated that he lived about a 1/4 of a mile south from the property. Darlene West, 5212 S. 164: Ms. West said that the citizens worked so long and hard on the SAO, and there has to be a strong and valid reason for not following it. She said that she felt Mr. Lowe had not shown any valid reasons for not doing the deep- seeded testing to find out how stable his property is. It is an unstable hill and there are a lot of springs and water on the hill. She continued by saying that there has been a lot made of the WSDOT drains and she is not sure if maintaining those is a requirement to maintain Mr. Lowe's property. WSDOT does not give a darn, they have put in writing that their drains were to only maintain the road. They have no intention of upgrading it to do anything else. Mr. Haggerton asked which direction her residence was located from the property. Ms. West said she was located south of the property. Richard Stevens, 800 Bellevue Wy, Suite 400: Mr. Stevens said that he is an attorney representing adjacent property owners, Mr. & Mrs. Iverson and Chris Thakler. Their properties back up to Mr. Lowe's property. If Mr. Lowe's proposal is denied or if he is required to complete a deep seeded analysis, it has an affect on his client's property and everybody who abuts that property. Everyone agrees that the danger at that shallow level is no problem because Mr. Lowe's proposal will get the water off of the property. The problem is somewhere below and would affect his clients' property. If it's determined that this is a danger, it will devalue the neighboring properties. If Mr. Lowe's geotechnical engineers are discounted, it should be on some pretty clear and substantial evidence. All they have is a peer review by Mr. Clayton, who is not licensed in this state as a geotechnical engineer. Therefore, great weight should not be placed on something .that is purely speculative. The language in the SAO talks about two different types of situations in which they would allow alteration of an area of potential geologic /3 Planning Commission Minutes Page 11 October 14, 1993 instability. The second situation says that the area of potential geologic instability can be modified, or the project can be designed so that any potential impact to the project and surrounding properties is eliminated. The question is, what is it that needs to be eliminated. Mr. Clayton has advocated that they have to show that the geologic instability is eliminated and Mr. Stevens said that he didn't feel that could ever be shown under any circumstances. Mr. Stevens continued by saying that it is WSDOT's legal responsibility to take care of the wells. The Planning Commission and perhaps City Council, should urge WSDOT to take care of that problem. But that problem does not affect Mr. Lowe's houses any more than it affects his client's houses. If there is a deep - seeded problem, it is going to affect everyone. The short term problem, everyone agrees, will get fixed by the project itself. It seems that the position of the Planning Department is that unless the problem is completely solved, Mr. Lowe cannot do something that is going to help. Everyone agrees that getting the water off the property is going to help and his clients would like to see that stabilized. Mr. Malina clarified that if the applicant falls under the criteria of either item #1 or #2, then that is what triggers this process. Mr. Stevens said that was not his understanding from the staff report. If he has to show that the instability will be eliminated, Mr. Stevens said he didn't think anyone could show that. Everyone agrees that Mr. Lowe has eliminated whatever impacts his project will have on instability. This project will do nothing but make this area safer. Mr. Malina asked if Mr. Clayton was not licensed in the State of Washington. Mr. Stevens said that was his understanding. Mr. Haggerton asked if Mr. Steven's clients have suffered any instability on their property. Mr. Stevens said not that he knows of. Mr. Meryhew asked how long his clients have owned their property. Mr. Stevens said that one client has owned it for several decades, and Mr. Thaekler has owned his less time. There are four properties that abut this project and he represents the owners of two of them. Bob Levinson, Earth Consultants, 1805 136 Pl, Bellevue 98005: Mr. Levinson said that he didn't get involved in this situation with an agenda. He stated that he has been involved in the last few months. He stated that this hillside is probably the most studied hillside in the Puget Sound area. He added that he doubted that the deep- seeded potential slide plain exists anywhere near Mr. Twelker's first line. In reading the initial Dames and Moore report, they talked about a clay silth that failed. The information that IA Planning Commission Minutes October 14, 1993 Page 12 he has seen on Mr. Lowe's property shows about 65 feet of sand. There are two different types of geologic conditions there. It is unlikely that a old sheer plain would go through two different types of geologic conditions. Mr. Levinson said that he has reason to believe that the deep seeded plain does not exist. He said that he is in agreement with everyone with regard to the short term solution when a property is developed, and it is done with a geotechnical engineer, it will always stabilize the slope because the water is being controlled. On the basis of his study, Mr. Levinson said that he thought Mr. Lowe should be able to develop his property. It would give stability to his client's property on the upper portion. He said that he talked to the citizen's committee regarding the SAO and.he is against the 15 %, 25 %, 40% slope type of designation that this city has because they do not fully address the potential problems. His recommendation to the citizen's committee was to try to define the sensitive areas and study those instead of going by slope percentages. Mr. Lowe's design on those footings is over -kill and not really needed. It will tend to control the water and stabilize the slope beneath his client's property. Mr. Malina asked if Mr. Levinson has read the City's Sensitive Areas Ordinance since it has been completed. Mr. Levinson said that he had not. He added that he read the portion that was mentioned in the soils report. Mr. Malina asked with regard to section 18.47, the two basic criteria, does the applicant fall into one of those two categories? Mr. Levinson said that there is evidence of past instability. At the same time, that is off the proposed property. With regard to the second criteria, the word, "eliminate" is very strong. You cannot eliminate anything, but you can minimize it. He said he would not have a problem making the statement that impacts based on this development have been minimized where it will not negatively effect any of the adjacent properties. Mr. Malina asked if he felt a deep- seeded study should be required. Mr. Levinson said that on this project, he feels that if the WSDOT situation is taken care of, no further work is needed on Mr. Lowe's property. Although there has been past instability, it is not due to this man's project. It's due to some outside influence. When he talked with the citizen's committee, he was referring to projects that would have impacts on adjacent properties. Mr. Erickson, 16040 51 Ave. S.: He said that his property probably has more frontage on the edge of the hill than anyone IS Planning Commission Minutes Page 13 October 14, 1993 else's. There are springs all over in the hill. Before wetlands and sensitive areas were identified, he specialized in problem properties and putting structures on wet lots and on hillsides. There is a lot of water coming out of the hillside that needs to be addressed. He said that he was sure that controlling the water on the top would help stabilize the hill. He urged that the Council or city engineers get guarantees that the project is installed correctly. He said that he was not against developing the property as long is it is done correctly. Mr. Haggerton asked where the test hole is in relation to his property. Mr. Erickson said that it was on the lot adjacent to his. Mr. Haggerton asked what type of soil there was. Mr. Erickson said that it was sand and his lot is pure sand. Mr. Knudson asked what his specialty is. Mr. Erickson said it is in excavation. Mr. Knudson asked if he has seen success in developing problem properties. Mr. Erickson said that if it is installed as engineered, then it will be o.k. Generally, he has not seen any failures if it has been installed correctly and maintained. WSDOT is going to have to take care of their system and maintain it. Ray Neilsen, 5108 S. 163 Pl.: Mr. Neilsen said that the rear of his property abuts this proposed project. He added that there has been some slippage up there. He said that in the 27 years that he has lived in his home the ground is slipping east, down the hill. Below the proposed property is a swamp all the time. Darlene West asked staff if they were asking the applicant to do the deep seeded testing on their property or on the property down hill from them? Mr. Clayton said that he didn't know what they would have to do to get the required information. There are no wells located on the property that are deep enough. There are wells down hill. Ms. West said it's not fair to expect WSDOT to maintain the drains they put in for a road. She said that no -one should expect WSDOT to support anything that changes what that hill was when they first put in their drain. Mr. Knudson said that because of the cut for the road, they caused some damage and Planning Commission Minutes Page 14 • October 14, 1993 impact, and they are responsible. It's a matter of maintaining what they have already destroyed. Mr. Meryhew said that the Commission's decision could not be based on what WSDOT is responsible for. Mr. Meryhew asked for clarification from Mr. Clayton regarding the comment that he is not licensed in the State of Washington. Mr. Clayton said that he is an engineering - geologist and they don't have licenses in the State of Washington. He is a registered geologist in California and Oregon and a certified engineering geologist in Oregon. Mr. Meryhew closed the public hearing. Mr. Meryhew called for a five minute break. Mr. Meryhew called the meeting back to order. He continued by saying that they were looking at a case of potential liability, safety and individual property rights. He said that he was concerned with the safety, but also looking for a reasonable win/win situation for everyone. The development proposed by the applicant should be certified as safe by a geotechnical engineer or geologist and that's in the SAO. Also, the applicant could provide a letter to the City which is covered in the SAO. Essentially, this letter with plans and specifications, stating that he /she understands and accepts the risk in developing an area with potential unstable soils and he /she will advise, in writing, prospective purchasers of the site or any prospective purchasers of structures or portions of structures on the site of the slide potential. In addition, the owner should execute a covenant within the bounds of the SAO, which that provides a legal description and a statement that the site is an area of potential instability and some of the risks that are associated with it; and any conditions or prohibitions on the development; any features or modifications of the design to address any anticipated soil changes; (inaudible) waive any claims of the owner or his successors for signs that they may have against the City of Tukwila. All of this is allowed within the SAO. In addition to that, the foundation designs that are proposed by Mr. Twelker in his letter to Mr. Lowe dated July 19th should be incorporated as additional safety. Mr. Malina said in his opinion, the Director did what he had to do because of the way the SAO is written. Mr. Meryhew said he agreed, and there is a difference of opinion. Mr. Malina said the appellant is in a catch 22 situation. As far as the peer review group goes, Shannon and Wilson' requirement of an off -site deep seeded study, as well as an on- site deep seeded study, is inappropriate. The appellant is only responsible for his portion. I7 Planning Commission Minutes Page 15 October 14, 1993 If the City or State wants to do an off -site study, that is up to them, it should not have to come out of the applicant's pocket. He added that he personally felt that the applicant has not complied to the conditions of the SAO as the way the SAO is written today. That is not to say that this may be amended at some future date. It may come back to the Planning Commission to be amended. But at this point, the SAO is written, the applicant has not complied and the Director of DCD did the correct thing. Mr. Haggerton said that when they developed the SAO, they paid close attention to the Class III, Class IV and Coal Mine Hazard areas. There was a reason for that. This site has clearly been identified as a Class IV area. The applicant is not responsible for anything off of his property. Because of the way it is classified and the testimony, there should be more on -site tests to prove the stability of the land. Mr. Knudson said that he has problems with the technicalities of what they are trying to achieve. Mr. Meryhew said that there has to be some risk involved, not everything can be guaranteed. Mrs. Craft said that the whole idea of a SAO is so that houses can be built with some responsibility. Part of the wording of the SAO makes it difficult to do that. The whole idea is to develop responsibly, and they are being asked to decide if the appellant's evidence lead to a clear conviction that a mistake was made by the director. She said that she didn't see how they could say that the director made a mistake, he was following the direction of the SAO. She added that she would like to see how they can help the appellant and what is the next step to make the development happen. Mr. Malina said that they can make the development happen. They are just showing that the director of DCD interpreted the SAO correctly. What the appellant is doing is appealing the decision of DCD and what has been demonstrated here is that the director of DCD was correct. Mr. Meryhew said that this is subject to interpretation. Mr. Malina said that it was apparent as far as 18.47(E), all he has to do is demonstrate and either /or situation. There's no question that there was a landslide and it is a hazard. The applicant; staff, peer review, and citizens know that there is serious concern there. The applicant does have a responsibility to create a covenant when the project is developed. He has a responsibility to the community and there's no doubt he can make the project work. What the Commission is concerned with is what is before them and the Commission can make a recommendation after they make a motion. Mr. Meryhew said that the SAO says that a detailed stability analysis needs to be completed. And what that consists of is what is being interpreted differently. IA Planning Commission Minutes Page 16 October 14, 1993 Mr. Malina said that the studies have to be within the boundaries of the project. Mr. Meryhew said that it could be interpreted as improving the site without removing eliminating every problem. There has to be a risk factor involved. Mr. Haggerton said that section 18.48 is the most important to him; "the detailed slope stability analysis must be done if it is a Class IV area ". A detailed slope stability analysis consists of a determination of what types of soils exist where the houses are going to be placed, how much water is in the hillside and how can it best be diverted so that it is not a detriment to that hillside. He added that common sense needs to be used along with the ordinance that they adopted. Every word cannot be analyzed. Mr. Meryhew said that is why he is suggesting that they ask for the certification from a geotechnical engineer or licensed geologist that the property is safe. They could also have a covenant that goes along with the land. Mr. Knudson said that it involves more than just someone buying the property, it's what impact will it have on adjacent properties. Mr. Meryhew said that he didn't believe that doing a detailed study on the property alone will provide anymore information than they already have. Mr. Haggerton said that it would provide more information, but would not guarantee it more. Mr. Mauna asked for clarification from staff if sensitive area properties have that indicated on titles to the property. Mr. Pace said that existing development has none of that. The only exception is for property owners who want deferment of their taxes. Mr. Haggerton asked what happens if the appeal is approved. Mr. Pace said that if the appeal is approved, staff cannot require further geotechnical analysis and they would proceed ahead. Mr. Haggerton 'said that apparently, the DCD director is. bound by the SAO and the way that it is written to require additional geotechnical studies. Mr. Mauna said that's what the peer review is recommending, but they are stepping out of 14) Planning Commission Minutes Page 17 October 14, 1993 the bounds of the project, and the applicant should not have to. There can be enough of a study done on the site if the deep seeded test were done in the right place. It's important to get the project completed. It's one thing if the slide never occurred, but that's why it has received this hazardous rating to begin with. Mr. Meryhew said that he still felt it was a question of interpretation and some risk still needs to be taken. The covenant also provides liability insurance for the City as stated in the SAO, section 18.48(d) & (e). During the straw vote, Messrs. Haggerton and Malina said they would deny the appeal and Mrs. Craft said she would probably deny the appeal; Messrs. Knudson and Meryhew said they would approve the appeal. MR. MALINA MOVED ON 93 -01 -APP: IT[I.T,CREST TO UPHOLD STAFF'S FINDINGS, CONCLUSIONS AND RECOMMENDATIONS TO DENY THE APPEAL. MR. HAGGERTON SECONDED THE MOTION. THE MOTION DID NOT PASS BY A VOTE OF 3 -2. MESSRS. HAGGERTON AND MALINA WERE IN FAVOR OF THE MOTION AND MESSRS. KNUDSON, MERYHEW AND MRS. CRAFT VOTED AGAINST TH1 MOTION. In order to clarify the motion and provide a positive motion: MR MERYHEW MOVED TO ACCEPT L93 -01 -APP- H LLCREST, APPEALING A DECISION THAT THE APPEAL BE APPROVED AND CARRIED BY A VOTE OF 3 -2 OF THE PLANNING COMMISSION. THE REASON FOR THE MOTION IS THAT '1`HE PLANNING COMMISSION BY A 3 -2 VOTE, FELT THAT THE INTENT AND PURPOSE OF THE SENSITIVE AREAS ORDINANCE HAD BEEN MET SUBJECT TO THEIR INTERPRETATION OF THE SAO AND UNDERSTANDING THAT THE INTERPRETATION IS A DIFFERENCE OF OPINION. WITH A 3 -2 VOTE, THEY FELT THAT THE TECHNICAL INFORMATION AND GEOLOGICAL STUDIES HAVE BEEN ADEQUATE TO MEET THE INTENT OF THE SAO AND THAT THE DCD DIRECTOR'S CONCLUSION AND ORIGINAL DENIAL WAS THE RESULT OF A DIFFERENT INTERPRETATION THAN THE PLANNING COMMISSION'S. Taking a formal vote: JIM HAGGERTON VOTES NO GEORGE MAUNA VOTES NO VERN MERYHEW VOTES YES JERRY KNUDSON VOTES YES JOYCE CRAFT VOTES YES. THE APPEAL IS APPROVED. h, Planning Commission Minutes October 14, 1993 Vern Meryhew adjourned the meeting. Prepared By, Sylvia Schnug 21 Page 18 .3; 1 i1 W 1 >- 0: Z w W b o 7. U a 0 W o U U < 1 >. 4 J ft 0 • < < U N W 1 Q " z 0 c 0 O M v OCT. 13, 1993 0 W > N W �' F- Z W � m c N U9 n 0 W W }' W d_ Y J } . 0 E Q 2 0 0 3 2 E 000 U (1. Q O >- F 0 0- z W 0 C W ¢ 1 0 w 0 C JZ O a. ¢ n. Z W Q 0 TO THE HEARING. N N } J d>. z Q'.1. o J d w I- 1- Z d W w (A N N w Q. N = w _ 3 w o ¢ W Z Q 1- > I z°" w O 0 z W ¢ = W U > Z W 0 O w 0= U 0- w ¢ O F' m w O ° ›. 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O J w N F- W >- a w C' a > z W 1- D U Z Q U Z 1' W m Q < o 1 w N a C0 >- ►- w Q z U) ?- m {- 1=- a J w >- 0 x W W w a w w W}..< - t w o ❑ 2 a 2 > 2 U cc 1 1 W 1 O ❑ W W O 1- i F. W F- 0 a 1- P. U 1 2 Z a > J PROHIBIT NO SENSE I 24 • CHRONOLOGY OF EVENTS & DATES HILLCREST BOUNDARY LINE A0J. BOUNDARY 11NE ADJUSTMENT 5 -17 -90 SITE SURVEY COMPLETED 5 -21 -90 MORITORIUM WAVER REQUESTED 5 -30 -90 1sT GEO. REPORT COMPLETED 7 -2 -90 MORITORIUM WAVER GRANTED 7 -0-90 PRELIM. PLANS SUBMITTED 6 -27 -90 GEO REPORTS COMPLETED 9 -0-90 GEOTECH REPORTS TO CITY 10 -20 -90 SITE SURVEY WITH LOTS 85 & 86 ADDED 1991 1 -21 -91 LETTER FROM CASCADE AT THE REQUEST OF THE CITY RE; GEOTECH & PLANS REVIEW 9 -6 -91 B.L.A. APPLICATION FILED 1 -6 -92 A.J. BREDBERG WETLAND STUDY 3 -26 -92 PLANNING COMM. HEARING 4 -20-92 WETLAND STUDY FILED WITH CITY 6 -15 -92 REASONABLE USE EXCEPTION REQUEST FILED WITH CITY 1993 1 -13 -93 LOWE DEFENDS POSITION THAT ADDITIONAL STUDIES ON OTHER PROPERTY . NOT HIS RESPONSIBILITY 3 -30 -93 SLOPE STABILITY ANALYSIS FILED WITH CITY 3 -30 -93 CONTRACT FOR PIER REVIEW OF SLOPE STABILITY ANALYSIS FILED WITH CITY LEROY C. LOW A.I... ARCMITCCT ' FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG • SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV \.%:. u1 I uKwua PRELIM PLANS REJECTED BY DARREN WILSON. LOTS ARE NOT CONTIGUOUS. LOWE DISAGREES. 7 -0-90 DARREN WILSON FINALLY CONCEEDS d ACCEPTS PRELIM. DOC. FOR B.L.A. 10 -20 -90 JACK PACE CANCELS MEETING 12 -31 -91 JACK PACE CANCELS MEETING 1 -10 -91 JACK PACE CANCELS MEETING 1 -16 -91 JACK PACE DEMANDS WETLAND STUDY 6wos AFTER B.L.A. SUBMITAL NEEDS INVENTORY OF WHAT IS TO BE DEWATERED. 3 -26 -92 JACK PACE DEMANDS PIER REVIEW OF ALL GEO REPORTS 22mos AFTER GEO REPORTS WERE SUBMITTED 9 -0 -90 DARREN WILSON - (NEVER SEEN THESE PLANS BEFORE) 6 -15 -92 PLANNING COMM HEARING 7 -30 -92 SHANNOM & WILSON GEOTECH PIER REVIEW 11 -24 -93 DEC JACK PACE DEMANDS ADDITIONAL GEOTECH STUDIES BE PREPARED ON PUGET WESTERN'S PROPERTY. 12 -9 -92 JAN FEB MAR SHANNON A WILSON DEMAND SLOPE STABILITY STUDY WITH A PIER REVIEW OF SAME. 3 -2 ■93 11-11S DEMAND APR IS MADE OVER TWO (2) YEARS AFTER GEOTECH REPORTS ORIGINALLY SUBMITTED. MAY JUN JUL AUG SEP OCT OCT 14T4 PLANNING COMMISSION PUBLIC HEARING NOV • DEC SET 23Ro MEETING CANCELED 52 eittne;l■•"ice Chapter 18.45 SENSITIVE AREAS OVERLAY ZONE • Sections. 18.45.010 18.45.020 18.45.030 18.45.040 18.45.060 18.45.080 *I 18.45.090 18.45.115 18.45.120 18.45.125 18.45.130 18.45.135 18.45.140 .;C Purpose. Sensitive area designation, rating methodologies, classifications and applicability. Interpretation. Sensitive area buffers. Procedures. Uses and standards. Sensitive areas tracts. Exceptions. Variances. Appeals. Recording required. Assurance device. Assessment relief. 18.45.010 Purpose.. (a) The purpose of the Sensitive Areas Overlay Zone is to establish special standards for the use and development of lands based on the existence of natural conditions thereon in order to protect environmentally sensitive areas, including the natural character of Tukwila's wooded hillsides. (b) Standards are hereby established to meet the following goals of protecting environmentally sensitive areas: (1) Minimize developmental impacts on the natural functions and values of these areas. (2) Protect quantity and quality of . water resources. (3) Minimize turbidity and pollution of wetlands and fish - bearing waters and maintain wildlife habitat (4) Prevent erosion and the loss of slope and soil stability caused by the removal of trees, shrubs, and root systems of vegetative cover. (5) Protect the public against avoidable losses, public emergency rescue and relief operations cost, and subsidy cost of public mitigation from landslide, subsidence, erosion and flooding. (6) Protect the community's aesthetic resources and distinctive features of natural lands and wooded hillsides. (7) • Prevent unlawful disturbance of archaeologic or geologic sites.with historic or prehistoric artifacts. (8) Balance the private rights of individual property owners with the preservation of envi- ronmentally sensitive areas. (9) Prevent . the loss of wetland and watercourse function and acreage, and strive for a gain over present conditions. (Ord 1599 §3(part) 1991) TITLE 18 — ZONING 18.45.020 Senstthro area designation — rating methodologies— classifications and app8Cablkity. (a) Applicability — This chapter applies to any use or development proposed on any legal lot of record, any portion of which is a sensitive area or a sensitive area buffer as defined in Sections 18.06.695 and 18.06.697, and specifically including one or more of the following and their buffers: (1) Abandoned coal mines; (2) Areas of potential geologic instability: Class 2, 3, 4 and seismic instability areas (Section 18.06.050 and subsection (e) of this section); (3) Wetlands (18.06.939); (4) Watercourses (18.06.935); (5) Areas that contain archaeological remnants of value to the archaeological research community, which includes but is not limited to colleges, universities or societies of professional archaeologists, or which is designated as important to save as a record of the area's past by the State Office of Archaeology and Historic Preservation. (b) Sensitive Areas Maps and Inventories (1) The distribution of many sensitive areas in Tukwila is displayed on the Sensitive Areas Maps, dated 1990, and on file with the Department of Community Development (DCD). (2) Studies, preliminary inventories and ratings of potential sensitive areas are on file with the DCD in the Sensitive Areas Notebook, dated May 1990. (3) The maps and preliminary inventories and ratings are hereby adopted by reference. The actual presence or absence of sensitive areas as defined by or otherwise referred to in this chapter and as determin by the City will govern. The actual ratings and buffers for any sensitive area will be determined by the City using the methodologies and procedures provided in this chapter for each type of sensitive area. (4) All revisions, updates and reprinting of sensitive areas maps, inventories, ratings and buffer shall conform to this chapter. (c) Wetlands — For the purposes of this chapter "wetlands" is defined in Section 18.06.939. A wetlan boundary is the line delineating the outer edge of wetland established by using the "Federal Manual fo Identifying and Delineating Jurisdictional Wetlands, dated January 10, 1989, as revised or updated, an jointly published by the U.S. Environmental Protectio Agency, the U.S. Fish and Wildlife Service, the U.S Army Corps of Engineers, and the U.S. Soi Conservation Service. Wetland types and rating criteria are listed below: (1) Type 1 wetlands, those wetlands whic meet any of the following criteria: (A) The presence of species listed by th federal government or State as endangered or threa ened, or the presence of critical or outstanding actu- habitat for those species, -• Printed August 4, 1993 Page 18-3 2!s TUKWILA MUNICIPAL CODE ) (B) Having forty to sixty percent perma- nent open water in dispersed patches with two or more classes of vegetation, (C) Equal to or greater than five acres in size and having three or more wetland classes, one of which may be substituted by permanent or open water; or (2) Type 2 wetlands, those wetlands which meet any of the following criteria: (A) Greater than one acre in size, (B) Equal to or less than one acre in size and having three or more wetland classes, (C) Equal to or less than one acre, that have a forested wetland class comprised of at least twenty percent coverage of total surface area, or (D) The presence of heron rookeries or raptor nesting trees, (E) The presence of native plant associa- tions of infrequent occurrence; (3) Type 3 wetlands, those wetlands which are equal to or less than one acre in size and that have two or fewer wetland classes. For the purposes of this section, the U.S. Fish and Wildlife Service's "Classification of Wetlands and Deepwater Habitats of the United States FWS/OBS- 79/31" (Cowardin et al., 1979), contains the descrip- tions of wetland classes and subclasses. (d) Watercourses — For the purposes of this chap- ter, "watercourses" is defined in Section 18.06.935. The City "Watercourse Study" (1990) includes the methodology and criteria that will be used for deter- mining watercourse ratings. Watercourse ratings are based on the existing habitat functions. Each segment or reach of a water- course is rated individually. The rating system will score a reach point total for each side of the water- course. Watercourse types, rating scores and rating criteria are described below. (1) Watercourse Types and Rating Scores. (A) Type 1 watercourse, twenty-one to thirty-three points; (B) Type 2 watercourse, eleven to twenty points; (C) Type 3 watercourse, three to ten (2) Watercourse Rating Criteria. (A) Instream Features. (I) Width of watercourse: A measure of the average width of the channel at the ordinary high water mark. (11) Channel capacity: Quantifies the ability of the channel to convey high flows without flooding. points; (iii) Channel stability: Measures the stability of the channel by evaluating evidence of bank failure, scour, and downcutting. (iv) Fish use and fish habitat: Anadromous species and resident salmonid need pro- tection measures if present. Rating depends on the number of different types of habitat present. (B) Corridor Quality. (1) Width of unmaintained vegetation: A measure of the width of unmaintained vegetation from the ordinary high water mark. (ii) Vegetation diversity: Quantifies the elements of terrestrial habitat associated with the watercourse corridor. (111) Corridor barrier function: Provides some measure of effectiveness of the buffer to limit intrusion and disturbance. (iv) Surrounding land use: Evaluation of the land use immediately outside the vegetated corridor. (e) Areas of Potential Geologic Instability - Areas of potential geologic instability are defined in Section 18.06.050, and are classified as follows: (1) Class 1 areas, where landslide potential is low, and which slope is less than fifteen percent; (2) Class 2 areas, where landslide potential is moderate, which slope is between fifteen and forty percent, and which are underlain by relatively perme- able soils; (3) Class 3 areas, where landslide potential is high, which include areas sloping between fifteen and forty percent, and which are underlain by relatively impermeable soils or by bedrock, and which also include all areas sloping more steeply than forty percent; (4) Class 4 areas, where landslide potential is very high, which include sloping areas with mappable zones of groundwater seepage, and which also include existing mappable landslide deposits regardless of slope; (5) Areas of potential seismic instability, with soft soils, loose sand and a shallow groundwater table. (f) Sensitive Areas Special Studies (1) Required. An applicant for a development proposal that includes sensitive areas shall submit those studies as required by the City to adequately identify and evaluate the sensitive area and its buffers. (2) Waiver. If there is agreement between the Director of the Department of Community Development (DCD) and the applicant concerning the sensitive area classification and type, the Director of DCD may waive the requirement for sensitive area studies. There must be substantial evidence that the sensitive areas classification is correct, that there will be no detrimental impact to the sensitive areas or buffers, and that the goals, purposes, objectives and requirements of this chapter will be followed. (3) Review of Studies. The Department of Community Development will review the informa- tion submitted in the sensitive area studies to verify the information, confirm the nature and type of the sensitive area, and ensure the study is consistent with this chapter. 2 7 rage 18-40 Printed August 4, 1993 (g) When this chapter imposes greater restrictions or higher standards upon the development or use of land than other laws, ordinances or restrictive covenants, the provisions of this chapter shall prevail. (h) All other relevant standards of this Code must also be met. • (Ord 1608 §7, 1991; Ord. 1599 §3(part), 1991) 18.45.030 Interpretation. The provisions of this chapter shall be held to be minimum requirements in their interpretation and application and shall be liberally construed to serve the purposes of this chapter. (Ord. 1599 §3(part), 1991) 18.45.040 Sensitive area buffers. (a) General (1) Any land alteration must be located out of the buffer areas as required by this section. Buffers in general are intended to: (A) Minimize long -term impacts of devel- opment on properties containing sensitive areas; (B) Protect sensitive areas from adverse impacts during development; (C) Preserve the edge of the sensitive area for its critical habitat value; and • (D) Prevent loading of potentially unstable slope formations. Land alteration is permitted for public access, supplemental planting and approved land uses as provided in Section 18.45.080. An undisturbed sensitive area or buffer may substitute for the yard setback and landscape requirements of Chapters 18.50 and 18.52. • (2) Wetland and watercourse buffers are intended to: (A) Provide shading to maintain stable water temperatures and vegetative cover for additional wildlife habitat; (B) Provide input of organic debris, and uptake of nutrients; (C) Provide an area to stabilize banks, to absorb overflow during high water events, and to allow for slight variation of aquatic system boundaries over time due to hydrologic or climatic effects; (D) Reduce erosion and increased surface water runoff; (E) Reduce loss of or damage to property; (F) Intercept fine sediments from surface water runoff and serve to minimize water quality impacts; (G) Preserve the edge for its habitat value; (H) Protect the sensitive area from human and domestic animal disturbance. (3) Buffers for areas of potential geologic instability are intended to: (A) Protect slope stability; and TITLE 18 — ZONING (B) Provide erosion control and attenua- tion of precipitation surface water and stormwater runoff; (C) Reduce loss of or damage to property; (D) Preserve the natural character of wooded hillsides where they exist. (b) Special Bu,ffer Studies — Applicants for a use or development on a legal lot of record within a sensi- tive area maximum buffer shall be required to conduct .a sensitive area study to provide a buffer analysis for the sensitive area. This study may be waived by the Director of the Department of Community Develop- ment (DCD) pursuant to Section 18.45.020(f) (2). (c) Ratings and Bufer Width —Ratings and appropriate buffers for wetlands and watercourses are listed below. (1) For wetlands: (A) Type 1, one - hundred -foot -wide buffer; (B) Type 2, fifty- foot -wide buffer; (C) Type 3, twenty-five- foot -wide buffer. (2) For watercourses, the buffer shall be as follows: and (A) Type 1, seventy-foot -wide buffer; (B) Type 2, thirty -five- foot -wide buffer; (C) Type 3, fifteen- foot -wide buffer; (3) Setbacks. (A) All commerdal and industrial devel- opments shall be set back fifteen feet and all residential development shall be set back ten feet. This setback shall be measured from the foundation to the buffer's edge. (B) The DCD Director may waive setback requirements when a site plan demonstrates there will be no impacts to the buffer zone.(See Figure 18 -4.) (4) Variation of Standard or Creation of Variable Width Wetland/Watercourse Buffers. (A) The DCD Director may reduce the standard wetland /watercourse buffers on a case -by- case basis, provided the buffer does not contain slopes fifteen percent or greater. The approved buffer width shall not result in greater than a fifty percent reduction in width, and the reduced buffer shall not be less than fifteen feet for wetlands and ten feet for watercourses. Any buffer reduction proposal must demonstrate to the satisfaction of the DCD Director that it will not result in direct or indirect, short-term or long -term adverse impacts to wetlands or watercourses, and that (1) The buffer is vegetated and includes an enhancement plan as may be required to improve the buffer function and value; or (ii) If there is no significant vegetation in the buffer, a buffer may be reduced only if an enhancement plan is provided. The plan must include using a variety of native vegetation that improves the functional attributes of the buffer and provides addi- tional protection for the wetland or watercourse func- tions and values. Printed August 4, 1993 Page 18-41 28 TUKWILA MUNICIPAL CODE (B) Buffers for all types of wetlands and watercourses will be increased when they are deter- mined to be particularly sensitive to disturbance or the proposed development will create unusually adverse impacts. Any increase in the width of the buffer shall be required only after completion of a wetland or wa- tercourse study by a qualified wetlands specialist or expert which documents the basis for such increased width. An increase in buffer width may be appropriate when: (i) The development proposal has the demonstrated potential for significant adverse impacts upon the wetland or watercourse which can be miti- gated by an increased buffer width; or (ii) The area serves as habitat for endangered, threatened, sensitive or monitor species listed by the federal government or the State. (C) Every reasonable effort shall be made to maintain the existing viable plant life in the buffers. Vegetation may be removed from the buffer as part of an enhancement plan approved by the Director of DCD. Enhancements will ensure that slope stability and wetland and watercourse quality will be maintained or improved. Any disturbance of the buffers for wetlands or Watercourses shall be replanted with a diverse plant community of native northwest species that are appropriate for the specific site as determined by the DCD Director. If the vegetation must be removed, or because of the alterations of the landscape the vegeta- tion becomes damaged or dies, then the applicant for a permit must replace existing vegetation along wetlands and watercourses with comparable specimens, approved by the DCD Director, which will reproduce the existing buffer value within five years. (D) The DCD Director shall require subse- quent corrective actions and long -term monitoring of the project if adverse impacts to regulated wetlands, watercourses or their buffers are identified. (d) Areas of Potential Geologic Instability (1) Each development proposal for a legal lot of record containing an area of potential geologic instability shall .be subject to a geotechnical report pursuant to the requirements of Sections 18.45.060 and 18.45.080(e)(4). The geotechnical report shall analyze and make recommendations on the need for and width of any buffers necessary to achieve the goals and requirements of this chapter. Development proposals shall then include the buffer distances as defined within the geotechnical report. ' (2) Buffers may be increased by the DCD Director when an area is determined to be particularly sensitive to the disturbance created by a development. Such a decision will be based on a City review of the report as prepared by a qualified geotechnical consultant and by a site visit. (Ord. 1608 §3, 1991; Ord. 1599 §3(part), 1991) 18.45.060 Procedures. When an applicant submits an application for any building permit, subdivision, short subdivision or any other land use review which approves a use, devel- opment or future construction, the location of any sen- sitive areas and buffers on the site shall be indicated on the plans submitted. When a sensitive area is identi- fied, the following procedures apply. The Director of the Department of Community Development (DCD) may waive item numbers 1, 2, 4 and 5 of the follow- ing if the size and complexity of the project does not warrant that step in the procedures and the Director grants a waiver pursuant to Section 18.45.020(f)(2). (1) Sensitive Areas Study and Geotechnical Report. The applicant shall submit the relevant study as required in Section 21.04.140 and this chapter. It is intended that sensitive areas studies and information be utilized by applicants in preparation of their proposals and therefore shall be undertaken early in the design stages of a project. (2) Planned Residential Development Permit. Any new residential subdivision, residential short subdivision, residential boundary line adjustment, or multiple family residential proposal which includes a sensitive area or its buffer on the site shall apply for a planned residential development permit and meet the requirements of Chapter 18.46. (3) Denial of Use or Development. A use or development will be denied if it is determined by the .DCD_ Director:that_ the. applicant cannot 'ensure that potential dangers and costs to future inhabitants of the development, adjacent and ,local: properties,. and L.Tiikwila are minimized and mitigated. to an . acceptable -level. (4) Pre - development Conference. The applicant, specialist(s) of record, contractor, and department representatives will be required to attend preconstruction conferences prior to any work on the site. (5) Construction Monitoring. The specialist(s) of record shall be retained to monitor the site during construction. (6) On -site Identification. The DCD Director may require the boundary between a sensitive area and its buffer and any development or use to be permanently identified with fencing, or with a wood or metal sign with treated wood, concrete or metal posts. Size will be determined at the time of permitting, and wording shall be as follows: "Protection of this natural area is in your care. Alteration or disturbance is prohibited pursuant to Section 18.45. Please call the City of Tukwila for more information." (Ord. 1608 §4, 1991; Ord. 1599 §3(part), 1991) 18.45.080 Uses and standards. (a) General flies— The uses set forth in this entire section, including subsections (a) through (h), 29 Page 18-42 Printed August 4, 1993 • j lir 1TTLE18- ZONING 150' . WETLAND BUFFERS Typ. 1.100 toot wide buffer Typ. 2 -50 loot wide buffer Typo 3-25 loot wide butler .-...4- 10'SETDACK I1 TYPE 3 ::::':•::•::.::.. 1 '\ WATERCOURSE':;; ;':° °. •• \\ \ \ °.4—,0 sEIBACK I 14---ORDINARY HIGH I WATER MARK WATERCOURSE' �� "\ I BUFFERS Typ. 1 -70 foot wld. buffer • \ Typ. 2 -35 foot wld. buffer 1\ •`'1 . Typo 3 -15 foot wld. butter • \— 57TH AVE S. • • • • • • • ;: • • • .•• .•. •. • • • • • • • • • & • • • • • • ::••• —z 0 18' --t SAMPLE RESIDENTIAL SENSITIVE AREA SITE PLAN SUBMITTAL FIGURE 18-4 3e _) 31 and the following general uses, may be located within a sensitive area or buffer, subject to the provisions of Chapter 21.04 and of this section: (1) Maintenance and repair of existing uses and facilities provided no alteration or additional fill materials will be placed or heavy construction equipment used in the sensitive area or buffer; (2) Nondestructive education and research; (3) Passive recreation and open space; (4) Maintenance and repair of essential streets, roads, rights-of-way, or utilities; (5) Actions to remedy the effects of emergencies that threaten the public health, safety or welfare. (b) Permitted Uses Subject To Administrative Review — The following uses may be permitted only after administrative review and approval by the Director of the Department of Community Develop- ment (DCD): (1) Maintenance and repair of existing uses and facilities where alteration or additional fill materials will be placed or heavy construction equipment used; (2) Construction of new essential streets and roads, rights-of-way and utilities; (3) New surface water discharges to wetlands or watercourses or their buffers from detention facilities, presettlement ponds or other surface water management structures may be allowed provided that the discharge meets the clean water standards of RCW 90.48 and WAC 173.200 and 173.201 as amended, and does not increase the rate of flow to the wetland or watercourse beyond the level of the existing rate; (4) Regional stormwater detention areas may be allowed if use results in no decrease in rating of resource and enhances existing values and functions. Design shall be subject to the standards of this section and other applicable City standards; (5) Enhancement or other mitigation including landscaping. (c) Wetlands (1) General (A) No use or development may occur in a Type 1 and Type 2 wetland or its buffer except as specifically allowed by subsections (a), (b) and (h) of this section. Any use or development allowed is subject to the standards of this section. (B) Only isolated Type 3 wetlands can be altered or relocated, and then only with the permission of the DCD Director. A mitigation or enhancement plan must be developed and must comply with the standards of compensatory mitigation required in this chapter. (C) Mitigation plans shall be completed for any proposals for dredging, filling, alterations and t relocation of wetland habitat allowed in subsections (a), (b) and (h) of this section. TITLE 18 — ZONING (2) Compensatory Mitigation. (A) The mitigation plan shall be developed as part of a sensitive area study by a specialist approved by the DCD Director. Wetland and /or buffer alteration or relocation may be allowed only when a mitigation plan clearly demonstrates that the changes would be an improvement of wetland and buffer quantitative and qualitative functions. The plan shall follow the performance standards of this chapter and show how water quality, wildlife and fish habitat, and general wetland quality would be improved. (B) In order to achieve the City's goal of no net loss of wetland functions and acreage, alteration of wetlands will require the applicant to provide a restoration, enhancement or creation plan to compen- sate for the impacts to the wetland and will compen- sate at a ratio of 1.5 to 1. (C) Mitigation Location. (i) On -site compensation shall be provided, except where the applicant can demonstrate that: a. The hydrology and ecosystem of the original wetland and those who benefit from the hydrology and ecosystem will not be damaged by the on -site loss; and b. On -site compensation is not scientifically feasible due to problems with hydrology, soils, waves or other factors; or c. Compensation is not practical due to potentially adverse impact from surrounding land uses; or d. Existing functional values at the site of the proposed restoration are significantly greater than lost wetland functional values; or e. That established regional goals for flood storage, flood conveyance, habitat or other wetland functions have been established and strongly justify Location of compensatory measures at another site. (ii) Off -site compensation shall occur within the same watershed where the wetland loss occurred. (iii) In selecting compensation sites, applicants shall pursue siting in the following order of preference: a. Upland sites which were for- merly wetlands; b. idled upland sites generally having bare ground or vegetative cover consisting primarily of exotic introduced species, weeds or emergent vegetation; c. Other disturbed upland. (D) Mitigation Standards. The scope and content of a mitigation plan shall be decided on a case - by -case basis. As the impacts to the sensitive area increase, the mitigation measures to ,offset these impacts will increase in number and complexity. The Printed August 4, 1993 Page 18-43 TUKWILA MUNICIPAL CODE components of a complete wetlands mitigation plan are as follows: (i) Baseline information of quantita- tive data collection or a review and synthesis of exist- ing data for both the project impact zone and the proposed mitigation site; (ii) Environmental goals and objec- tives that describe the purposes of the mitigation measures. This should include a description of site - selection criteria, identification of target evaluation species and resource functions; (iii) Performance standards of the spe- cific criteria for fulfilling environmental goals, and for beginning remedial action or contingency measures. They may include water quality standards, species richness and diversity targets, habitat diversity indices, or other ecological, geological or hydrological criteria; (iv) Detailed construction plan of the written specifications and descriptions of mitigation techniques. This plan should include the proposed construction sequence and construction management, and be accompanied by detained site diagrams and blueprints that are an integral requirement of any development proposal; (v) Monitoring and /or evaluation pro- gram that outlines the approach for assessing a completed project. An outline shall be included that spells out how the monitoring data will be evaluated by agencies that are tracking the mitigation project's progress; (vi) Contingency plan identifying potential courses of action, and any corrective measures to be taken when monitoring or evaluation indicates project performance standards have not been met; (vii) Performance security or other assurance devices as described in Section 18.45.135. (E) Mitigation Timing. Where feasible, compensatory mitigation projects shall be completed prior to activities that will permanently disturb wet- lands and immediately after activities that will temporarily disturb wetlands. Construction of com- pensatory projects shall be timed to reduce impacts to existing wildlife, flora and water quality, and shall be completed prior to use or occupancy of the activity or development. (3) Essential Utilities. • (A) Essential utilities must be constructed to minimize or, where possible, avoid wetland distur- bance. (B) All construction must be designed to protect the wetland and its buffer against erosion, uncontrolled drainage, restriction of groundwater movement, slides, pollution, habitat disturbance, any loss of flood carrying and storage capacity, and excava- tion or fill detrimental to the environment. (C) Upon completion of installation of essential utilities, wetlands must be restored to pre- project configuration, replanted as required and provided with maintenance care until newly planted vegetation is established. (D) All crossings must be designed for shared facilities in order to minimize adverse impacts and reduce the number of crossings. (4) Essential Streets, Roads and Rights - of-Way. (A) Essential streets, roads and rights -of- way must be designed and maintained to prevent erosion and avoid restricting the natural movement of groundwater. (B) Essential streets, roads and rights -of- way must be located to conform to the topography so that minimum alteration of natural conditions is neces- sary. The number of crossings shall be limited to those necessary to provide essential access. (C) Essential streets, roads and rights -of- way must be constructed in a way which does not adversely affect the hydrologic quality of the wetland or interrelated stream habitat. Where feasible, crossings must allow for combination with other essential utilities. (D) Upon completion of construction, the area affected must be restored to an appropriate grade, replanted according to a plan approved by the Director of DCD, and provided with care until newly- planted vegetation is established. (5) Public Use and Access. (A) Public access shall be limited to trails, boardwalks, covered or uncovered viewing or seating areas and displays, and must be located in areas which have the lowest sensitivity to human disturbance or alteration, and (B) Public access must be specifically developed for interpretive, educational or research purposes by, or in cooperation with, the City or as part of the adopted Tukwila Parks and Open Space Plan. (C) No motorized vehicle is allowed within a wetland or its buffer except as required for necessary maintenance, agricultural management or security. (D) Any public access or interpretive displays developed in a wetland and its buffer must, to the extent possible, be connected with a park, recreation or open -space area. (E) Vegetative edges, structural barriers, signs or other measures must be provided wherever necessary to protect sensitive areas by limiting access to designated public use or interpretive areas. (F) Access trails must incorporate design features and materials which protect water quality and allow adequate surface water and groundwater movement. (G) Access trails must be located where they do not disturb nesting, breeding, and rearing areas and buffer areas, and must be designed so that sensitive plant and critical wildlife species are protected. Page 18-44 Printed August 4, 1993 3Z (6) Dredging, Digging or Filling. (A) Dredging, digging or filling within a wetland and its buffer may occur only with the per- mission of the DCD Director and only for the following purposes: (1) Uses permitted by subsections (a), (b) and (h) of this section; or (11) Maintenance of an existing wet - (iii) Enhancement or restoration of habitat in conformance with an approved mitigation plan identified in a sensitive area study; or (iv) Natural system interpretation, education or research when undertaken by, or in cooperation with, the City; or (v) Flood control or water quality enhancement by the City; or (vi) Maintenance of existing water quality controls, for normal maintenance needs and for any diversion, rerouting, piping or other alteration permitted by this chapter. (B) Any dredging, digging or filling shall be performed in a manner which will minimize sedi- mentation in the water. Every effort will be made to perform such work at the time of year when the impact can be lessened. (C) Upon completion of construction, the area affected must be restored to an appropriate grade, replanted according to a plan approved by the Director of DCD and provided with care until newly - planted vegetation is established. (d) Watercourses (1) General. (A) No use or development may occur in a watercourse or its buffer except as specifically allowed by this section. Any use or development allowed is subject to the standards of this section. (B) Diverting or rerouting may only occur with the permission of the DCD Director and an approved mitigation plan. (C) Any watercourse which has critical wildlife habitat, or is necessary for the life cycle or spawning of salmonids, shall not be rerouted unless it can be shown that the habitat will be improved for the benefit of the species. (D) Any watercourse which has no criti- cal wildlife habitat may be rerouted if the waters flowing from the new configuration can be shown to do so in a manner that does not in any way adversely affect the habitat of a downstream salmonid- bearing water. (2) Mitigation. (A) Plans. Mitigation plans shall be com- pleted for any proposals of dredging, filling, diverting and rerouting of watercourses. land; or TITLE 18 — ZONING (B) Plan Contents. The mitigation plan shall be developed as part of a sensitive area study by a specialist approved by the DCD Director. The plan must show how water quality, treatment, erosion control, pollution reduction, wildlife and fish habitat, and general watercourse quality would be maintained or improved. All such plans must be approved by the DCD Director. (C) Mitigation Standards. The scope and content of a mitigation plan shall be decided on a case- by -case basis. As the impacts to the sensitive area increase, the mitigation measures to offset these impacts will increase in number and complexity. The components of a complete mitigation plan are as follows: (1) Baseline information of quantitative data collection or a review and synthesis of existing data for both the project impact zone and the proposed mitigation site; (II) Environmental goals and objectives that describe the purposes of the mitigation measures. This should include a description of site - selection criteria, identification of target evaluation species and resource functions; (iii) Performance standards of the specific criteria for fulfilling environmental goals, and for beginning remedial action or contingency measures. They may include water quality standards, species richness and diversity targets, habitat diversity indices, or other ecological, geological or hydrological criteria. The following shall be considered the minimum performance standards for approved stream alterations: a. Maintenance or improvement of stream channel dimensions, including the components of depth, width, length and gradient of the original location, b. Bank and buffer configuration should be restored to an equal or enhanced state of the original stream, c. The channel, bank and buffer areas shall be replanted with native ,vegetation which replicates or improves the original in species, sizes and densities, d. The stream channel bed and the biofiltration systems shall be equivalent to or better than in the original stream, e. The original fish and wildlife habitat shall be maintained or enhanced, f. Relocation of a watercourse shall not result in the new sensitive area or buffer extending beyond the development site and onto adjacent property without the agreement of the affected property owners, g A watercourse may be rerouted; 3 Printed August 4, 1993 Page 18-45 TUKWILA MUNICIPAL CODE (iv) Detailed construction plan of the written specifications and descriptions of mitigation techniques. This plan should include the proposed construction sequence and construction management, and be accompanied by detailed site diagrams and blueprints that are an integral requirement of any development proposal; (v) Monitoring and /or evaluation program that outlines the approach for assessing a completed project. An outline shall be included that spells out how the monitoring data will be evaluated by agencies that are tracking the mitigation project's process; (vi) Contingency plan identifying potential courses of action, and any corrective measures to be taken when monitoring or evaluation indicates project performance standards have not been met; (vii) Performance security or other assurance devices as described in Section 18.45.135. (D) Mitigation Timing. DCD- approved plans must have the mitigation construction completed before the existing watercourse can be modified. (3) Essential Utilities. (A) Essential utilities must be constructed to minimize, or where possible avoid, disturbance of the watercourse and its buffer. (B) All construction must be designed to protect the watercourse and its buffer against erosion, uncontrolled drainage, restriction of groundwater movement, slides, pollution, habitat disturbance, any loss of flood carrying capadty and storage capacity, and excavation or fill detrimental to the environment. (C) Upon completion of installation of essential utilities, watercourses and their buffers must be restored to pre - project configuration, replanted as required and provided with maintenance care until newly planted vegetation is established. (D) All crossings must be designed for shared facilities in order to minimize adverse impacts and reduce the number of crossings. (4) Essential Streets, Roads and Rights -of -Way. (A) Essential streets, roads and rights -of- way must be designed and maintained to prevent erosion and avoid restricting the natural movement of groundwater. (B) Essential streets, roads and rights -of- way must be located to conform to the topography so that minimum alteration of natural conditions is necessary. The number of crossings shall be limited to those necessary to provide essential access. (C) Essential streets, roads and rights -of- way must be constructed in a way which does not adversely affect the hydrologic quality of the watercourse and its buffer. Where feasible, crossings must allow for combination with other essential utilities. (D) Upon completion of construction, the area affected must be restored to an appropriate grade, replanted according to a plan approved by the Director of DCD, and provided with care until newly - planted vegetation is established. (5) Public Use and Access. (A) Public access shall be limited to trails, boardwalks, covered or uncovered viewing and seating areas, and displays and must be located in areas which have the lowest sensitivity to human disturbance or alteration. (B) Public access must be specifically developed for interpretive, educational or research purposes by, or in cooperation with, the City or as part of the adopted Tukwila Parks and Open Space Plan. (C) No motorized vehicle is allowed within a watercourse or its buffer except as required for necessary maintenance, agricultural management • or security. (D) Any public access or interpretive dis- plays developed along a watercourse and its buffer must, to the extent possible, be connected with a park, recreation or open -space area. (E) Vegetative edges, structural barriers, signs or other measures must be provided wherever necessary to protect watercourses and their buffers by limiting access to designated public use or interpretive areas. (F) Access trails must incorporate design features and materials which protect water quality and allow adequate surface water and groundwater movement. (G) Access trails must be located where they do not disturb nesting, breeding and rearing areas and must be designed so that sensitive plant and critical wildlife species are protected. (6) Piping. (A) Piping of any watercourse should be avoided. Piping may be allowed in any watercourse if it is necessary for access purposes. Piping may be allowed in Type 3 watercourses if the applicant com- plies with the conditions of this section, including: (1) Providing excess capacity to meet needs of the system during a one - hundred -year flood event; and (II) Providing flow restrictors, and complying with water quality and existing habitat - enhancement procedures. (B) No process that requires maintenance on a regular basis will be acceptable unless this main- tenance process is part of the regular and normal facilities maintenance process or unless the applicant can show funding for this maintenance is ensured. (C) Piping in a watercourse sensitive area shall be limited and shall require approval of the DCD Director. Piping projects shall be performed pursuant to the following applicable standards: Page 18-46 Printed August 4, 1993 A (1) The conveyance system shall be designed to comply with the standards in current use and recommended by the Department of Public Works. (ii) Where allowed, piping shall be limited to the shortest length possible as determined by the Director of DCD to allow access onto a property. (iii) Where water is piped for an access point, those driveways or entrances shall be consoli- dated to serve multiple properties where possible, and to minimize the length of piping. (iv) When required by the Director of DCD, watercourses under drivable surfaces shall be contained in an arch culvert using oversize or super - span culverts for rebuilding of a strearnbed. These shall be provided with check dams to reduce flows, and shall be replanted and enhanced according to a plan approved by the Director of DCD. (v) When necessary to provide for fish passage, fish ladders shall be one -foot vertical rise to ten -foot horizontal distance, or as approved by the State Department of Fisheries. (vi) Stormwater runoff shall be de- tained and infiltrated to preserve the watercourse channel's dominant discharge. (vii) All construction shall be designed to have the least adverse impact on the watercourse, buffer and surrounding environment. (viii) Piping shall be constructed during periods of low flow, or as specified by the State Department of Fisheries. (ix) Water quality must be as good or better for any water exiting the pipe as for the water entering the pipe, and flow must be comparable. (7) Dredging, Digging or Filling. (A) Dredging, digging or filling within a watercourse or its buffer may occur only with the permission of the DCD Director and only for the following purposes: (i) Uses permitted by Section (ii) Maintenance of an existing water - (iii) Enhancement or restoration of habitat in conformance with an approved mitigation plan identified in a sensitive area study; (iv) Natural system interpretation, education or research when undertaken by, or in cooperation with, the City; (v) Flood control or water quality enhancement by the City; (vi) Maintenance of existing water quality controls, for normal maintenance needs and for any diversion, rerouting, piping or other alteration permitted by this chapter. 18.45.080; course; TITLE 18 — ZONING (B) Any dredging, digging or filling shall be performed in a manner which will minimize sedi- mentation in the water. Every effort will be made to perform such work at the time of year when the impact can be lessened. (C) Upon completion of construction, the area affected must be restored to an appropriate grade, replanted according to a plan approved by the Director of DCD, and provided with care until newly - planted vegetation is established. (e) Areas Of Potential Geologic Instability (1) General The uses permitted in the under- lying zoning district may be undertaken on sites which contain areas of potential geologic instability subject to the standards of this section and the requirements of a geotechnical study. (2) Exemptions. Any temporary slope which `'has been created through legal grading activities may be . :'regraded without application of this chapter under an approved permit. (3)':' Alterations. '.(A) -• Prior to permitting alteration of an area P otential geologic instability, the applicant must i<::demonstrate one of the following: 4:E(1) • There is no evidence of past insta- bility or earth movement in the vicinity of the proposed development, and quantitative analysis of 4;slope stability indicates no significant risk_ to the :proposed development or surrounding properties; or _ 4 (ii):- The area of potential geologic in- stability can be modified or the project can be designed so that any potential impact to the project and surround - ing properties is eliminated, slope stability is not 'N - decreased, ann"fhe increase in surface water discharge ?.:or sedimentation shall not decrease slope stability. (B) Where any portion of an area of poten- (tial geologic instability is cleared for development, a landscaping plan for the site shall include tree replanting with an equal mix of evergreen and deciduous trees, -;;preferably native, and approved by the Director of °;;:DCD. Replacement vegetation shall be sufficient to ;` ..provide erosion and stabilization protection. (4) Geotechnlcal Report • (A) The applicant shall submit a geotech- nical report appropriate to both the site conditions and ;.:the proposed development A geotechnical investiga- :: Uon shall be required for development in Class 2, Class Class 4 areas, and any areas identified as seismic or Coal Mine Hazard Areas unless waived pursuant to Section 18.45.020(f) (2). (B) Geotechnical reports for Class 2 areas shall include at a minimum a site evaluation review of available information regarding the site and a surface 'reconnaissance of the site and adjacent areas. 'Subsurface exploration of site conditions is at the discre- tion of the geotechnical consultant. Printed August 4, 1993 .R.80183-47 3t, TUKWILA MUNICIPAL CODE (C) Geotechnical reports for Class 3, Class 4 and Coal Mine Hazard Areas shall include a site evaluation review of available information about the site, a surface reconnaissance of the site and adjacent areas, and a subsurface exploration of soils and hydrol- ogy conditions. Detailed slope stability analysis shall be done if the geotechnical consultant recommends it in Class 3 or Coal Mine Hazard Areas, and must be done in Class 4 areas. (D) Seismic hazard areas shall include an evaluation of site response and liquefaction potential for the proposed development area. For one -story or two - story single-family dwellings, this evaluation may be based on the performance of similar structures under similar foundation conditions. For proposed develop- ments including occupied structures other than one - story and two-story single - family dwellings, the eval- uation shall include sufficient subsurface exploration to provide a site coefficient (S) for use in the static lateral force procedure described in the Uniform Building Code. (E) Applicants shall retain a geotechnical engineer to prepare the reports and evaluations required in this subsection. The geotechnical report and completed site evaluation checklist shall be prepared in accordance with the generally accepted geotechnical practices, under the supervision of and signed and stamped by the geotechnical engineer. The report shall be prepared in consultation with the appro- priate City department. Where appropriate, a geologist must be included as part of the geotechnical consulting team. The report shall make specific recommenda- tions concerning development of the site. (F) The geotechnical engineers required under this subsection must meet the qualification stan- dards approved by the Director of DCD. Applicants shall provide a list of qualifications of the firm or individuals who will be doing the technical studies, and those shall be approved by the Director of DCD. If the engineers' credentials are not sufficient, the City may require applicants to use a different engineer or firm which does meet the City's standards. (G) The opinions and recommendations contained in the report shall be supported by field observations and, where appropriate or applicable, by : :terature review conducted by the geotechnical engineer which shall include appropriate explorations, such as borings or test pits, and an analysis of soil characteristics conducted by or under the supervision of the engineer in accordance with standards of the American Society of Testing and Materials or other applicable standards. If the evaluation involves geologic evaluations or interpretations, the report shall be reviewed and approved by a geologist. (H) An independent review of geotechni- cal reports will be required per Section 21.04.140. M, (5) Dtsclosures, Declarations and. Covenants. (A) It shall be the responsibility of the applicant to submit, consistent with the findings of the geotechnical report, structural plans which were pre- pared and stamped by a structural engineer. The plans and specifications shall be accompanied by a letter from the geotechnical engineer who prepared the geotechni- cal report stating that in his /her judgment, the plans and specifications conform to the recommendations in the geotechnical report; the risk of damage to the pro- posed development site from soil instability will be minimal subject to the conditions set forth in the report; and the proposed development will not increase the potential for soil movement. (B) Further recommendations signed and sealed by the geotechnical engineer shall be provided should there be additions or exceptions to the original recommendations based on the plans, site conditions or other supporting data. If the geotechnical engineer who reviews the plans and specifications is not the same engineer who prepared the geotechnical report, the new engineer shall, in a letter to the City accom- panying the plans and specifications, express his or her agreement or disagreement with the recommenda- tions in the geotechnical report and state that the plans and specifications conform to his or her recommenda- tions. (C) The architect or structural engineer shall submit to the City, with the plans and specifica- tions, a letter of notation on the design drawings at the time of permit application stating that he or she has reviewed the geotechnical report, understands its recommendations, has explained or has had explained to the owner the risks of loss due to slides on the site, and has incorporated into the design the recommenda- tions of the report and established measures to reduce the potential risk of injury or damage that might be caused by any earth movement predicted in the report. '(D) The applicant, or the owner of the site If the applicant is not the owner, shall submit a letter to the City, with the plans and specifications, stating that he or she understands and accepts the risk of develop- ing in an area with potential unstable soils and that he or she will advise, in writing, any prospective purchasers of the site, or any prospective purchasers of structures or portions of a structure on the site, of the slide potential of the area. ' (E)" The owner shall execute a covenant, running with the land, on a form provided by the City. The City will file the completed covenant with the King County Department of Records and Elections at the expense of the applicant or owner. A copy of the recorded covenant will be forwarded to the owner. This covenant shall include: (i) The legal description of the prop- erty; (11) A statement: • Page 18-48 Printed August 4, 1993 37 38 a. Explaining that the site is in an area of potential instability, b. Of the risks associated with development on the site, c. Of any conditions or prohibi- tions on development, and d. Of any features in this design which will require maintenance or modification to address anticipated soil changes; (iii) A statement waiving any claims the owner or his /her successors or assigns may have against the City for any loss or damage to people or property, either on or off the site, resulting from soil movement arising out of the issuance of any permit(s) authorizing development on the site; (iv) The date of issuance and number of the permit authorizing the development. (6) Assurance Devices. Whenever the City determines that the public interest would not be served by the issuance of a permit in an area of poten- tial geologic instability without assurance of a means of providing for restoration of areas disturbed by, and repair of property damage caused by, slides arising out of or occurring during construction, the DCD Director may require assurance devices pursuant to Section 18.45.135. (7) Construction Monitoring. (A) The applicant shall retain a geotechni- cal engineer to monitor the site during construction. The applicant shall preferably retain the geotechnical engineer who prepared the final geotechnical recom- mendations and reviewed the plans and specifications. If a different consultant is retained by the owner, the new geotechnical engineer shall submit a letter to the City stating whether or not he /she agrees with the opinions and recommendations of the original geotechnical engineer. Further recommendations, signed and sealed by the geotechnical engineer, and supporting data shall be provided should there be exceptions to the original recommendations. (B) The geotechnical engineer shall moni- tor, during construction, compliance with the recom- mendations in the geotechnical report, particularly site excavation, shoring, soil support for foundations includ- ing piles, subdrainage installations, soil compaction and any other geotechnical aspects of the construction. Unless otherwise approved by the City, the specific recommendations contained in the soils report must be implemented by the owner. The geotechnical engi- neer shall make written, dated monitoring reports on the progress of the construction to the City at such timely intervals as shall be specified. Omissions or deviations from the approved plans and specifications shall be immediately reported to the City. The final construction monitoring report shall contain a statement from the geotechnical engineer that, based upon his or her professional opinion, site observations and testing during the monitoring of the construction, the TITLE 18 — ZONING completed development substantially complies with the recommendations in the geotechnical report and with all geotechnical- related permit requirements. Occupancy of the project will not be approved until the report has been reviewed and accepted by the DCD Director. (8) Conditioning and Dental of Use or Devel- opments. (A) Substantial weight shall be given to ensuring continued slope stability and the resulting public health, safety and welfare in determining whether a development should be allowed. (B) The City may impose conditions that address site -work problems which could include, but are not limited to, limiting all excavation and drainage installation to the dryer season, or sequencing activities such as installing erosion control and drainage systems well in advance of construction. A permit will be denied if it is determined by the DCD Director that the development will increase the potential of soil move- ment that results in an unacceptable risk of damage to the proposed development, its site or adjacent proper- ties. (f) Abandoned Mine Areas (1) Development of a legal lot of record con- taining an abandoned coal mine area may be permitted when a geotechnical report shows that significant risks associated with the abandoned mine workings can be eliminated or mitigated so that the site is safe. Approval shall be obtained from the DCD Director before any building or land - altering permit processes begin. (2) Any building setback or land alteration shall be based on the geotechnical report. (3) The City may impose conditions that address site -work problems which could include, but are not limited to, limiting all excavation and drainage installation to the dryer season, or sequencing activities such as installing drainage systems or erosion controls well in advance of construction. A permit will be denied if it is determined that the development will increase the potential of soil movement or result in an unacceptable risk of damage to the proposed develop- ment or adjacent properties. (g) Areas Of Important Geological Or Archaeological Evidence (1) Development on a legal lot of record determined to have historic or prehistoric geological or archaeological evidence, shall be prohibited until that evidence has been studied or researched for any valu- able information about our history. Removal or salvage of the evidence shall be done in accordance with RCW 27.53, and shall be performed in a timely manner. (2) Once the geologic or archaeological evi- dence or articles have been studied or researched, or the importance of the site is declared to be marginal or not of use to the scientific community, development shall be allowed on the site. Development shall not Printed August 4, 1993 Page 18-49 TUKWILA MUNICIPAL CODE begin on such a site until the DCD Director gives approval. (h) Permitted Uses Subject to Exception Approval. Other uses may be permitted upon receiving a reason- able use exception pursuant to Section 18.45.115. A use permitted through a reasonable use exception shall conform to the procedures of this chapter and be consistent with the underlying zoning. (Ord. 1608 § §1, 5, 1991; Ord 1599 §3(part), 1991) 18.45.090 Sensitive areas tracts. (a) In development proposals for planned resi- dential or mixed area use developments, short subdi- visions or subdivisions, and boundary line adjustments and binding site plans, applicants shall create sensitive areas tracts, in lieu of an open space tract, per the standards of Section 18.46.080. (b) Applicants proposing development involving uses other than those listed in subsection (a) of this section, on parcels with sensitive areas or their buffers, may elect to establish a sensitive areas tract which shall be: (1) If under one ownership, owned and main- tained by the ownership, which protection of the tract; (2) Held in common ownership by multiple owners who shall collectively be responsible for main- tenance of the tract; or (3) Dedicated for public use if acceptable to the City or other appropriate public agency. (Ord. 1599 §3(part), 1991) 18.45.115 Exceptions. (a) General. With the approval of the Director of DCD, isolated wetlands that are four hundred square feet or smaller in area, and which are low in value according to the rating methodology used in the City's Water Resource Rating and Buffer Study, may not require the compensatory mitigation standards of this chapter. (b) Piping. Piping will be allowed in Type 1 and Type 2 watercourses' only where relocation or alter- ation of a watercourse is denied and would result in denial of alLreasonable. use. (c) Reasonable Use Exceptions. —(1)--If-application-of-this-chapter would deny all reasonable use of the property containing wetlands, watercourses or their buffers, the property owner or the proponent of a development proposal may apply for a reasonable use exception. (2) The application for a reasonable use excep- tion shall be in a format specified by and filed with the Department of Community Development (DCD). Requirements may include an environmental impact statement pursuant to WAC 197.11 -400. Reasonable use exceptions shall be decided by the Planning Commission following a public hearing noticed as specified in Chapter 18.92. (3) If the applicant demonstrates to the satisfac- tion of the Planning Commission that application of the provisions of this chapter would deny all reasonable use of the property, development may be allowed which is consistent with the general purposes of this chapter and the public interest (4) The Commission, in granting approval of the reasonable use exception, must determine that: (A) No reasonable use with less impact on the sensitive area and its buffer is possible; (B) There is no feasible on -site alternative to the proposed activities, including reduction in size or density, phasing of project implementation, change in timing activities, revision of road and lot layout, and/or related site planning activities that would allow a reasonable economic use with fewer adverse impacts to the sensitive area and its buffer, (C) As a result of the proposed develop- ment there will be no increased or unreasonable threat of damage to off -site public or private property and no threat to the public health, safety or welfare on or off the development proposal site; (D) Alterations permitted shall be the minimum necessary to allow for reasonable use of the property; (E) The proposed development is compat- ible in design, scale and use with other development with similar site constraints in the immediate vicinity of the subject property; (F) Disturbance of sensitive areas has been minimized by locating the necessary alterations in the buffers to the greatest extent possible; (G) The inability to derive reasonable use of the property is not the result of actions by the appli- cant in segregating or dividing the property and creating the undevelopable condition after the effective date of the ordinance from which this chapter derives; and (H) Any approved alteration of a sensitive area under this section shall be subject to conditions as established by this chapter and will require mitigation under an approved mitigation plan. If a development is approved as a reasonable use, the Board of Architectural Review's process, review and standards shall be applied. (Ord 1599 §3(part), 1991) 18.45.120 Variances. (a) The Board of Adjustment shall review requests pursuant to Chapter 18.72 for variance from the standards of this chapter unless excepted by Section 18.45.115. (b) If a variance is granted, it shall be the mini- mum necessary to accommodate the permitted uses of the underlying zoning districts proposed by the appli- cation, and the scale of the. use may be reduced as necessary to meet this requirement. (Ord. 1599 §3(part), 1991) Page 18-50 Printed August 4, 1993 18.45.125 Appeals. (a) Any aggrieved party who objects to or ': agrees with Department of Community Development (DCD) decisions or conditions for development in a sensitive area shall appeal to the Planning Commission. Any such appeal shall be made in writing within ten days of the interpretation, condition or decision being appealed, and shall set forth the basis for the appeal. (b) In considering appeals of decisions or condi- tions, the following shall be considered: (1) The intent and purposes of the sensitive areas ordinance from which this chapter derives; (2) Technical information and reports consid- ered by the Department of Community Development; and (3) Findings of the DCD Director which shall be given substantial weight. (Ord. 1599 §3(part), 1991) 18.45.130 Recording required. The property owner receiving approval of a use or development pursuant to this chapter shall record the City- approved site plan clearly .delineating the wetland, watercourse, areas of potential geologic instability or abandoned mine and their buffers designated by Sections 18.45.020 and 18.45.040 with the King County Division of Records and Elections. The face of . the site plan must include a statement that the provisions of this chapter, as of the effective date of the ordinance from which this chapter derives or thereafter amended, control use and development of the subject property, and provide for any responsibility of the property owner for the maintenance or correction of any latent defects or deficiencies. (On1. 1599 §3(part), 1991) 4o 18.45.135 Asstsa ice device. (a) In appropriate circumstances, the Director of the Department of Community Development may require a letter of credit or other security device accept- able to the city, to guarantee performance and mainte- nance requirements of this chapter. All assurances shall be on a form approved by the City Attorney. (b) When alteration of a sensitive area is approved, the Director of the Department of Community Development may require an assurance device, on a form approved by the City Attorney, to cover the monitoring costs and correction of possible deficiencies. Monitoring of alterations may be required for up to five years. (c) Release of the security does not absolve the property owner of responsibility for maintenance or correcting latent defects or deficiencies. (Ord. 1599 §3(part), 1991) TITLE 18 — ZONING 18.45.140 Assessment relief. (a) Fair Market Value. The King County Assessor shall consider sensitive area regulations in determining the fair market value of land under RCW 84.34. (b) Current Use Assessment. Established sensi- tive area tracts, as defined in Section 18.06.695 and provided for in Section 18.45.090, shall be classified as open space and owners thereof may qualify for current use taxation under RCW 18.34; provided, such landowners have not received density credits, or set- back or lot size adjustments as provided in Chapter 18.46. (c) Special Assessments. Landowners who qualify under subsection (b) of this section shall also be exempted from special assessments on the sensitive area tract to defray the cost of municipal improvemer s such as sanitary sewers, storm sewers and water mains. (Ord. 1599 §3(part), 1991) Printed August 4, 1993 Page 18-51 City of Tukwila John W. Rants, Mayor Department of Community Development Rick Beeler, Director STAFF REPORT TO 'nth, PLANNING COMMISSION Prepared October 5, 1993 HEARING DATE: October 14, 1993 FILE NUMBER: 93 -01 -APP Hillcrest 90- 13- BLA/91 -3 -APRD L93 -0063 LOCATION: Slade Way & S. 160th Street APPELLANT: LeRoy Lowe A.I.A. Architect REQUEST: Appeal a decision by the De.partment of Community Development for slope stability requirements in the Sensitive Areas Ordinance, Section 18.45.080 (E). STAFF: Jack Pace, Senior Planner Daniel Clayton, Shannon/Wilson, Inc. PAGE ATTACHMENTS: +' A. Appellant's letter - July 20, 1993 5 9 B. Planning Division letter - July 12, 1993 6 3 C. Neil H. Twelker letter - June 22, 1993 6 5 D. LeRoy. Lowe letter - May 25, 1993 9 5 E. Planning Division letter - May 14, 1993 10 2 F. LeRoy Lowe letter - May 4, 1993 10 8 G.. LeRoy Lowe letter - March 30, 1993 118 H. Cascade Geotechnical, Inc. - March 22, 1993 13 2 I. LeRoy Lowe letter - March 25, 1993 13 6 J. Planning Division letter - March 15, 1993 142 K. Shannon/Wilson, Inc. - November, 1992 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 4313670 • Fax (206) 4313665 dl Staff Report to the L93- 0063 :Hillcrest Planning Commission Page 2 FINDING S BACKGROUND On July 2, 1990, a public hearing was held for a waiver request by the applicant from the City's Sensitive Areas Moratorium. The petitioner proposed to re- plat /short plat 5 single - family lots for development. The moratorium Ordinance No. 1544 precluded filing of new development - related applications in sensitive areas, including subdivisions or short sub - divisions, without a waiver. The applicant's property ha slopes over 15 percent. The waiver was granted with conditions by the City Council. Those conditions allowed Mr. Lowe to submit a Boundary Line Adjustment application. On October 16, 1990, the City received applications for a short plat and a boundary line adjustment from LeRoy Lowe. After reviewing both applications, staff concluded that only a boundary line adjustment is required. On June 28, 1991, the SAO became effective. Until this time, staff could not finalize the boundary line adjustment submitted by Mr. Lowe. As a requirement of the SAO an APRD (Administrative Planned Residential Development) was required concurrently with the existing application. The APRD application was submitted on September 9, 1991. On March 19, 1992, the applicant appealed the director's decision regarding the Sensitive Areas Ordinance requirements (TMC 18.45.125). The Planning Commission's conclusions are listed below and denied the appeal. * Based upon TMC 18.45.060 the applicant is required to do a wetland study as part of the boundary line adjustment review. The applicant does not meet the waiver criteria contained in TMC 18.45.020 (F). The proposed development plan could have detrimental impacts on the wetland. * As noted in the report, the applicant has provided adequate geotechnical soil reports for the initial review. Further work may need to be done once the wetland study has been completed. In addition, further stormwater studies need to be done to determine the impacts on down -slope properties. * At this time, the applicant has not provided the needed information to address this issue. Once the wetland study has been completed, there may be justification to use this section of the ordinance. Q2 Staff Report to the L93- 0063:Hillcrest Planning Commission Page 3 DECISION CRITERIA Administrative decisions by the Department of Community Development may be appealed as follows: 18.45.125: Appeals (a) Any aggrieved party who objects to or disagrees with Department of Community Development (DCD) decisions or conditions for development in a sensitive area shall appeal to the Planning Commission. Any such appeal shall be made in writing within ten days of the interpretation, conditions or decision being appealed, and shall set forth the basis for the appeal. (b) in considering appeals of decisions or conditions, the following shall be considered: The intent and purposes of the sensitive areas ordinance from which this chapter derives; Technical information and reports considered by the Department of Community Development; and Findings of the DCD Director which shall be given substantial weight. The SAO contains the following requirements for interpretation: TMC 18.45.030: Interpretation: The provision of this chapter shall be held to be minimum requirements in their interpretation and application and shall be liberally construed to serve the purposes of this chapter. The applicant is appealing staff's interpretation of Section 18.45.080(e)(4)(c). (C) Geotechnical reports for Class 3, Class 4 and Coal Mine Hazard Areas shall included a site evaluation review of available information about the site, a surface reconnaissance of the site and adjacent areas, and a subsurface exploration of soils and hydrology conditions. Detailed slope stability analysis shall be done if the geotechnical consultant recommends it in Class 3 or Coal Mine Hazard Areas, and must be done in Class 4 areas. In ._.Attachment .A,.. the applicant has explained why he is appealing staff's interpretation of the ordinance. He has stated:, "We appeal your consultants interpretation of the SAO and disagree with his opinion regarding the slope stability analysis prepared by Cascade Geotech.. Mr. Lamb of Staff Report to the L93- 0063:Hillcrest Planning Commission Page 4 Cascade has stated that this site is stable and the potential for landsliding on this property has been addressed." As reflected by the numerous attachments, their has been on -going discussion with the applicant and staff regarding this issue. Attachment A through K contain the applicant's and staff's technical material and position regarding slope stability analysis. In his letter, Mr. Lowe disagrees with the evaluation of the adequacy of the slope stability analysis performed by Cascade Geotechnical Consultants and offers the following opinions as additional basis for his appeal: * Three professional engineers have advised him that, "there will be no adverse impact to the project or surrounding properties"; * "Construction would have a stabilizing effect in controlling on -site water and thus, add a dimension of slope stability"; * "Mr. Lamb of Cascade has stated that the site is stable and the potential for landsliding on this property has been addressed"; and * Various foundation designs are available that will eliminate any impact to the project as a result of geologic instability." In reviewing Mr. Lowe's letter and attachments, There is no basis for revising the conclusions presented in the peer review and summarized in the consultant's letter of June 28, 1993 to the City of Tukwila. With the exception of the last point listed above, the opinions that Mr. Lowe has offered in his appeal were previously discussed with Mr. Lowe and were considered in our letter of June 28, 1993. To reiterate staff's position, both shallow and deep- seated stability analyses are necessary to evaluate the geologic stability of the proposed project. As of this time, no deep- seated analysis has been performed for the project, although the applicant has been advised repeatedly of the importance staff has placed on this analysis in evaluating the geologic stability of the property. In staff's opinion, a deep- seated analysis is critical to this evaluation because: * The proposed development is situated on an old landslide deposit, with the scarp of the slide crossing the property immediately west of the proposed residences. 44 Staff Report to the L93- 0063:Hillcrest Planning Commission Page 5 * Surficial soils in several areas downslope from the site appear to be unstable, including an area which is affecting Slade Way to the south of the proposed development. * Slope indicator data from the vicinity of Slade Way immediately downslope of the property provide questionable evidence of minor, relatively deep- seated slope movement since the 1960 landslide. * A large deep- seated slope failure occurred in 1960 on an adjoining property a short distance downslope. Recurrent movement on this landslide, if it were to occur, might threaten the stability of the site. * Three static an dynamic slope stability analyses conducted during the 1980's indicate that the deep- seated slope stability of the area immediately downslope from the proposed Hillcrest Development is marginal, at best, and likely to. diminish over time if the hillside dewatering system operated by WSDOT continued to deteriorate. The slope stability analysis report prepared by Cascade Geotechnical does not address these issues nor does it provide rationale why these considerations were not addressed. Considering the evidence that deep- seated sliding cold potentially affect the project, staff does not believe that the requirements or intent of the SAO have been met. Lacking documentable evidence to the contrary, it would appear prudent to assume that the site is at risk from a deep- seated slope failure and that such a failure could pose a considerable risk to the proposed development and its occupants. With respect to Mr. Lowe's opinion that the foundation design options presented in his appeal could be used to eliminate any impact to the project resulting from geologic instability, we believe that he has not provided an adequate basis for this conclusion. It is staff's opinion that a meaningful evaluation of foundation feasibility and design for this site would require a deep- seated slope stability analysis along with an understanding of the depth of potential movement that could result from sliding. This information has not been provided by the applicant, nor has it been developed to our knowledge. Consequently, there does not appear to be any basis to conclude that foundations could be designed to eliminate the risk of irreparable damage to the structure or possible risk to human health or safety. Along with Mr. Lowe's appeal, his consultants have provided information about a proposed relocation of the wetlands currently on site to a location on the lower part of the hillside. Although staff has concerns about the effect of such a wetland on the d' Staff Report to the Planning Commission Page 6 L93- 0063:Hillcrest overall slope stability, this aspect of the project has not been considered in our review because it is our understanding that the placement and design of wetlands is not relevant to the requirements for a BLA. CONCLUSIONS As noted in criteria discussion, the Planning Commission needs to consider three items in making their decisions; 1) Intent/Purposes of the Sensitive Areas Ordinance. The applicant has not addressed the following purposes statements contained in the Sensitive Areas Ordinance: (1) Minimize developmental impact on the natural functions and values of these areas. (4) Prevent erosion and the loss of slope and soil stability caused by the removal of trees, shrubs, and root systems of vegetative cover. (5) Protect the public against avoidable losses, public emergency rescue and relief operations cost, and subsidy cost of public mitigation from landslide, subsidence, erosion and flooding. 2) Technical Information As reflected by the numerous reports contained in the attachments, both staff and the applicant have hired technical experts to review this site. Staff believes that both shallow and deep- seated stablity analyses are necessary to evaluate the geologic stability of the proposed project. As of this time, no deep- seated analysis has been performed for the project. 3) DCD Director shall be given substantial weight The City has retained Shannon & Wilson, Inc. to provide peer review of technical material. Based upon their expertise and review with other governmental agencies, the DCD Director has concluded the applicant must demonstrate one of the following: 1) There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no significant risk to the proposed development or surrounding properties; or Staff Report to the Planning Commission L93- 0063:Hillcrest Page 7 .2) .. .The area of potential geologic instability.can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability. RECOMMENDATION Based upon the evidence and evidence to overturn the Director's interpretation, staff recommends the appeal be denied. . r, LEROY C. LOWE A.I.A. ARCHITECT I.O. SOX 1241 SEATTLE. WASHINGTON SSM JUL 2 .1 1n3 JACK PACE SENIOR PLANNER CITY OF TUKWILA 6300 SOUTHCENTER BLVD. TUKWILA, WA. 98188 DEAR MR. PACE : JULY 20, 1993 1 HAVE REVIEWED THE IMPACT OF CONSTRUCTING RESIDENCES ON THE HILLCREST PROPERTY WITH THREE PROFESSIONAL ENGINEERS. THEIR OPINION IS THAT THERE WILL BE NO ADVERSE IMPACT TO THE PROJECT OR SURROUNDING PROPERTIES; TO THE CONTRARY, CONSTRUCTION WOULD HAVE A STABILIZING EFFECT IN CONTROLLING ON SITE WATER AND THUS ADD'A DIMENSION OF SLOPE STABILITY. WE APPEAL YOUR CONSULTANTS INTERPRETATION OF THE SAO AND DISAGREE WITH HIS OPINION REGARDING THE SLOPE STABILITY ANALYSIS PREPARED BY CASCADE GEOTECH. MR. LAMB OF CASCADE HAS STATED THAT THIS SITE IS STABLE AND THE POTENTIAL FOR LANDSLIDING ON THIS PROPERTY HAS BEEN ADDRESSED. The SAO indicates that "prior to permitting alteration of an area of potential geologic instability, the applicant must demonstrate one of the following: 1) There is no evidence of past instability or earth movement in the vicinity of the_ proposed development, and quantitative analysis of slope stability indicates no signifi- cant risk to the proposed development or surrounding properties; or RESPONSE : 1) THE HILLCREST PROPERTY HAS NOT BEEN THE SUBJECT OF EARTH MOVEMENT IN RECENT HISTORICAL TIME AND THE HILLSIDE BELOW SLADE WAY HAS BEEN STABLE ' FOR SOME 32 YEARS. THE QUANTITATIVE ANALYSIS OF . SLOPE STABILITY PREPARED BY GEORGE LAMB INDICATES THERE IS NO SIGNIFICANT RISK TO THE PROPOSED DEVELOPMENT OR SURROUNDING PROPERTIES. • • 2) The area of potential geologic instability can be modified or the project can be de- signed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability." RESPONSE : 2) THE DEVELOPMENT OF THE PROPERTY HAS BEEN DESIGNED SO THERE IS NO IMPACT TO THE PROJECT AND SURROUNDING PROPERTY. SLOPE STABILITY WILL BE INCREASED BY THE CONTROL OF SURFACE WATER WITH THE CONSTRUCTION OF OUR STORM WATER DETENTION SYSTEM. WATER DISCHARGE WILL BE CONTROLED AND SEDIMENTATION ELIMINATED WITH THIS WATER DETENTION SYSTEM. WE ARE INCLUDING WITH THIS LETTER, FOUNDATION DESIGN OPTIONS THAT WILL ELIMINATE ANY IMPACT TO THE PROJECT AS A RESULT OF GEOLOGIC INSTABILITY. ALSO INCLUDED ARE LETT RS FROM MR. TWELKER & .BREDBERG VERY TRULY YOURS LEROY C. LOWE A.I.A. �, � Woo cf_7,01 1 -=F Y. HILLCIZ6T ) /L' U. C_ /CZ C:eDA/C - TL -I‘16 REGIST� 1 ARCHITECT LEROY C. LOWE STATE OF WASHINGTON -2 j11es.1702Cib'cS7- /612C, Cee -f f/NG ,1:4// 50;y__//LD,5A 9:101 4770- ArAMeTr-,4 .BOLTS`k L : C, ., _. �/oz fwvi� G # kS/&-M v 4LL •.\ AA„ a SECTION V& -/C v/5,LAc2M&YT DETAIL v/. -. tet= ' - Arc Po/ V7Z5 ? q'o'Qc . 477c g4 7/A/ :1-c").4L A-14)/ ¢0..4iSI .Q iBo 7af @ c i—R7(-/A.KC)4T/ON..: PL -4A/ ELEVATION .cfc4z,o.: / a"' -- - -- - : J • -. 1 m -m. C...... 1 r r •./. nILccPEsT REGISTERED ARCHITECT LEROY C. LOWE STATE Of WASHINGTON T ', /12'0442 2C. A F7::Jc3" Z/Z2)2Y- PL 1/ .(21 ¢-Qp• C. ZPIj= 'tel.' 7X11 7 �- YS�'"5- z5 -y FS -"7C./• :ELEVATION • SECTION • C, LJ__ �-LAS--�J� 7t�.--�Qfi�i L.1�L_✓'1�_rvr��r�+�� DETAIL 1 111 %11 •ua • 14/(X'aD737.07;c7-G-3"11-721f2v.5 C2C.KI54E-4.7:4%/C/Gi 737,4= -A------- SI1011 IP a I APFIVA/JV 4,1.041 • _vv AU -401. morammariacfra HILLCIZ6T REGIIERED ARCHITECT LEROY C. LOWE STATE Of WASHINGTON •7A4v1/14.47-= pz...ximntp M277fi-451-1.577. — --1Cf=7-7157._•5_1.57- -- --F;e2Vc71/A/c-: 1c=1 -4/V ZAW 4E /r/1c. 7 - Apr t _ZX - =11 = •Zave1245,i r c r, I. at;iZ7A _ccnc,:zitiz".7 r Lel dP 11, 44, IL. ;2Z47 SECTION /11= 41. 57/VZT5 " , e.• 7_117 AP ligaliW • aizTi ;44 dr•.0 dip • I -ZI'525 .1-7. 04. 1/".C7 -13nLZ.5 ;-„ '''Cle44- / 040 -4'71-‘171) -.E.zrAfr_21A/ ELEVATION L5C.AL /P - /2- cr* 'edberg -c` Associates, Inc. Post Office Box 1337 (206) 858 -7055 Gig Harbor, WA 98335 FAX (206) 858 -2534 r July 20, 1993 NEE Leroy Lowe WM P.O. Box 3972 Bellevue, WA 98009 soassexas =Cemat RE: Wetland creation Tukwila site Dear Lowe: Class I11 wetlands can be created on the Tukwila site by detaining the surface sheet flow to create hydric conditions and supplementing the existing vegetation with wetland species. The wetland can be created with out creating areas of surface water and running water. It will require specific design and implementation, but I feel it can be done with the team of geoengineers and engineers currently working on the project. The intent will be to compact the soils and establish low (less than 1 foot) berms to create hydric conditions. Willow, black cottonwood and westem red cedar will be planted to help stabilize the slope. A conceptual mitigation plan can be provided when needed. 54 If you have any questions please feel free to call me. Si fcerel Ali NSCSS • . A.J. Bre S "‘ieesI.J1,1511►',,, • P . '2 1 Telephone: (206) 284 2410 Neil H. Twelker and Associates, Inc. Consulting Soils Engineers 5645 42nd Ave. West Mail to :. P.0 Box 99086 Seattle, Washington 98199 July 19, 1993 Leroy C. Lowe, A.I.A P.O. Box 1241 Seattle, Washington 98111 Re: Residential Foundation Design Hillcrest Residential Development Slade Way near South 160th St. Tukwila, Washington Dear Mr. Lowe: In several recent discussions which we have had (including the meeting with representatives of the City of Tukwila on June 21, 1993, the question of the interpretation of the applicable City Ordinance has arisen. The following paragraphs are intended to explore this matter in greater detail, and to develop a foundation system which would resolve the doubts of the City personnel regarding the feasibility of the project. The part of the Ordinance in question states: .or, The area of potential geologic instability can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability." (City of Tukwila Regulation) The first question to be resolved here is: What is the area of potential instability? While we continue to believe that the loss of Slade Way, which would constitute a threat to all of the properties on the uphill side of Slide Way (some 300 families), is a catastrophe of simply unthink- able proportions which would never be allowed to happen, it is nevertheless true that some people are viewing this as a real possibility. If this were to occur, the potential area of instability could conceivably extend to the base of the scarp on the upper part of the Hillcrest site. In the long run, it would not matter if the ancient slide which has been hypothesized by some members of the Geotech- nical community had actually occurred, or if the extent of previous slide movement is represented by the present location of Slade Way. In either case, the threat posed to uphill property by uphill progression of a major landslide is the same (i.e., damage to improvements and Toss of property values). Leroy C. Lowe July 19, 1993 Page 2 Although the improvement of drainage on the Hillcrest property would be of some limited value (while producing no adverse impacts on adjacent properties), we must assume that, in the face of a total breakdown of the drainage system which has thus far maintained the stability of the area, nothing within reason could be done on the Hillcrest property to actually stabilize the ground below Slade Way ; consequently, the only recourse here would be to design the structures "...so that any potential impact to the project and surrounding properties is eliminated .. . Now comes the question: What is the nature of the impact? From the behavior of the slope during the past several years, we conclude the impact to the new development would consist of Toss of support of the structures, either in part or in whole, over a period of months, or (probably) years. The effect would not be limited to Hillcrest, however; it would extend to many other properties as well. A significant aspect of the potential threat to all uphill locations is that it would not proceed with such speed that it would be a threat to human life. Can the projects be designed to eliminate the potential impact of future instability? The answer to this question is, "Yes, at moderate cost." The problem is to restore foundation support to those areas of the structure in which support has been lost. This could require either support to certain areas of a building (where support has been lost by progressive breaking up of the soil formation) • or it could mean the releveling of an entire building because of a massive rotational slip. In either case, the buildings would be designed to resist the stresses imposed by the loss of support and to be releveled by jacking against concrete pads placed beneath the bearing elements, or by jacking against portions of the foundation. Three specific design proposals have been advanced and have been reviewed by us for use in these projects. These are (1) Separation of the stemwall from the spread footing, with bolted brackets at closely spaced intervals, in which jacks would be placed to relevel the structure, (2) Separation of the foundation plate from the stemwall, with the superstructure to be raised by jacking (as in the above system); and (3) If the structure is to be supported on augercast piles, the jacks would be placed between the grade beams and the piles. Any of these sys- tems (as illustrated in working drawings which you have prepared and forwarded to us) could be employed with every prospect of success, and at comparatively modest cost. Of course, it could be argued that if the slide were to go on and on and on, that it would swallow up any and all attempts to correct for the loss of support. It would appear, however, that the large number of dwelling units which would be affected (nearly all of which would have been built without the accommodating features described above) by uncontrolled extension of 'the slide mass would mandate a restoration of the original 1960's drainage system which proved so effective for so many years. A possible remaining question is that of the interpretation of the wording of the Ordinance: Does " .. potential impact to surrounding properties . . ." refer to the impact on adjacent properties produced• by the proposed development, or does it refer to the impact produced on adjacent properties by the hypothetical landslide? It seems very clear to us that the wording is meant to apply to the impact produced by the development, rather than the landslide. (The "impact" of the devel- opment would be entirely favorable, because of the improved surface and subsurface drainage.) The alternative interpretation (i.e., that the development must address the impact of the landslide on the adjacent properties) would be so outlandish as to assign to a developer the task of guaranteeing eternal life and prosperity to everyone within the distance of a cannon shot. In the discussion of foundation types and possible methods of mitigation of future damages, we feel obliged to point out that at the present stage of the development only the question of lot line EXPIRES 31t31 / q cr 7- 1 Leroy C. Lowe July 19. 1993 Page 3 revision is under consideration; specific design proposals for general outline, substructure type, foundation support, and other details are to be dealt with in future stages of the project. The decision to allow a lot line revision will presumably be based on the perceived practicality of a foundation system which will provide the necessary degree of support for the structures to be constructed here, and the method which would be used to correct any loss of grade which might be occasioned by continued deterioration of the 1961 drainage system. The releveling methods outlined above have been used by us on many occasions: we are able to state unequivocably that they are simple, economical, and effective. We note also that the various Geotechnical consultants whose input has been sought for this devel- opment have recommended a variety of foundation systems, ranging from simple spread footings to augercast piles. While, we continue to believe that this site presents no unusual or difficult prob- lems, and that the development can be executed by spread footings, simple drainage procedures, and a modest amount of regrading, we have no overwhelming objection to the use of ultraconservative foundation systems, nor do we believe that they would interfere with the cor- rection of future damages brought about by a reactivation /extension of the old landslide. Wetlands Substitution We have been in contact with A. J. Bredberg of Bredberg and Associates, Inc. regarding the formation of a wetlands on the slope below Slade .Way (as a substitute for the wetlands to be lost in the development of the Hillcrest property). His proposal calls for the for- mation of a Class III wetland, in which hydric soils would be created by compacting the existing soils to reduce their permeability, and surface runoff impeded slightly by very low berms. Exist- ing vegetation would then be supplemented with wetland species. We note that the effect of this program would be to create a local alteration of drainage within the surficial soil layers, producing a perennial increased moisture content in the uppermost layers. To the degree that infiltration of surface water is impeded by compaction of the surface soils, the effect of the wetland addition would actually be beneficial to the stability of the hillside. In any event, we believe that the wetlands aspect of the Hillcrest development could have no adverse effect whatever on the groundwater con- dition which has been responsible for the landslide below Slade Way. We shall be pleased to discuss our conclusions at greater length with you or other interested parties at any time or in any place. N HT: nt Very truly yours, NEIL H. TWELKER AND ASSOC.. INC. Neil H. Twelker, Pres. 67 City of Tukwila John W. Rants, Mayor Department of Community Development Rick Beeler, Director July 12,1993 Leroy Lowe P.O.Box 1241 Seattle, Wa 98111 RE: Soil Geotechnical Evaluation of proposed Lowe BLA Dear Mr.Lowe: At the June 21,1993 meeting Mr.George Lamb of Cascade Geotechnical Consultants, disgreed with with our interpretation of the first part of item 2 of Section 18.45.080(e) of the SAO. Attached to this letter is the results of our rewiew of the interpretation..Under the SAO , you have 10 days from the date of this leter to appeal the decision (TMC Section 18.45.125). Should you have any questions, please feel free to call or write. Sincere dY ck Pace enior Planner cc:Ron Cameron, City Engineer ATTACHMENT B 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 4313670 • Fax (206) 4313665 Ell' SHANNON &WILSON, INC. June 28, 1993 City of Tukwila Department of Public Works 6300 Southcenter Boulevard, Suite 100 Tukwila, Washington 98188 Attn: Mr. Jack Pace, Senior Planner RE: SOILS AND GEOTECHNICAL EVALUATION OF PROPOSED HILLCREST DEVELOPMENT, TUKWILA, WASHINGTON SEATTLE EVERETT KENNEWICK FAIRBANKS ANCHORAGE SAINT LOUIS In our peer review of the slope stability analysis report for the proposed Hillcrest Development we indicated that, in our opinion, the applicant has not adequately addressed the potential for landsliding on this property, nor has he demonstrated that the property meets the requirements set forth in the City of Tukwila Sensitive Areas Ordinance (SAO). Specifically, we indicated that the requirements of Section 18.45.080E of the SAO have not been met. This section of the SAO indicates that "prior to permitting alteration of an area of potential geologic instability, the applicant must demonstrate one of the following: 1) There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no signifi- cant risk to the proposed development or surrounding properties; or 2) The area of potential geologic instability can be modified or the project can be de- signed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability." A meeting was held on June 21, 1993 between the City of Tukwila and Mr. LeRoy Lowe, the applicant, to discuss our review and what steps need to be taken to proceed with application to develop the property. At that meeting the applicant's geotechnical consultant, Mr. George Lamb of Cascade Geotechnical Consultants, disagreed with our interpretation of the first part of Item 2 of Section 18.45.080E of the SAO. As we understand his argument, Mr. Lamb interpreted this clause to mean the following: • 400 NORTH 34TH STREET • SUITE 100 P.O. BOX 300303 SEATTLE, WASHINGTON 98103 206.632.8020 FAX 206.633.0777 r JUN 2 91993 I:) F. v . :_ ._: '= .. i; _ NT • W- 6367 -02 City of Tukwila Attn: Mr. Jack Pace, Senior Planner June 28, 1993 Page 2 SHANNON &WILSON. INC. The area of potential geologic instability can be modified or the project can be designed so that any. potential' impact resulting from the project to the project itself or to the surrounding properties is eliminated. Based on his interpretation of the wording of this clause, Mr. Lamb felt that this condition has been met for the proposed Hillcrest Development and, consequently, that this requirement of the SAO has been fulfilled. In contrast, we interpret this clause to also require that any potential impact to the project from potential geologic instability must be eliminated by design or site modification. Our review indicated that this requirement has not been demonstrated for the proposed development. In order to resolve this difference of opinion, Mr. Jack Pace advised the applicant that the City would review the intent of the wording and consult with the geotechnical consultant who prepared the study on which the SAO was based to determine what their intent had been in this wording. At your request, we contacted Mr. Donald Tubbs of GeoEngineers, Inc., the author of the report that was used in. preparing sections of the SAO pertaining to landslide. hazards. Mr. Tubbs advised us that GeoEngineers, Inc. did not write the clause in question. However, he said that their intent was that for development in an area of potential geologic instability, an applicant should be required to demonstrate that through either site modification or project design there would be no adverse impact to the development as a result of geologic instability. We also reviewed the Sensitive Areas Ordinance for King County to determine what require- ments the county has for developments on areas of potential geologic instability. The King County ordinance specifies that alterations to property in landslide hazard areas may only be allowed in the following circumstances: 1) If the development proposal will not decrease slope stability on adjacent properties; and 2) If the landslide hazard area can be modified or the development proposal can be designed so that the landslide hazard to the project and adjacent property is eliminated or mitigated and the development proposal on that site is certified as safe by a licensed geologist or geotechnical engineer. W- 6367 -02 City of Tukwila Attn: Mr. Jack Pace, Senior Planner June 28, 1993 Page 3 SHANNON &WILSON, INC: Based on this review, we believe that the intent of the clause in the Tukwila SAO is clearly stated and is consistent with both Geo Engineers' intent and interpretation and with the wording of a similar provision in the King County Sensitive Areas Ordinance. Therefore, it is our opinion that there is no reason to revise the conclusions presented in our review of the slope stability analysis for the proposed Hillcrest Development. Sincerely, SHANNON & WILSON, INC. Daniel N. Clayton,A.E.G. Senior Associate DNC:WPG/eet ' W63 67-02.LT1 /W63 67-1kdie et W-6367-02 Telephone: (206) 284 2410 Ne i I H. Twelker and Associates, Inc. Consulting Soils Engineers 5645 42nd Ave. West Mail to : P.0 Box 99086 Seattle, Washington 98199 June 22, 1993 City of Tukwila 6300 South Center Blvd, Suite 100 Tukwila - Wa 98188 Attn: Jack Pace, Dept of Cummunity Development Re: Hillcrest Development (Leroy Lowe) Dear Jack: In reviewing our conference of yesterday (June 21, 1993), it has occurred to me that we seemed to be talking past each other, and that we (all of us) somehow failed to center on the real issues which are involved in this dispute. I would like to take one last shot at a real focus on just what is involved here, not as much for Leroy Lowe as for the entire community; to do this I am taking the liberty of addressing you directly. One remark that Dan Clayton (Shannon and Wilson) made stands out in my mind: "Slade Way is just a peanut on the back of an elephant!" If he is viewing Slade way as a load, and the slope as the beast which must bear the load, I agree with him entirely. In this respect Slade Way is even less than a peanut. But, it depends on one's point of view; if Dan is thinking of Slade Way as a load which could contribute to a reactivation of that prehistoric slope failure which he has hypothesized, Slade Way is as nothing, as close to nothing as anything could be. On the other hand, to a resident of the area, who must depend on Slade Way for thoroughfare, fire and police protection, sewerage, electricity, water, communications and other vital links to the outside world, Slade Way is a great deal more than a peanut. Let us go back to the facts on which we can all agree: 1. The 1960 slide was instigated by an excavation near the base of the slope. 2. None of the ground which now lies uphill of Slade Way was involved in this activity. 3. For a period of more than 30 years the drainage system installed by WashDOT kept the formerly unstable earth mass in equilibrium. 4. The recent evidences of unstable earth in the vicinity of Slade Way are associated with deterioration of the drainage system installed over 30 years ago. JUN 2 8 1993 COMMUNITY 43 City of Tukwila June 22. 1993 Page 2 While I am not entirely certain as to just when it was that the concept of the prehistoric slide (with its 3000 -ft long headscarp, with groundwater emerging from its base) made its appearance, I nevertheless have the impression that the potential implications of this feature did not play an important part in any of the developments which have taken place in the area during the decades since the slide occurred. (And if these implications were considered, they certainly appear to have been discounted.) Like Dan Clayton, I can easily visualize a horde of young geologists becoming involved in a never - ending exploration into the mysteries of ice - marginal deposition; it sounds like an absolutely fascinating project, especially when one considers that the answer to the real problem is already known! (Just repair the old 1960 drainage system.) The final truth must be self- evident to everyone: Unless Slade Way moves, the Hillcrest property is not going to move. If the City of Tukwila (or the State of Washington ) is going to maintain the drainage system which has been shown to be so effective in protecting Slade Way and the ground which lies uphill of it, the Hillcrest property is not going to be endangered by a reactivation of either the 1960 slide or a reactivation of the hypothesized "prehistoric slide." During this meeting you refused on several occasions to answer the question as to what the City's intentions are regarding the future of Slade Way. I cannot help but think that if the City of Tukwila intends to let Slade Way fall into disuse it should (out of courtesy to its citizens) so state without delay. . Very truly yours, NEIL H. TWELKER AND ASSOC.. INC. by N HT: nt cc: Leroy Lowe Neil H. Twelker, Pres. MEMORANDUM TO: DATE: RON CAMERON / JACK PACE MAY 25, 1993 PROJECT: HILLCREST BLA SUBJECT: SLOPE STABILITY ANALYSIS GENTLEMEN : RECEIVED MAY 2 6 1993 CITY OF TUKVI'1LA MAYOR'S OFFICE BOUND TOGETHER WITH THIS MEMORANDUM ARE THE FOLLOWING DOCUMENTS FOR YOUR REVIEW AND COMMENT. I. SLOPE STABILITY ANALYSIS PREPARED BY ; CASCADE GEOTECH INC. DATED : 4 -30 -93 2. PEER REVIEW OF SLOPE STABILITY ANALYSIS (CASCADE GEO) PREPARED BY : NEIL TWELKER P.E. PHD 5 -3 -93 3. TRANSMITTAL LETTER PREPARED BY : JACK PACE 5 -14 -93 4. SOILS & GEOTECHNICAL EVALUATION OF PROPOSED LEROY LOWE DEVELOPMENT PREPARED BY : DAN CLAYTON / PAUL GRANT DATED : 5 -5 -93 5. CLARIFICATION OF SLOPE STABILITY ISSUES PREPARED BY : NEIL TWELKER P.E. PHD DATED : 5 -17 -93 6. PEER REVIEW OF "OUR" EVALUATION OF THE HILLCREST TUKWILA PROPERTY PREPARED BY : CASCADE GEOTECH DATED : 5 -19 -93 7. LETTER OF REVIEW & COMMENT PREPARED BY : CASCADE GEOTECH INC. DATED : 5 -20 -93 THESE DOCUMENTS ARE PROVIDED TO YOU FOR YOUR REVIEW AND COMMENT. LEROY C . LOWE A.1 A.I.A. LEROY C. LOWE A.I.A. ARCHITECT r.0. •OR 1341 •C•rVLS. ■■•■41wG1.O14 9•111 CASCADE GEOTECHNICAL INC. 12016115TH AVENUE N.E., BLDG. H (206) 821.5080 KIRKLAND, WASHINGTON 98034 FAX: (206) 820.6953 March 22, 1993 Job No. 9005 -13G Mr. Leroy C. Lowe, A.I.A. P.O. Box 1241 Seattle, Washington 98111 Reference: Slope Stability Analysis Hillcrest Residential Development Slade Way, South of South 160th Street Tukwila, Washington Dear Mr. Lowe: As requested, we have completed a slope stability analysis for the above site. The following letter summarizes our analysis and presents conclusions regarding the present and post - construction stability of the north -south trending, east facing slope. We understand that this analysis will be used for a Boundary Line Adjustment application. SCOPE Per the proposal for services letter dated February 24, 1993, our scope of work consisted of: • reviewing existing geotechnical reports previously prepared for this project • analyzing the present stability of the north -south trending slope on your property by constructing two cross sections oriented perpendicular to the slope. • preparation of this letter stating the results of our analysis. PROJECT DESCRIPTION Slope profiles were derived from an undated topographic site plan titled " Hillcrest," that was prepared by Leroy C. Lowe. The site plan is scaled at 1" =30' and topography is shown in two foot intervals. Property lines and adjoining roads Slade Way and South 160th Street are clearly delineated. Based on our understanding of the project, development will consist of the construction of five (5) single family residences, with driveway access from Slade Way and South 160th Street. All five residences are to be located in the gently sloping area at the base of the 2(H):1(V) slope. Several areas on the slope are moderately steeper than 2(H):1(V). CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G We reviewed two geotechnical reports previously prepared for this project by Cascade Geotechnical Inc.' We also reviewed a report prepared by Shannon & Wilson, Inc.2 which addresses geotechnical implications for this project, Slade Way, and Klickitat Drive. SLOPE STABILITY Methodology We analyzed the stability of two sections of the north -south trending slope, for both pre - construction and post- construction conditions. These sections were chosen based on their proximity to previously obtained subsurface information, as well as their ability to represent the most critical areas of the slope. We performed both shallow and deep seated failure analyses. Cross section A -A' begins in the southwest property corner and extends northeasterly across the entire property. Cross section B -B' begins approximately 200 feet north of the southwest property corner and also extends northeasterly across the entire property. Both cross sections extend twenty -five (25) feet beyond the eastern and western property lines. For more details, please refer to the site plan and cross sections contained in the appendices. Utilizing the computer program SB- SLOPE, we have determined minimum factors of safety against.slope failure under both static and pseudostatic (seismic) loading conditions. SB -SLOPE incorporates Bishop's Simplified method of slices. To mimic a severe seismic event, a horizontal inertial force equal to 0.2 times the total weight of the potential sliding mass was used in the pseudostatic analysis. This inertial value was selected based on Seattle D.C.L.U. recommended values', which are generally accepted as conservative. Parameters Scil parameters were conservatively estimated based on previously obtained subsurface soils information. Six soil types were chosen that we believe accurately characterize actual soil conditions. The typical soil strengths selected are based on our experience with similar soils in this area. The pre - construction analysis incorporated a ground water surface based on elevations ' Cascade Geotechnical Inc., May 30, 1990, Preliminary Geotechnical Investigation for Hillcrest Slade Way, South of South 160th Street, Tukwila, Washington; and addendum report dated August 27, 1990. 2 Shannon & Wilson Inc., November 23, 1992, "Geotechnical Review of Proposed LeRoy. Lowe Development, Tukwila, Washington." 3 Seattle Department of Construction and Land Use Director's Rule 3 -93, March, 1, 1993. ' 2 CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G encountered during our subsurface investigation in May and August of 1990, and springs, which were observed at the toe of the slope. The post - construction analysis incorporated the benefits of the drainage enhancements shown on the plans. The phreatic surface is assumed to be lowered to the dense sands after drainage is installed. The following table summarizes the soil parameters used in our analysis. SOIL PROPERTIES Soil Type & Classification Approx. Depth from Surface Dry Unit Weight (pcf) Cohesion (psf) 1, (SP) dry sand, loose 0 to 12 feet: upper slope 96 0 31 2, (SP) dry sand, medium dense 12 to 25 feet: upper slope 101 0 34 3, (SP /ML), wet sand and silt, med. dense to dense 25 to 33 feet: upper slope 38 to 45 feet: lower slope 108 200 30 4, (SP), wet sand, dense 33 to 45 feet: upper slope 45 to 65 feet: lower slope 110 0 37 5, (SP /SM) wet silty sand, loose 0 to 25 feet: lower slope 96 500 31 6, (SP /SM) wet silty sand, medium dense ` 25 to 38 feet: lower slope 1 101 500 34 RESULTS Pre - construction With respect to deep failures, our results show that the slope is presently stable under static and pseudostatic (with seismic loading) conditions. With respect to shallow failures, our results 3 Cab CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G show that the slope is stable under static conditions, but unstable during pseudostatic conditions. The following tables summarize the results of our analysis. Pre - construction Factors of Safety Cross Sections Deep Seated Shallow Static Pseudostatic Static Pseudostatic A -A' 1.87 1.20 1.16 0.75 B -B' 1.66 1.10 1.04 0.69 The shallow, pseudostatic failure surface for section A -A' began four (4) feet west of the crest of the slope and extended approximately forty (40) feet eastward beyond the toe. The shallow, pseudostatic failure surface for section B -B' began and ended at the crest and toe of slope, respectively. Both shallow failure surfaces were, at their deepest point, only approximately three (3) feet deep. The relatively low factors of safety for shallow, pseudostatic conditions are a result of the oversteepened slope created during the past ancient landslide, which left the fine to medium grained sands on the slope face very near their angle of repose. The deep seated, pseudostatic failure surface for section A -A' began at the crest of the slope and continued approximately forty (40) feet eastward beyond the toe. The deep seated, pseudostatic failure surface for section B -B' began forty (40) feet west of the crest of the slope and terminated eastward at the toe. Both deep seated failure surfaces were, at their deepest point, approximately twenty (20) feet deep. Post- construction With respect to deep failures, our results show that the slope will be significantly improved by the planned drainage. The slope will be stable under static and pseudostatic conditions. Shallow failures may still occur on the slope due to the slope being very near the angle of repose. The following tables summarize the results of our analysis. 4 CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G Post - construction Factors of Safety Cross Section Deep Seated Static Pseudostatic Section A -A' 2.18 1.35 Section B -B' 1.85 • 1.25 The failure surfaces occurred at approximately the same location as in the preconstruction analysis. CONCLUSIONS Under pre - construction conditions, we conclude that the slope will be stable except for shallow instability during a maximum probable seismic event that may result in mass wasting of several feet of soil directly on the slope face. This shallow mass wasting can be characterized by localized slumping and not movement of the entire slope. With post- construction improvements, we conclude that the slope will be generally stable for deep failures, under both static and pseudostatic conditions. The post construction improvements include the French drains shown in the plans. Shallow failures may still occur under strong seismic loading. We conclude that the proposed Boundary Line Adjustment for this site will not affect the suitability of the site development provided our recommendations contained in the Recommendations Section of this report are carefully followed. RECOMMENDATIONS Building Setbacks To protect property from shallow failures (localized sloughing) during a maximum probable seismic event, we recommend the following: • The exclusion of any building development within forty -five (45) feet of toe of the slope. .5 CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G GENERAL We recommend that we engaged to review the final plans when they become available to confirm that our recommendations have been properly interpreted and to provide any additional or alternate recommendations as necessary. We expect the on -site soil conditions to reflect our findings; however, some variations may occur. Should soil conditions be encountered that cause concern and /or are not discussed herein, Cascade Geotechnical Inc. should be contacted immediately to determine if additional or ,alternate recommendations are required. This letter has been prepared for the exclusive use of Leroy Lowe, for specific application to the proposed Hillcrest Residential Development located at Slade Way, South of South 160th Street, Tukwila, Washington, in accordance with generally accepted soils and foundation engineering practices. No other warranty, expressed or implied, is made. Thank you for this opportunity to work with you on this project. If you have any questions, feel free to contact us at any time. Respectfully submitted, CASCADE GEOTECHNICAL INC. George E. Lamb, P.E. President 6 Larry Jackson Project Manager HILLCREST SLOPE STABILITY ANALYSIS SITE PLAN SOUTH 16°TH S7gEfT TEST PIT (05/15/90) HAND AUGER (05/23/90) X TEST BORING (07/24/90 & 07/25/90) FROM A LANDSCAPE SITE PLAN BY LEROY C. LOWE. ALA. ARCHITECT (03/18193) AND FROM TEST HOLE LOCATION MAP BY CASCADE GEOTECHNICAL (05/29190) CASCADE GEOTECHNICAL INC. 1) U1611)t11AV1I111111 (,1111/:. 11 x:111.16.1 'n �, . 1.I141,140 W \)IwGIJN 56,114 1 \1 I /'/y. f. •'�� �,1 lob No 9303.05G SCALE : 1' = 50 LOCATION!: ARE APPROXIMATE Do" 04/08193 eY HLA [611. 04;08193 G•ol 7Z HILL 'REST SLOPE STABILITY ANALYSIS CROSS SECTION A - A' O Q 0 ^O 0 r) N N 0 ra INI I 1 111 l.11 llld I I I I3N11 AIH3dONd I II _° 0 0 0 N Q I 1 I 1 1 0 0 0 0 0 at 0 n at rrl • N N N O gr 0 o N 0 OI 0 0 U °- Z. Q . -- U Z I 0= Wes- 0 • W = -° 0 U• = Q _ • • _ 0 8 in cu 01 cm 1NI7 A .1U3d01.1d ) (11 1 ) (I) 1 1 1 I 74 I I •cr 0 — — l) 0 = 0 0 CV 0 cs, 0 1-• co 11. — 0 = 1 Ca 0 0 0 0 0 VI CV 01 C.1 CV CV CV IS j 0 0 CV 0 0 7 01 LOCATIONS ARE APPROXIMATE 0 01 0 o Telephone: (206) 284 2410 Neil l-L Twelker and Associates, Inc. Consulting Soils Engineers 5645 42nd Ave. West Mail to : P.O Box 99086 Seattle, Washington 98199 May 3, 1993 Leroy C. Lowe, A.I.A P.O. Box 1241 Seattle, Washington 9811.1 Re: Review of Slope Stability Analysis by Cascade Geotechnical Hillcrest Residential Development Slade Way near South 160th St. Tukwila, Washington Dear Mr. Lowe: At your request we have performed a review of an analysis of slope stability prepared by Cascade Geotechnical Inc. (March 22, 1993) for your proposed residental development near the intersecton of Slade Way and S. 160th St. in Tukwila, Washington. We present herewith a report of our findings, conclusions, and recommendations. Site Description The site, which has 50 ft of frontage on S.160th St. and 300 ft of frontage on Slade Way, is an irregular- shaped tract having overall dimensions of 300 ft east -west by 500 ft north - south. It consists of three principal topographic elements: (1) an upper very gently sloping area along the west boundary of the tract, the width of which varies from 20 ft at the north to approxi- mately 180 ft at the south; (2) a steeply sloping central area whose topographic relief varies from 20 ft at the south to 32 ft at the north; and (3) a gently sloping, poorly drained area occupying the westerly half of the property, whose total relief is 20 to 24 ft, with an average declivity of 15 to 20 per cent. Two well- defined streams cross the property from west to east, fed by springs at the base of the steep slope. The site is devoid of structures, with the exception of a sewer which has been constructed in an easement along the south property line, to a manhole at the southeast corner of the property, and thence northerly in Slade Way. The property is covered by scattered deciduous trees, a few small cedar trees, and a sparse understory of native brush. Soils within the site are known (from subsurface explorations performed by Cascade Geotechnical Inc.) to consist of the following principal soil units: 1. The upper (i.e., most recent) unit consists of soft sedimentary peat covering much or most of the poorly drained lower area to depths of less than a foot to as much as five feet. Z5. Leroy C. Lowe. A.I.A May 3. 1993 Page 2 2. The underlying unit consists of moderately dense to dense sand, the upper foot or two of which is somewhat loosened from weathering. 3. A thin unit of stiff silt was found between 23 and 28 ft in one test boring (located in the low area). The lowermost unit encountered consists of dense sand. Goundwater was encountered at depths varying from 1 to 2 ft in the low area and at depths of 25 to 30 ft in the westerly upland. An important feature of the site is the emergence of groundwater at the base of the steep slope. Cascade Geotechnical Inc. Stability Analyses In their report of March 22, 1993, Cascadce Geotechnical begins by mentioning that they had reviewed their own previous reports and also, a report prepared by Shannon and Wilson dated November 23, 1992. They go on to outline their methodology, soil values, and the results of their studies, including both static and pseudostatic (seismic) reviews of stability. The method employed was the "Simplified Method of Slices," developed by Alan Bishop (based on the original "Method of Fellenius "). Two cross sections were selected, to be representative of the most critical areas of the slope, and because of their proximity to locations at which soil conditions had already been determined. The parameters for the strength and density of the soil were given for 6 classifications of inorganic soils found at the site. No soil parameters were assigned to the peat, presumably because it was expected to play no part in the scenario of a hypothetical slope failure. The Cascade Geotechnical studies showed the preconstruction site to be very stable for static and pseudostatic (seismic at 0.2 times the acceleration of gravity) conditions in the deep- seated failure mode. The deep- seated failure mode was taken from 4 ft west of the crest of the steep slope and extending 40 ft east of the toe of the slope, while the shallow zone study addressed only the actual steep slope itself, an area which is very near the "angle of repose" of a granular soil. The shallow failure mode was calculated to be only barely stable for static conditions, and not at all stable for the pseudostatic seismic review. With the site drained in accordance with the Cascade Geotechnical recommendations, the stability of the deep- seated failure mode is improved somewhat; the results of the shallow failure mode are not reported. Cascade Geotechnical concludes its report with a recommendation to exclude "any building develop- ment within 45 feet of the toe of the slope," in order to avoid the consequences of a shallow zone failure. Although the Cascade study included a review of the Shannon and Wilson report of Nov. 23, 1992, they did not comment on any of the conclusions presented in that report. Conclusions On the basis of our review of the available information (including an examination of the site) we draw the following principal conclusions: 1. The soil parameters used in the Cascade Geotechnical report are believed to be conservative and generally representative of the conditions which prevail at the site. 2. The method of analysis employed is an accepted one, and the results of their study compare very well with our intuitive appraisal of the site and with our shorthand methods of calcu- lation. We note in particular that the most serious failure mode (deep- seated movement) is very unlikely in any event (whether drained or undrained, static or seismic). 7', Leroy C Lowe A.I.A May 3. 1993 Page 3 3. That the safety factor in shallow zone mode is low is not all surprising, inasmuch as the declivity of the steep slope is very near the angle of repose (i.e., the declivity or pitch at which any additional material would simply roll or slide to the base of the slope). The amount of material involved in a failure of this kind is comparatively small, and the consequences correspondingly minor. Moreover, it is a fairly simple matter to reinforce those parts of the slope which are critical to structures placed at or near the toe, or to provide catchment for the displaced material, a fact which would obviate the need for the 45 ft set -back recommended by Cascade Geotechnical. 4. No analysis was applied specifically to the gently sloping lower area, presumably on the grounds that an area with a declivity as low as this, and with a dense soil structure, must be inherently stable, even if it does have a high ground water table. We concur with this view; we see no problem with the stability of the lower area, whether in the static condition or in the seismic condition. 5. The peat which covers much of the lower area is totally incapable of support of any major or settlement - critical structure; all major improvements must be founded on the underlying moderately dense sand (a comparatively simple undertaking). The simplest and most econom- ical site preparation procedure will be to conduct an initial program of shallow drainage, after which the peat may be removed from the building area and the structure placed on spread footings on the native sand formation. We see no need for the use of piling foundations for structures in any part of the site. We believe that any drainage of the soil units, or expediting of the escape of surface water, will have a favorable effect on the stability of the entire area. 6. We note that the poorly drained lower area, besides having outstanding capabilities for the breeding of mosquitoes, possess a special status under the designation of "Wet Land," making it necessary to find a way to execute the project to the satisfaction of the regulating agencies who sit in judgment of such matters. Two methods may considered for the treatment of the wetland problem: (1) removal of the peat only from the building footprint, with the remainder of the site covered with organic materials such as wood chips or bark (to allow foot traffic), or (2) stripping of the entire site and regrading with imported inorganic materials. Inasmuch as the law requires that "lost" wetlands be replaced with a 50 per cent increase in wetland, it would be necessary to provide a poorly drained area of an appropriate size at some other location in either case. You have advised us that you own or have access to property below Slade Way, to the north of the 1960 slide mass which can be converted to "wetland" by altering its topography so that it becomes poorly drained. 7. The Shannon and Wilson Report Cascade Geotechnical reviewed the Shannon and Wilson report but did not comment on it; we, on the other hand, feel compelled to note two important points of disagreement with that report, as presented in the following paragraphs. 8. Indications of Prehistoric Landslide On page 3 of the S and W Nov. 23, 1993, report they state, "The steep slope that separates the upland and lowland portions of the site is interpreted to be the southern end of the headscarp of an ancient extensive landslide. This landslide escarpment extends about 3000 feet northwest of the site and typically displays about 30 to 40 feet of topographic relief. At the base of the escarpment, the ground is typically hummocky and poorly drained and commonly is the site of springs and boggy conditions, as it is on the Lowe property. This disrupted drainage probably has contributed to the numerous smaller slides within the ancient slide mass." At this juncture we direct Leroy C.' Lowe. A.I,A May 3. 1993 Page 4 attention to the fact that the steep slope (or scarp) which traverses the Lowe Property is a feature whose declivity exceeds that which could be expected in a "normal" landscape whose contours are developed from erosion from sheet flow and surface weathering. When one encounters a feature of this kind, an explanation for its presence is called for. The two most common reasons for the occurrence of an abnormally steep slope are (1) it is the head scarp of a large scale landslide, or (2) it has been developed by "sapping," (Le., the result of the emergence of groundwater at the toe of the slope). The conditions reported by Shannon and Wilson describe the scarp throughout its length as having springs and seeps at its base; nevertheless, they unhesitatingly attribute the presence of the scarp to an "ancient landslide." We are unable to dispute (on the basis of direct evidence) the existence of an ancient landslide; however, we can point to the presence of the springs and seeps at the base of the scarp as a "smoking gun," easily capable of having caused the feature in question. The great length of the steep slope, together with the presence of groundwater emergence at its toe throughout its length, lead us to conclude that sapping (rather than mass earth movement) has been responsible for the formation of the scarp. It appears that in their search for an explanation of a physical feature, Shannon and Wilson have overlooked an obvious and ongoing natural process in favor of an unsupported hypothesis whose implications are somewhat sinister (in that they cast doubt on the history of the Hillcrest property). 9. Shannon and Wilson call attention to the presence of "bowed and leaning tree trunks" in the easterly part of the Hillcrest area as evidence of shallow soil creep. While shallow creep could very well be taking place in the upper organic soil units, it should be pointed out that the trees in this area consist mainly of alders, whose growth direction is influenced primarily by the search for Tight and air. 10. Influence of Recent (1960) Landslide Shannon and Wilson allude (at considerable length) to the large landslide which occurred between Slade Way and Klickitat Drive in 1960. We were present almost at the moment when that slide commenced; it was instigated by large- scale removal of material from the slope (for use in preparing the Andover industrial site). A very high groundwater table also played a prominent part in the Toss of hillside stability. The slide was eventually brought under control by an elaborate network of drainage which included horizontal drains and deep well pumps. From the early 60's to the mid 80's the system appears to have been effective in preventing noticeable movement within the slope; however, a number of small slides and set -downs in the area (even extending into the right -of -way of Slade Way) have led to a series of studies by WSDOT, Dames and Moore, and Geoengineers, all of which concluded that the drainage system was undergoing an "ongoing deterioration," and that ".... without a functional drainage system there would be an unacceptable risk of deep - seated slope failure at the site." Shannon and Wilson then proceed to attach the feasibility of the Hillcrest development to the restoration of the drainage system which had demonstrated so well its ability to maintain stability for several decades. They imply that i f the 1960 landslide were to become reactivated (because of the deterioration of its drainage system) and i f the slide mass were to extend laterally into Slade Way to the boundaries it has been known to occupy, and if the slide mass were extended even more to the west (involving earth which had not been previously disturbed, and if the new boundaries of the unstable mass somehow were to "join up with" the those of prehistoric slide which they have postulated (we believe mistakenly), then the improvements of the Hillcrest development would be at risk. While we are able to understand and sympathize with the reluctance of a responsible engineer to acquiesce to the construction of a new development on ground which has fallen under the Leroy C. Lowe: A,I.A May 3. 1993 Page 5 cloud of potential instability, we nevertheless feel compelled to call attention to the following points which we believe to over -ride and outweigh all other considerations: a. Slade Way, which lies between the Hillcrest property and the 1960 landslide, is a very important (if not absolutely vital) thoroughfare in this area. In addition to providing access for local traffic and emergency vehicles, it also contains important buried utilities without which any number of households in the area could not survive. b. The unfolding of the scenario outlined above would threaten a considerable number of existing residences in the same manner that the Hillcrest development would be threatened. Barring a fortuitous intervention by a "public spirited angel," it appears that the City of Tukwila must either (1) undertake the restoration of the deteriorating drainage system or (2) have on hand a contingency program to include the condem- nation of existing residences (and relocation of their occupants), and the rerouting of the utilities which serve residents located beyond the area which would be directly affected by the contemplated reactivation of the 1960 landslide. Likewise, it also appears that the residents whose lives would be disrupted should be concerned with their own agenda: (1) If forced to move. where would they go? (2) What recourse could they pursue following the Toss of Slade Way? (3) What steps could be taken to prevent the loss of Slade Way? and (4) How can they force the City of Tukwila to perform the tasks ordinarily accep ed by city government? In all this confusion, it is obvious that the least affected of II parties would be the owners of the Hillcrest property (which, after all, is nothi g more than poorly drained vacant land). c. The success of the means of st demonstrated over a period of thre proper maintenance and/or replace timely manner. This is a civic res large number of residential propert community, cannot be placed on th In summary, we believe that whatever stability p have are of very minor importance, that they are with at the time of development by fairly straightf We shall be pleased to discuss our conclusions w convenience. � `.`' =.S /7• s. 4) i/S• -q 5757 1.-- • N HT: nt ilizing the 1960 landslide area has been well decades. Continued stability simply requires that ent of the drainage installations be performed in a onsibility which, because of its great extent, the es involved, and the potential impact on the shoulders of a private citizen. oblems the proposed Hillcrest development might onfined to the actual site, and that they can be dealt rward methods of drainage and site preparation. h you or other interested parties, at your Very truly yours, NEIL H. TWELKER AND ASSOC.. INC. Neil H. Twelker, Pres. May 14,1993 City of Tukwila Department of Community Development Leroy Lowe P.O. Box 1241 Seattle, WA 98111 John W. Rants, Mayor RE: Soil geotechnical evaluation of proposed Lowe BLA Dear Mr. Lowe: Attached to Wilson,Inc. demonstrate the City of Rick Beeler, Director this letter is the peer review completed by Shannon & In their conclusions they do not believe the studies that the property meets the requirements set forth in Tukwila's Sensitive Areas Ordinance. Under the SAO, you may appeal this decision (TMC Section 18.45.125). Given the complexity of this issue, I suggest a meeting to review the positions of the consultants and discuss ways in which to resolve the issues. Should you have any questions, please feel free to call or write. Sincerely, Jack Pace Senior Planner cc: Ron Cameron, City Engineer .6300 Southcenter Boulevard. Suite #100 • Tukwila. Wa.ShInPton 02122 • 1206) 411•. 7670 • Fay! 12061431-3665. .1.1°W SHANNON WILSON, INC. SEATTLE ® GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS FAIRBANKS ANCHORAGE SAINT LOUIS May 5, 1993 City of Tukwila Department of Public Works 6300 Southcenter Boulevard, Suite 100 Tukwila, Washington 98188 Attn: Mr. Jack Pace, Senior Planner RE: SOILS AND GEOTECHNICAL EVALUATION OF PROPOSED LEROY LOWE DEVELOPMENT, TUKWILA, WASHINGTON At your request, we have reviewed the slope stability analysis report dated March 22, 1993 by Cascade Geotechnical, Inc. for the proposed Hillcrest Residential Development, Slade Way South of 160th Street, Tukwila, Washington. We understand that this report was .prepared to fulfill a requirement of the City of Tukwila Sensitive Areas Ordinance (SAO) for an application for a Boundary Line Adjustment for the subject property. As part of our review, we have reviewed three previous geotechnical reports for the Hillcrest Development dated May 30, 1990, August 27, 1990, and April 24, 1991, and a supplemental letter dated July 27, 1992, all prepared by Cascade Geotechnical, Inc. We also have referred to our geotechnical review of the proposed Hillcrest Development dated November 23, 1992, and our letter of February 1, 1993. These latter documents recommended that slope stability analyses be performed to assess the potential for both shallow and deep seated landsliding, and stressed the need for careful evaluation of the potential for landsliding on this property before proceeding with any development that might endanger human health or safety. It is our opinion that these recommendations are consistent with the requirements set forth in the City of Tukwila SAO. REVIEW COMMENTS The March 22, 1993 report by Cascade Geotechnical, Inc. describes the methodology, soil parameters used, and results of pre- and post - construction stability analyses for both shallow and deep - seated sliding under static and pseudostatic conditions. These analyses were conducted specifically to evaluate the stability of the north -south trending, steep slope that runs through the center of the property. Two cross sections were reportedly developed and analyzed, both 400 NORTH 34TH STREET • SUITE 100 P.O. BOX.300303 SEATTLE, WASHINGTON 98103 206.632.8020 FAX 206.633.67 77 �4VP�•J •t • r. W- 6367 -01 �1 City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 2 SHANNON &WILSON. INC. trending in a northeasterly direction across the property and extending 25 feet beyond the western and eastern property lines. Cascade Geotechnical's report concludes that with respect to deep - seated stability, the site is stable under both pre - and post construction conditions, and that deep - seated slope stability will be significantly improved by the planned drainage. With respect to shallow slope stability, only the pre - construction factors of safety are presented. These show the site to be marginally stable under static conditions and unstable under seismic loading. The report states that shallow failures may still occur under post - construction conditions, both under both static and pseudostatic conditions, and therefore recommends a 45 -foot construction setback from the toe of the steep slope. This setback would appear to require relocation of three of the proposed residences shown on the site plan. In reviewing this stability analysis report, we have identified three main issues which are not addressed, and which in our opinion must be considered in an evaluation of the stability of this property. These issues, as itemized below, revolve around a central issue; the stability analyses were performed for the steep north - trending slope rather than for the property as a whole. ► There does not appear to be adequate subsurface data to define the stratigraphic and groundwater conditions below the site in such a manner that they can be used in analyzing the deep - seated slope stability of the property. It is our understanding that three borings ranging from 26.5 to 31.5 feet deep were used, along with several shallow test pits and hand auger borings, to define subsurface conditions and to develop the cross sections for the site. Our primary concern with this is that there are no borings that extend deep enough to intercept the deep confining strata or the underlying confined aquifer which was a primary cause of landsliding immediately to the east of the site. Consequently, the depth of this confined aquifer is not known, nor is there any information on the hydraulic head within the confined aquifer. ► The report does not address the potential for reactivation of landsliding immediately downslope from the property in the stability analyses. There is also no acknowledg- ment in the report that such a problem might exist. Multiple studies for the area immediately downslope from the proposed Hillcrest Development indicate that the stability of this area is dependent on an extensive hillside dewatering system. This system, which is about 30 years old, is reportedly deteriorating and its capability W- 6367 -01 82. City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 3 SHANNON &WILSON. INC. to lower the hydrostatic head acting on the hillside is diminishing. Consequently, the factor of safety for hillside stability can be assumed have dropped below a level that several years ago was considered by the Washington Department of Transporta- tion and Dames & Moore to be unacceptably low for a proposed residential devel- opment. Without a basis to conclude otherwise, it appears reasonable to assume that the factor of safety for deep seated slope stability of the proposed Hillcrest Development would be similar to that which has been determined for the property immediately downslope. In our opinion, slope failure of the area immediately downslope could readily impact the stability of the proposed Hillcrest Development, and must be considered in evaluating the deep - seated stability of the property. ► The site subsurface data do not define the depth of the slip plane of the ancient landslide that is interpreted from geomorphic expression and previous studies to underlie the eastern half of the property. While the stability analysis report indi- cates a deep - seated failure surface at a depth of 20 feet, this appears to be iri reference to a hypothetical failure plane below the steep slope west of the proposed building sites. We believe that in order to adequately assess the deep seated stability of this site it will be necessary to further evaluate the evidence of a deep seated slide plane under the site, including a determination of its depth and soil and groundwater conditions above and below it. This information would help to resolve uncertainties regarding the potential for slide reactivation or involvement of the property in a deep - seated failure. Because the stability analysis report only addresses the potential for sliding of the steep north - trending slope in the center of the property, and ignores the potential significance of a deeper seated slide, we believe it would be inappropriate at this time to critically review the stability analyses that have been performed. From a cursory review, however, it appears that the stability analyses are conservative with respect to shallow soil conditions and seismic loading conditions. However, the following additional issues that should be addressed or clarified before a critical review can be accomplished. ► The report does not include all of the assumptions that were used in developing the cross sections or in performing the analyses. Specifically, the following are not provided: a) it is not clear how the cross sections and soil parameters were devel- oped below the maximum depth of soil borings of 31.5 feet; b) assumed slip surfaces are not shown, c) groundwater levels used in the analysis are not shown; and d) it is not clear how the considerable subsurface detail shown in the vicinity of the steep slope was derived. W- 6367 -01 City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 4 SHANNON FJWILSON. INC. ► Cascade Geotechnical's report of August, 27, 1990, indicates that a hydrostatic head which reached the ground surface was noted during drilling of one of the borings. This observation suggests that confined groundwater conditions exist in the relatively shallow strata underlying the site strata. Similarly, the report also indicates that there may be a number of separate, confined aquifers at depth. It would be appropriate to evaluate the potential significance of these confined zones to the stability of the site if this has not already been done. • The planned drainage enhancements that have been assumed in the post - construction stability analyses are not shown on the copy of the plans that we received. Without knowing what these proposed enhancements are, it is not possible to provide an opinion on the validity of this assumption. However, the report assumes that these drainage enhancements would lower the groundwater to the level of the dense sand. According to the table of soil properties presented on page 3, this level would correspond to depths of 33 to 45 feet below the upper slope and 45. to 65 feet below the lower slope. It is not clear to us what type of drainage has been proposed to accomplish such a groundwater drawdown, but this does not appear to be consistent with our previous understanding that relatively shallow French drains were to be used. ► The slope stability analyses reportedly used groundwater levels measured during the summer of 1990, rather than higher water levels that would be expected during the winter months. While this may not make much difference for the analyses of post - construction conditions, it could be an important factor in the analysis of present -day conditions. A higher water table is often an overriding factor in initiating landsliding sliding because of increased pore -water pressures. ► There is no discussion of the impact of site grading or excavation, either of which could significantly affect shallow slope stability during or after construction. CONCLUSIONS The slope stability analysis report for the proposed Hillcrest Development addresses the stability of the north - trending steep slope that runs through the center of the property rather than the stability of the property as a whole. In our opinion, the primary concern for stability of the property is related to the potential for a deep seated failure that could affect much if not all of W- 6367 -01 134° City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 5 SHANNON &WILSON, INC. the property, rather than the smaller, more localized type of failures which are addressed in this report. With respect to the stability analyses that have been performed, it appears that these analyses are generally conservative in assessing the stability of the north - trending steep slope. However, this cannot be substantiated without additional documentation of the assumptions that were used in the stability analyses. Depending on what these assumptions were, additional subsurface data may be required to derive a meaningful stability analysis of this slope. In our opinion, the slope stability analyses that have been performed do not adequately address the potential for landsliding on this property, nor do they demonstrate that the property meets the requirements set forth in the City of 'Tukwila SAO. Specifically, Section 18.45.080E of the SAO states that prior to permitting alteration of an area of potential geologic instability, the applicant must demonstrate either: 1) There is no evidence of past instability or earth mo\,:ment in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no significant risk to the proposed development or surrounding properties; or 2) The area of potential geologic instability can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability. Our review indicates that the proposed development does not meet the first condition, and because the potential for deep seated sliding has not been addressed, it has not been demonstrated to meet the second condition required for permitting a development in an area of potential geologic instability. W- 6367 -01 City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 6 Please call if there are any questions regarding this letter. Sincerely, SHANNON & WILSON, INC. Daniel N. Clayton, C.E.G. Senior Associate Reviewed by: W. Paul Grant, P. E. Vice President DNC:WPGIdnc W6367-01 .LTR/W 6367- 1kdllkd SHANNON F,WILSON. INC. W- 6367 -01 Telephone: (206) 284 2410 Ne i I R Twelker and Associates, Inc. Consulting Soils Engineers 5645 42nd Ave. West Mail to : • P.0 Box 99086 Seattle, Washington 98199 May 17, 1993 Leroy C. Lowe, A.I.A P.O. Box 1241 Seattle, Washington 98111 Re: Clarification of Slope Stability Issues Hillcrest Residential Development Slade Way near South 160th St. Tukwila, Washington Dear Mr. Lowe: We are in receipt of the "peer review" recently made by Shannon and Wilson of the March 22, 1993 report of Cascade Geotechnical. Our comments and further clarification of the issues which are involved in this discussion are presented in the following paragraphs. The Shannon and Wilson position in this latest report is exactly the same as in their previous report (November 23, 1992), namely that the site is threatened by a reactivation of the 1960 slide because of the deterioration of the WSDOT drainage system at the base of the hill. Inasmuch as the terms " deep - seated sliding," deep - seated aquifers," and "confined groundwater tables" have been bandied about without explanatory detail, and because the discussion of the problem will once more take place before persons not totally familiar with those terms or their true significance, we feel obliged to present the following additional clarifying information. Deep - Seated Slide Studies On page 4 of the Cascade Geotechnical Report (March 22, 1993) the approximate positions of the two deep - seated slide circles for the pseudostatic case are described. The report does not state that the static circles were in the same (or nearly the same location); however, we consider it only logical that the positions of the critical circles would not differ greatly for the two conditions of analysis. We note also that the report contains no other direct reference to the location of the circles for the static analysis. The failure surfaces referred to are described as, "..for section A -A' (it) began at the crest of the slope and continued approximately forty (40) feet eastward beyond the toe. The deep - seated, pseudostatic failure surface for section B -B' began forty (40) feet west of the crest of the slope and terminated eastward at the toe. Both deep - seated failure surfaces were, at their deepest point, approximately twenty (20) feet deep." Leroy C. Lowe May 17. 1993 Page 2 The positions of these critical circles are illustrated in the attached cross - section of the site, which extends from the westerly margin of the site, through Slade Way to Klickitat Drive at the base of the .' hill. It is evident .that Cascade was concerned only with the possibility of slides on the actual property, whereas Shannon and Wilson are principally concerned with a recurrence of slide activity on the much deeper failure surface of the 1960 episode, an approximate location of which is also shown on the attached cross section. We direct attention to the fact that the known boundary of slide activity stops well short of the Hillcrest property; in fact, it does not actually extend to the pave- ment of Slade Way at this location (although it has crossed Slade Way to the south of the property. The disparity of focus is clearly stated in the Shannon and Wilson "Conclusions" (page 4 and page 5): "... the primary concern for stability of the property is related to the potential for a deep- seated failure that could affect much if not all of the property, rather than the smaller more localized type of failures which are addressed in this (the Cascade) report." On page 3 of their May 5, 1993, letter Shannon and Wilson state, "The site subsurface data do not define the depth of the slip plane of the ancient landslide that is interpreted from geomorphic expression and previous studies to underlie the eastern half of the property." The slip plane which they have inferred is also shown (approximate position) in the attached cross section. It is of the greatest importance to recognize that the Shannon and Wilson slip surface is not a feature which has been directly observed; it is based (as they state) on an "interpretation of geomorphic expression." This is an interpretation with which we take the strongest exception, even though it appears that the other firms which have reported on the problem (including Cascade Geotechnical) may also have made the same blunder. As we have stated in our letter of May 3, 1993, an abnormally steep slope can either be explained as having been created by massive earth movement, or by differential ero- sion (sapping). The presence of a water table at the base of the slope, and the vigorous springs which exit there can easily explain the presence of the steep slope. To postulate that a massive earth movement has created a feature with a length of some 3000 ft, is to reduce the presence of ground- water throughout the toe of that feature to the status of "mere coincidence." (What an amazing coincidence!) With regard to the City of Tukwila requirements quoted on page 5 of the S and W report, we offer the following comments: "There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no significant risk to the proposed development or surrounding properties." (City of Tukwila Regulation) Evidence of past instability is confined to the area below Slade Way, and to the south of the subject property. While the interpretation of the word "vicinity" might be open to some discussion, it appears to us that the importance of Slade Way (as a corridor for emergency vehicles and vital utilities) •is so great that it is inconceivable that it will be allowed to be seriously disrupted, or even that its "rescue" would be shunted off to the shoulders of a private citizen. We point out once more that the stability of the area for more than three decades is absolute proof that the means of protecting this valuable area are sufficiently understood that remedial efforts can be undertaken at any time (without the elaborate and expensive geotechnical studies so dearly beloved of Shannon and Wilson). • We call attention to a report prepared by GeoEngineers in 1982 in which they state (page 8), "... . it appears that the overall site stability in the aproxiamtely 20 years since these drains have been constructed has been good and that the drainage system continues to be generally effective in Leroy C. Lowe May 17. 1993 Page 3 preventing deep - seated movements. It is of prime importance, however, that the system remain fully operational. If the drainage system should deteriorate so that artesian pressures can again increase beneath the hillside, the risk of deep - seated and extensive earth movements can be expected to significantly increase." With the security of Slade Way assured, ft is a simple matter to vouch for the safety of the site from deep- seated sliding. The Cascade report covers (and indorses) the stability of the site itself; moreover, we believe that it follows automatically that development of the site would improve local soil and groundwater conditions and would therefore improve the stability of the general area. "The area of potential geologic instability can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability." (City of Tukwila Regulation) Given the safety of Slade Way, the only area of potential geologic instability is the steep slope which trends north -south through the center of the property. The height of this feature varies from 20 to a maximum of 32 feet; to modify it to improve its stability is a comparatively simple exercise in regrading, which when combined with the drainage improvements of the project, would have far- reaching effects on the stability of the neighborhood. The Cascade recommendation for a 45 -ft set- back from the toe of the slope apparently assumes that the slope would be left in its present over - steepened condition; however, we see no reason why such a feature should be allowed to remain. A very modest regrading at the toe of the slope, combined with a simple subsurface drainage system consisting of perforated pipes placed in shallow trenches will provide all the stability which could be asked for (either static or seismic pseudostatic). We note that surface water runoff, groundwater discharge, and sedimentation are at present totally uncontrolled, leaving the steep slope entirely unprotected; a correction of this state of affairs would automatically accompany the development of the property, to the overall improvement of this and neighboring properties. In summary, we point out that the misinterpretation of a Iandform has led Shannon and Wilson to conclude erroneously that the two vital requirements of the City of Tukwila ordinance are not met by this project, whereas, if the correct interpretation is applied, the requirements are met. We welcome the opportunity to discuss this directly with any or all interested parties. H' T wEct Very truly yours, \Yri$\ ? •s ,fj NEIL H. TWELKER AND ASSOC.. INC. f 1 A I i 5757 STV. • SIONAL, S4L) Neil H. Twelker, Pres. IEXPIRED 31131 -9 9-4 . Ind: Cross Section Through the Hillside KH T : of 134 May 19, 1993 Job No. 9005 -13G Leroy Lowe PO Box 1241 Seattle, WA 98111 SUBJECT: Peer Review of Our Evaluation of Your Tukwila Property Dear Leroy: I have reviewed the letter from Shannon and Wilson which you dropped off yesterday and wish to make a few brief comments. Feel free to share these with the City if you wish. The peer review basically restates the City's position that development of your property will not be permitted until you correct any stability problems on the street and the property below the street. It seems clear to me that the C' ty staff will not willingly issue a permit for the development of this property. The City and Shannon and Wilson indicate that the owner of your property is responsible for maintaining the hillside from the street to the freeway. They also believe that the scarp on your side is unquestionably the head scarp of that movement, although Neil Twelker's analysis indicates that other reasonable possibilities exist. I have enjoyed working with you on this project, but I feel it would be a disservice to you to continue to perform analyses which will continue to be ignored. The peer review starts with the assumption that you must address the stability of the entire region, rather than that portion which now involves your property. During our conversations, and during our work on this matter we have repeatedly emphasized that we are analyzing the stability of the slope on your property. That is, the area over which you have some control. An attempt to dewater a lower aquifer from wells on your property would require that you discharge large quantities of water which the City will not allow to enter the City drainage system. It would also, according to Shannon and Wilson's theory, provide a major advantage to both the City, which maintains the street, and the lower property owner by stabilizing their property at no cost to themselves. I believe that the responsibility for the street and lower property must be established, before any work is undertaken to correct it. I do not see that it is your responsibility to fix a problem brought about by the excavation at the toe of the slope, the failure of the Highway Department's drainage system, and the failure of the City to protect the street from encroachment from below. So You have a political problem not a technical problem. Your property can't be used until the City allows the boundary adjustment and the city staff won't allow that regardless of the technical facts. I will be happy to work with you to evaluate and provide solutions for the technical problems when we get to that point. In the meantime I would be very reluctant to waste your money on additional technical work at the site. Once you have obtained the permits, and I am confident that you will, I will be happy to work with you on the details of developing this property. Good luck Leroy. Keep me posted. Sincerely, CASCADE GEOTECHNICAL, INC. George E. Lamb, P.E. • President GEL:pg a� CASCADE GEOTECHNICAL INC. 12016 115TH AVENUE N.E., BLDG. H (206) 821.5080 KIRKLAND, WASHINGTON 98034 FAX: (206) 820.6953 May 20, 1993 Leroy Lowe PO Box 1241 Seattle, WA 98111 SUBJECT: Hillcrest Residential Development Dear Leroy: I have reviewed the letter dated March 14 from the City of Tukwila and the attached review dated May 5 from Shannon and Wilson. I have also discussed the situation with Neil Twelker and reviewed the draft of his letter responding to the "peer review" by Shannon and Wilson. I have also re- examined all our previous work on the property and the previous reports by others in our files. From these I have reached the following conclusions: • The Shannon and Wilson "peer review" of May 5, 1993 fails to address the stability of your property in itself. The review concentrates on stability downhill from Slade Way. In regard to your property itself, the review does state that "from a cursory review, however, it appears that the stability analyses are conservative with respect to shallow soil conditions and seismic load conditions ". The review also requests further information regarding our methodology. This information is in our file and will be forwarded if needed. • My review reinforces my professional opinion that your property is stable at the present time. • My review also reinforces my professional opinion that the property will be even more stable after it is developed as we recommended. • The only realistic expectation of danger to your property would be if the City of Tukwila fails to protect your property against the loss of lateral support. This could occur if the City permitted Slade Way to fail completely and thus remove lateral support from your property. Barring the possibility of this irresponsibility action by the City, your property is stable and can be developed as planned. CASCADE GEOTECHNICAL INC. I trust that this brief note restates my position clearly enough to resolve most of the confusion. Good luck. Sincerely, CASCADE GE NC. v -t 071- t) x) 4 George E. Lamb;.,f_,i, President GEL:pg • EXPIRES 11 -21.93 jY 0 n P 0 U-) H A ••0 1 11 0 r r A fil 0 -r N m ? O (DD N fD a- n N N C 2 o n m m w 0. a CD n9 w y cn O a 014 S3 a O � C C O to _ w O S o ? ° m y S N a o • 07- CD CT 7 • 3f° 0 m = m v° 5 w a 10 O N N O m =. O o O fD £ O ° !D m N m p 5 m 3 w = Om • o w = '� S. m w C fD 5 w y m a a O 7 0 7 a) • 5. o. D w co o 3• vo 5 m £ m y a. ? w O Q . o. SLADEWAV y 5. —1 n co c 3 a) o w N 0 o .. 5. a) £ O N cD O m -0 v C d O • o av y m cD a L /CA-/ OR/ •N .41 7 0 o o. o. O < o • m 3 a 3 m s •£ w =(0 2: • (D y O Er • °, l0 May 14,1993 City of Tukwila John W. Rants, Mayor Department of Community Development Rick Beeler, Director Leroy Lowe P.O. Box 1241 Seattle, WA 98111 RE: Soil geotechnical evaluation of proposed Lowe BLA Dear Mr. Lowe: Attached to this letter is the peer review completed by Shannon & Wilson,Inc. In their conclusions they do not believe the studies demonstrate that the property meets the requirements set forth in the City of Tukwila's Sensitive Areas Ordinance. Under the SAO1 you may appeal this decision (TMC Section 18.45.125). Given the complexity of this issue, I suggest a meeting to review the positions of the consultants and discuss ways in which to resolve the issues. Should you have any questions, please feel free to call or write. Sincerely, J�ick Pace Senior Planner cc: Ron Cameron, City Engineer ATTACHMENT E '6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665 SHANNON W! ON'N� SEATTLE EVERETT K TENVIRONMENTAL 3L FAIRBANKS ANCHORAGE SAINT LOUIS May 5, 1993 City of Tukwila Department of Public Works 6300 Southcenter Boulevard, Suite 100 Tukwila, Washington 98188 Attn: Mr. Jack Pace, Senior Planner RE: SOILS AND GEOTECHNICAL EVALUATION OF PROPOSED LEROY LOWE DEVELOPMENT, TUKWILA, WASHINGTON At your request, we have reviewed the slope stability analysis report dated March 22, 1993 by Cascade Geotechnical, Inc. for the proposed Hillcrest Residential Development, Slade Way South of 160th Street, Tukwila, Washington. We understand that this report was prepared to fulfill a requirement of the City of Tukwila Sensitive Areas Ordinance (SAO) for an application for a Boundary Line Adjustment for the subject property. As part of our review, we have reviewed three previous geotechnical reports for the Hillcrest Development dated May 30, 1990, August 27, 1990, and April 24, 1991, and a supplemental letter dated July 27, 1992, all prepared by Cascade Geotechnical, Inc. We also have referred to our geotechnical review of the proposed Hillcrest Development dated November 23, 1992, and our letter of February 1, 1993. These latter documents recommended that slope stability analyses be performed to assess the potential for both shallow and deep seated landsliding, and stressed the need for careful evaluation of the potential for landsliding on this property before proceeding with any development that might endanger human health or safety. It is our opinion that these recommendations are consistent with the requirements set forth in the City of Tukwila SAO. REVIEW COMMENTS The March 22, 1993 report by Cascade Geotechnical, Inc. describes the methodology, soil parameters used, and results of pre- and post - construction stability analyses for both shallow and deep- seated sliding under static and pseudostatic conditions. These analyses were conducted specifically to evaluate the stability of the north -south trending, steep slope that runs through the center of the property. Two cross sections were reportedly developed and analyzed, both 400 NORTH 34TH STREET •SUITE 100 P.O. BOX 300303 SEATTLE, WASHINGTON 98103 206.632.8020 FAX 206.633.6777 ti ....�., KA •1 0 72;93 W- 6367 -01 City of Tukwila • Attn: Mr. Jack Pace May 5, 1993 • Page 2 SHANNON &WILSON, INC. trending in a northeasterly direction across the property and extending 25 feet beyond the western and eastern property lines. Cascade Geotechnical's report concludes that with respect to deep - seated stability, the site is stable under both pre- and post construction conditions, and that deep - seated slope stability will be significantly improved by the planned drainage. With respect to shallow slope stability, only the pre - construction factors of safety are presented. These show the site to be marginally stable under static conditions and unstable under seismic loading. The report states that shallow failures may still occur under post - construction conditions, both under both static and pseudostatic conditions, and therefore recommends a 45 -foot construction setback from the toe of the steep slope. This setback would appear to require relocation of three of the proposed residences shown on the site plan. In reviewing this stability analysis report, we have identified three main issues which are not addressed, and which in our opinion must be considered in an evaluation of the stability of this property. These issues, as itemized below, revolve around a central issue; the stability analyses were performed for the steep north - trending slope rather than for the property as a whole. ► There does not appear to be adequate subsurface data to define the stratigraphic and groundwater conditions below the site in such a manner that they can be used in analyzing the deep - seated slope stability of the property. It is our understanding that three borings ranging from 26.5 to 31.5 feet deep were used, along with several shallow test pits and hand auger borings, to define subsurface conditions and to develop the cross sections for the site. Our primary concern with this is that there are no borings that extend deep enough to intercept the deep confining strata or the underlying confined aquifer which was a primary cause of landsliding immediately to the east of the site. Consequently, the depth of this confined aquifer is not known, nor is there any information on the hydraulic head within the confined aquifer. ► The report does not address the potential for reactivation of landsliding immediately downslope from the property in the stability analyses. There is also no acknowledg- ment in the report that such a problem might exist. Multiple studies for the area immediately downslope from the proposed Hillcrest Development indicate that the stability of this area is dependent on an extensive hillside dewatering system. This system, which is about 30 years old, is reportedly deteriorating and its capability W- 6367 -01 City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 3 SHANNON &WILSON. INC. to lower the hydrostatic head acting on the hillside is diminishing. Consequently, the factor of safety for hillside stability can be assumed have dropped below a level that several years ago was considered by the Washington Department of Transporta- tion and Dames & Moore to be unacceptably low for a proposed residential devel- opment. Without a basis to conclude otherwise, it appears reasonable to assume that the factor of safety for deep seated slope stability of the proposed Hillcrest Development would be similar to that which has been determined for the property immediately downslope. In our opinion, slope failure of the area immediately downslope could readily impact the stability of the proposed Hillcrest Development, and must be considered in evaluating the deep - seated stability of the property. ► The site subsurface data do not define the depth of the slip plane of the ancient landslide that is interpreted from geomorphic expression and previous studies to underlie the eastern half of the property. While the stability analysis report indi- cates a deep - seated failure surface at a depth of 20 feet, this appears to be in reference to a hypothetical failure plane below the steep slope west of the proposed building sites. We believe that in order to adequately assess the deep seated stability of this site it will be necessary to further evaluate the evidence of a deep seated slide plane under the site, including a determination of its depth and soil and groundwater conditions above and below it. This information would help to resolve uncertainties regarding the potential for slide reactivation or involvement of the property in a deep - seated failure. Because the stability analysis report only addresses the potential for sliding of the steep north - trending slope in the center of the property, and ignores the potential significance of a deeper seated slide, we believe it would be inappropriate at this time to critically review the stability analyses that have been performed. From a cursory review, however, it appears that the stability analyses are conservative with respect to shallow soil conditions and seismic loading conditions. However, the following additional issues that should be addressed or clarified before a critical review can be accomplished. ► The report does not include all of the assumptions that were used in developing the cross sections or in performing the analyses. Specifically, the following are not provided: a) it is not clear how the cross sections and soil parameters were devel- oped below the maximum depth of soil borings of 31.5 feet; b) assumed slip surfaces are not shown, c) groundwater levels used in the analysis are not shown; and d) it is not clear how the considerable subsurface detail shown in the vicinity of the steep slope was derived. W- 6367 -01 . asp City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 4 SHANNON FiWILSON, INC. ► Cascade Geotechnical's report of August, 27, 1990, indicates that a hydrostatic head which reached the ground surface was noted during drilling of one of the borings. This observation suggests that confined groundwater conditions exist in the relatively shallow strata underlying the site strata. Similarly, the report also indicates that there may be a number of separate, confined aquifers at depth. It would be appropriate to evaluate the potential significance of these confined zones to the stability of the site if this has not already been done. ► The planned drainage enhancements that have been assumed in the post - construction stability analyses are not shown on the copy of the plans that we.received. Without knowing what these proposed enhancements are, it is not possible to provide an opinion on the validity of this assumption. However, the report assumes that these drainage enhancements would lower the groundwater to the level of the dense sand. According to the table of soil properties presented on page 3, this level would correspond to depths of 33 to 45 feet below the upper slope and 45 to 65 feet below the lower slope. It is not clear to us what type of drainage has been proposed to accomplish such a groundwater drawdown, but this does not appear to be consistent with our previous understanding that relatively shallow French drains were to be used. ► The slope stability analyses reportedly used groundwater levels measured during the summer of 1990, rather than higher water levels that would be expected during the winter months. While this may not make much difference for the analyses of post- construction conditions, it could be an important factor in the analysis of present -day conditions. A higher water table is often an overriding factor in initiating landsliding sliding because of increased pore -water pressures. ► There is no discussion of the impact of site grading or excavation, either of which could significantly affect shallow slope stability during or after construction. CONCLUSIONS The slope stability analysis report for the proposed Hillcrest Development addresses the stability of the north - trending steep slope that runs through the center of the property rather than the stability of the property as a whole. In our opinion, the primary concern for stability of the property is related to the potential for a deep seated failure that could affect much if not all of W- 6367 -01 City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 5 SHANNON 6WILSON. INC. the property, rather than the smaller, more localized type of failures which are addressed in this report. With respect to the stability analyses that have been performed, it appears that these analyses are generally conservative in assessing the stability of the north - trending steep slope. However, this cannot be substantiated without additional documentation of the assumptions that were used in the stability analyses. Depending on what these assumptions were, additional subsurface data may be required to derive a meaningful stability analysis of this slope. In our opinion, the slope stability analyses that have been performed do not adequately address the potential for landsliding on this property, nor do they demonstrate that the property meets the requirements set forth in the City of Tukwila SAO. Specifically, Section 18.45.080E of the SAO states that prior. to permitting alteration of an area of potential geologic instability, the applicant must demonstrate either: 1) There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no significant risk to the proposed development or surrounding properties; or 2) The area of potential geologic instability can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability. Our review indicates that the proposed development does not meet the first condition, and because the potential for deep seated sliding has not been addressed, it has not been demonstrated to meet the second condition required for permitting a development in an area of potential geologic instability. W- 6367 -01 City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 6 Please call if there are any questions regarding this letter. Sincerely, SHANNON & WILSON, INC. Daniel N. Clayton, C.E.G. Senior Associate Reviewed by: W. Paul Grant, P. E. Vice President DNC: WPG /dnc W6367 -01.LTR/W6367- 1kd /Ikd SHANNON EIWILSON, INC. W- 6367 -01 MEMORANDUM TO: RON CAMERON / JACK PACE DATE: MAY 4, 1993 PROJECT: HILLCREST BLA SUBJECT: PIER REVIEW SLOPE STABILITY STUDY ENCLOSED FOR YOUR REVIEW & COMMENT IS A PIER REVIEW OF GEORGE LAMB'S SLOPE STABILITY STUDY PREPARED BY NIEL TWELKER P.E. YOUR EARLY RESPONSE WILL BE APPRECIATED. THANK YOU. L. LOWE LEROY C. LOWE A.I.A. ARCHITECT PO ■O• 12•1 ••ATTL•. WA•MINOTON •15111 ATTACHMENT F Telephone: (206) 284 2410 Neil R Twelker and Associates, Inc. Consulting Soils Engineers Leroy C. Lowe, A.I.A P.O. Box 1241 Seattle, Washington 98111 5645 42nd Ave. West Mail to : P.0 Box 99086 Seattle, Washington 98199 May 3, 1993 Re: Review of Slope Stability Analysis by Cascade Geotechnical Hillcrest Residential Development Slade Way near South 160th St. Tukwila, Washington Dear Mr. Lowe: At your request we have performed a review of an analysis of slope stability prepared by Cascade Geotechnical Inc. (March 22, 1993) for your proposed residental development near the intersecton of Slade Way and S. 160th St. in Tukwila, Washington. We present herewith a report of our findings, conclusions, and recommendations. Site Description The site, which has 50 ft of frontage on S.160th St. and 300 ft of frontage on Slade Way, is an irregular- shaped tract having overall dimensions of 300 ft east -west by 500 ft north - south. It consists of three principal topographic elements: (1) an upper very gently sloping area along the west boundary of the tract, the width of which varies from 20 ft at the north to approxi- mately 180 ft at the south; (2) a steeply sloping central area whose topographic relief varies from 20 ft at the south to 32 ft at the north; and (3) a gently sloping, poorly drained area occupying the westerly half of the property, whose total relief is 20 to 24 ft, with an average declivity of 15 to 20 per cent. Two well- defined streams cross the property from west to east, fed by springs at the base of the steep slope. The site is devoid of structures, with the exception of a sewer which has been constructed in an easement along the south property line, to a manhole at the southeast corner of the property,, and thence northerly in Slade Way. The property is covered by scattered deciduous trees, a few small cedar trees, and a sparse understory of native brush. Soils within the site are known (from subsurface explorations performed by Cascade Geotechnical Inc.) to consist of the following principal soil units: 1. The upper (i.e., most recent) unit consists of soft sedimentary peat covering much or most of the poorly drained lower area to depths of Tess than a foot to as much as five feet. Ana Leroy C. Lowe. A,I.A May 3. 1993 Page 2 2. The underlying unit consists of moderately dense to dense sand, the upper foot or two of which is somewhat loosened from weathering. 3. A thin unit of stiff silt was found between 23 and 28 ft in one test boring (located in the low area). The lowermost unit encountered consists of dense sand. Groundwater was encountered at depths varying from 1 to 2 ft in the low area and at depths of 25 to 30. ft in the westerly upland. An important feature of the site is the emergence of groundwater at the base of the steep slope. Cascade Geotechnical Inc. Stability Analyses In their report of March 22, 1993, Cascadce Geotechnical begins by mentioning that they had reviewed their own previous reports and also, a report prepared by Shannon and Wilson dated November 23, 1992. They go on to outline their methodology, soil values, and the results of their studies, including both static and pseudostatic (seismic) reviews of stability. The method employed was the "Simplified Method of Slices," developed by Alan Bishop (based on the original "Method of Fellenius "). Two cross sections were selected, to be representative of the most critical areas of the slope, and because of their proximity to locations at which soil conditions had already been determined. The parameters for the strength and density of the soil were given for 6 classifications of inorganic soils found at the site. No soil parameters were assigned to the peat, presumably because it was expected to play no part in the scenario of a hypothetical slope failure. The Cascade Geotechnical studies showed the preconstruction site to be very stable for static and pseudostatic (seismic at 0.2 times the acceleration of gravity) conditions in the deep- seated failure mode. The deep- seated failure mode was taken from 4 ft west of the crest of the steep slope and extending 40 ft east of the toe of the slope, while the shallow zone study addressed only the actual steep slope itself, an area which is very near the "angle of repose" of a granular soil. The shallow failure mode was calculated to be only barely stable for static conditions, and not at alt stable for the pseudostatic seismic review. With the site drained in accordance with the Cascade Geotechnical recommendations, the stability of the deep- seated failure mode is improved somewhat; the results of the shallow failure mode are not reported. Cascade Geotechnical concludes its report with a recommendation to exclude "any building develop- ment within 45 feet of the toe of the slope," in order to avoid the consequences of a shallow zone failure. Although the Cascade study included a review of the Shannon and Wilson report of Nov. 23, 1992, they did not comment on any of the conclusions presented in that report. Conclusions On the basis of our review of the available information (including an examination of the site) we draw the following principal conclusions: 1. The soil parameters used in the Cascade Geotechnical report are believed to be conservative and generally representative of the conditions which prevail at the site. 2. The method of analysis employed is an accepted one, and the results of their study compare very well with our intuitive appraisal of the site and with our shorthand methods of calcu- lation. We note in particular that the most serious failure mode (deep - seated movement) is very unlikely in any event (whether drained or undrained, static or seismic). Leroy C. Lowe.A.I.A May 1993 Page 3 3. That the safety factor in shallow zone mode is low is-not all surprising, inasmuch as the declivity of the steep slope is very near the angle of repose (Le., the declivity or pitch at which any additional material would simply roll or slide to the base of the slope). The amount of material involved in a failure of this kind is comparatively small, and the consequences correspondingly minor. Moreover, it is a fairly simple matter to reinforce those parts of the slope which are critical to structures placed at or near the toe, or to provide catchment for the displaced material, a fact which would obviate the need for the 45 ft set -back recommended by Cascade Geotechnical. 4. No analysis was applied specifically to the gently sloping lower area, presumably on the grounds that an area with a declivity as low as this, and with a dense soil structure, must be inherently stable, even if it does have a high ground water table. We concur with this view; we see no problem with the stability of the lower area, whether in the static condition or in the seismic condition. 5. The. peat which covers much of the lower area is totally incapable of support of any major or settlement - critical structure; all major improvements must be founded on the underlying moderately dense sand (a comparatively simple undertaking). The simplest and most econom- ical site preparation procedure will be to conduct an initial program of shallow drainage, after which the peat may be removed from the building area and the structure placed on spread footings on the native sand formation. We see no need for the use of piling foundations for structures in any part of the site. We believe that any drainage of the soil units, or expediting of the escape of surface water, will have a favorable effect on the stability of the entire area. 6. We note that the poorly drained lower area, besides having outstanding capabilities for the breeding of mosquitoes, possess a special status under the designation of "Wet Land," making it necessary to find a way to execute the project to the satisfaction of the regulating agencies who sit in judgment of such matters. Two methods may considered for the treatment of the wetland problem: (1) removal of the peat only from the building footprint, with the remainder of the site covered with organic materials such as wood chips or bark (to allow foot traffic), or (2) stripping of the entire site and regrading with imported inorganic materials. Inasmuch as the law requires that "lost" wetlands be replaced with a 50 per cent increase in wetland,,it would be necessary to provide a poorly drained area of an appropriate size at some other location in either case. You have advised us that you own or have access to property below Slade Way, to the north of the 1960 slide mass which can be converted to "wetland" by altering its topography so that it becomes poorly drained. 7. The Shannon and Wilson Report Cascade Geotechnical reviewed the Shannon and Wilson report but did not comment on it; we, on the other hand, feel compelled to note two important points of disagreement with that report, as presented in the following paragraphs. 8. Indications of Prehistoric Landslide On page 3 of the S and W Nov. 23, 1993, report they state, "The steep slope that separates the upland and lowland portions of the site is interpreted to be the southern end of the headscarp of an ancient• extensive landslide. This landslide escarpment extends about 3000 feet northwest of the site and typically displays about 30 to 40 feet of topographic relief. At the base of the escarpment, the ground is typically hummocky and poorly drained and commonly is the site of springs and boggy conditions, as it is on the Lowe property. This disrupted drainage probably has contributed to the numerous smaller slides within the ancient slide mass." At this juncture we direct to Leroy C. Lowe. A.I.A May a 1993 Page 4 attention to the fact that the steep slope (or scarp) which traverses the Lowe Property is a feature whose declivity exceeds that which could be expected in a "normal" landscape whose contours are developed from erosion from sheet flow and surface weathering. When one encounters a feature of this kind, an explanation for its presence is called for. The two most common reasons for the occurrence of an abnormally steep slope are (1) it is the head scarp of a large scale landslide, or (2) it has been developed by "sapping," (i.e., the result of the emergence of groundwater at the toe of the slope). The conditions reported by Shannon and Wilson describe the scarp throughout its length as having springs and seeps at its base; nevertheless, they unhesitatingly attribute the presence " of the scarp to an "ancient landslide." We are unable to dispute (on the basis of direct evidence) the existence of an ancient landslide; however, we can point to the presence of the springs and seeps at the base of the scarp as a "smoking gun," easily capable of having caused the feature in question. The great length of the steep slope, together with the presence of groundwater emergence at its toe throughout its length, lead us to conclude that sapping (rather than mass earth movement) has been responsible for the formation of the scarp. It appears that in their search for an explanation of a physical feature, Shannon and Wilson have overlooked an obvious and ongoing natural process in favor of an unsupported hypothesis whose implications are somewhat sinister (in that they cast doubt on the history of the Hillcrest property). 9. Shannon and Wilson call attention to the presence of "bowed and leaning tree trunks" in the easterly part of the Hillcrest area as evidence of shallow soil creep. While shallow creep could very well be taking place in the upper organic soil units, it should be pointed out that the trees in this area consist mainly of alders, whose growth direction is influenced primarily by the search for Tight and air. 10. Influence of Recent (196Q) Landslide Shannon and Wilson allude (at considerable length) to the large landslide which occurred between Slade Way and Klickitat Drive in 1960. We were present almost at the moment when that slide commenced; it was instigated by large - scale removal of material from the slope (for use in preparing the Andover industrial site). A very high groundwater table also played a prominent part in the loss of hillside stability. The slide was eventually brought under control by an elaborate network of drainage which included horizontal drains and deep well pumps. From the early 60's to the mid 80's the system appears to have been effective in preventing noticeable movement within the slope; however, a number of small slides and set -downs in the area (even extending into the right -of -way of Slade Way) have led to a series of studies by WSDOT, Dames and Moore, and Geoengineers, all of which concluded that the drainage system was undergoing an "ongoing deterioration," and that ". . . . without a functional drainage system there would be an unacceptable risk of deep - seated slope failure at the site." Shannon and Wilson then proceed to attach the feasibility of the Hillcrest development to the restoration of the drainage system which had demonstrated so well its ability to maintain stability for several decades. They imply that i f the 1960 landslide were to become reactivated (because of the deterioration of its drainage system) and i f the slide mass were to extend laterally into Slade Way to the boundaries it has been known to occupy, and if the slide mass were extended even more to the west (involving earth which had not been previously disturbed, and if the new boundaries of the unstable mass somehow were to "join up with" the those of prehistoric slide which they have postulated (we believe mistakenly), then the improvements of the Hillcrest development would be at risk. While we are able to understand and sympathize with the reluctance of a responsible engineer to acquiesce to the construction of a new development, on ground which has fallen under the Leroy C. Lowe.. A,I,A May 3 1993 Page 5 cloud of potential instability, we nevertheless feel compelled to call attention to the following points which we believe to over -ride and outweigh all other considerations: a. Slade Way, which lies between the Hillcrest property and the 1960 landslide, • is a very important (if not absolutely vital) thoroughfare in this area. In addition to providing access for local traffic and emergency vehicles, it also contains important buried utilities without which any number of households in the area could not survive. b. The unfolding of the scenario outlined above would threaten a considerable number of existing residences in the same manner that the Hillcrest development would be threatened. Barring a fortuitous intervention by a "public spirited angel," it appears that the City of Tukwila must either (1) undertake the restoration of the deteriorating drainage system or (2) have on hand a contingency program to include the condem- nation of existing residences (and relocation of their occupants), and the rerouting of the utilities which serve residents located beyond the area which would be directly affected by the contemplated reactivation of the 1960 landslide. Likewise, it also appears that the residents whose lives would be disrupted should be concerned with their own agenda: (1) If forced to move, where would they go? (2) What recourse could they pursue following the toss of Slade Way? (3) What steps could be taken to prevent the loss of Slade Way? and (4) How can they force the City of Tukwila to perform the tasks ordinarily accepted by city government? In all this confusion, it is obvious that the least affected of all parties would be the owners of the Hillcrest property (which, after all, is nothing more than poorly drained vacant land). c. The success of the means of stabilizing the 1960 landslide area has been well demonstrated over a period of three decades. Continued stability simply requires that proper maintenance and /or replacement of the drainage installations be performed in a timely manner. This is a civic responsibility which, because of its great extent, the large number of residential properties involved, and the potential impact on the community, cannot be placed on the shoulders of a private citizen. In summary, we believe that whatever stability problems the proposed Hillcrest development might have are of very minor importance, that they are confined to the actual site, and that they can be dealt with at the time of development by fairly straightforward methods of drainage and site preparation. We shall be pleased to discuss our conclusions with you or other interested parties, at your convenience. 5757 •.) N HT: nt Very truly yours, NEIL H. TWELKER AND ASSOC.. INC. by Neil H. Twelker, Pres. MEMORANDUM TO: DATE: RON CAMERON / JACK PACE MAR . 30 , 1993 PROJECT: HILLCREST BLA SUBJECT: SLOPE STABILITY ANALYSIS GENTLEMEN : RECEIVED MAR 3 11993 DEVELOPi.vIEN / ENCLOSED ARE THE FOLLOWING DOCUMENTS 1 . SLOPE STABILITY ANALYSIS PREPARED BY CASCADE GEOTECH INC . 2. CONTRACT WITH NEIL TWELKER P.E. TO PROVIDE A PIER REVIEW OF CASCADE'S SLOPE STABILITY ANALYSIS THESE DOCUNENTS ARE PROVIDED TO YOU FOR YOUR REVIEW AND COMMENT. LEROY C . -LOWE A.I A.I.A. C .0 . NEIL TWELKER P . LEROY C. LOWE A.I.A. ARCHITECT n0. •OM 11 ..1 •••TTI•. WA•NINOTON ••111 ATTACHMENT G Telephone 284 2410 Neil H. Twelker and Associates, Inc. MAR 3 ? 1x93 Consulting Soils Engineers OEVELOPivIENT 5645 42nd Ave. West Mail to : P.0 Box 99086 Seattle, Washington 98199 March 26, 1993 LeRoy C. Lowe, AIA, Architect P.O. Box 3972 Bellevue, Wash. 98007 Re: Slope Stability, Proposed Hillcrest Residential Site Slade Way, Tukwila, Washington Dear Mr. Lowe: On numerous occasions during the past 6 months we have discussed with you the various factors involved in the appraisal of stability of your forthcoming residential development ( Hillcrest) on Slade Way in Tukwila, Washington. We present herewith (at your request) our proposal for a geotechnical report containing a review of the slope stability studies which have been per- formed by Cascade Geotechnical (including independent slope stability calculations if neces- sar y) . All work will be performed by Neil H. Twelker (personal history statement attached). We propose to invoice you at our regular rate for consulting services, $100 per hour. We estimate that our review of the Cascade Geotechnical studies can be accomplished with an expenditure of 4 hours. If independent slope stability calculations are required, an additional 4 hours could conceivably be expended. The report containing our findings and conclusions will require 2 to 3 hours, making the maximum anticipated amount $1100. • We will be in a position to commence our studies within 5 working days of your verbal authorization to proceed. LeRoy C. _owe, AIA March 26, 1993 Page 2 We trust this proposal meets with your approval, and look forward to this opportunity to be of assistance. 4/-7/a/flaveLP G�M'2 30 / 9�3 LE y C. Lew 3V / 4_ N HT :: nt Ind: Personal History Statement of Neil H. Twelker Very truly yours. NEIL H. TWELKER AND ASSOC.. INC. by Neil H. Twelker, Pres. PERSONAL HISTORY STATE1:ENT NEIL H. TWELKER BS .(Civil Engineering) AM (Soil Mechanics) PhD (Soil Mechanics, w /Geology Minor) University of Washington Harvard University 1942 1954 Harvard University 1958 Membe r American Society of Civil Engineers Geological Society of America Structural Engineers Association of Washington United States Committee on Large Dams Tau Beta Pi Sigma Xi Lecturer, Univ. Wash., College of Arch. & Urban Planning Registered Professional Engineer Washington (Civil) Alaska (Civil) Oregon (Civil) Experience Forty -eight years total experience in the application of soil mechanics to engineering works, including: Nine years with the Seattle District of the Corps of Engineers, with responsible charge of soils engineering on major and minor projects, including design and construction phases of Chief Joseph Dam and Albeni Falls Dam, preliminary investigations on numerous dam sites throughout the Pacific Northwest, airfields, levees, breakwaters, and building foundations. Two years with Shannon and Wilson, Soil Mechanics and Foundation Engineers, Seattle, Washington, with responsible charge of site investigations for Wells Dam (Columbia River, Washington), Tolt River Dam, Upper Baker River Dam, slope stability investigation for open pit copper mines in Utah and Nevada, and miscellaneous foundation and landslide investigations. Three years of combined private practice, teaching, and graduate study, including work on the St. Lawrence Seaway and consultation on Waterton Lakes Dam, Alberta. Thirty -four years in present practice. Soils and foundation engi- neering for buildings, bridges, roads, dams, and breakwaters. Over 6000 professional assignments in Washington, Oregon, Califor- nia, and Alaska, including waterfront and major industrial devel- opments for Port of Seattle, Port of Tacoma, and other governmen- tal and private organizations, and U.S. Navy at Whidbey Island and Adak Air Stations, and Puget Sound Naval Shipyard. Consultation on design and execution of numerous large and small diameter soft ground and hard rock tunnels, deep trenches, caissons, etc. Numerous remedial and underpinning projects for building founda- tions, and foundation and site preparation for residential and commercial structures of all sizes. Soil and foundation studies for Tri- cities Bridge (Columbia River). In Seattle: Crown Plaza Hotel, Westin Building, Warwick Hotel, Market Place North, Seattle Aquarium, Pacific Museum of Flight and restoration of Pike Place Market. In Portland, Ore.: Portland General Electric Headquarters Complex, KOIN Plaza, Marri- ott Hotel, Ben Franklin, and others. 111 W. CASCADE GEOTECHNICAL INC. 12016 115TH AVENUE N.E.. BLDG. H (206) 821.5080 KIRKLAND WASHINGTON 98034 FAX: 206 8 March 22, 1993 Job No. 9005 -13G Mr. Leroy C. Lowe, A.I.A. P.O. Box 1241 Seattle, Washington 98111 3 ( ) 820-6953 Reference: Slope Stability Analysis Hillcrest Residential Development Slade Way, South of South 160th Street Tukwila, Washington Dear Mr. Lowe: MAR 3 1 1993 DEVELOPMENT As requested, we have completed a slope stability analysis for the above site. The following letter summarizes our analysis and presents conclusions regarding the present and post - construction stability of the north -south trending, east facing slope. We understand that this analysis will be used for a Boundary Line Adjustment application. SCOPE Per the proposal for services letter dated February 24, 1993, our scope of work consisted of: • reviewing existing geotechnical reports previously prepared for this project • analyzing the present stability of the north -south trending slope on your property by constructing two cross sections oriented perpendicular to the slope. • preparation of this letter stating the results of our analysis. PROJECT DESCRIPTION Slope profiles were derived from an undated topographic site plan titled "Hillcrest," that was prepared by Leroy C. Lowe. The site plan is scaled at 1 " =30' and topography is shown in two foot intervals. Property lines and adjoining roads Slade Way and South 160th Street are clearly delineated. Based on our understanding of the project, development will consist of the construction of five (5) single family residences, with driveway access from Slade Way and South 160th Street. All five residences are to be located in the gently sloping area at the base of the 2(H):1(V) slope. Several areas on the slope are moderately steeper than 2(H):1(V). CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G We reviewed two geotechnical reports previously prepared for this project by Cascade Geotechnical Inc.' We also reviewed a report prepared by Shannon & Wilson, Inc.2 which addresses geotechnical implications for this project, Slade Way, and Klickitat Drive. SLOPE STABILITY Methodoloey We analyzed the stability of two sections of the north -south trending slope, for both pre- construction and post - construction conditions. These .sections were chosen based on their proximity to previously obtained subsurface information, as well as their ability to represent the most critical areas of the slope. We performed both shallow and deep. seated failure analyses. Cross section A -A' begins in the southwest property corner and extends northeasterly across the entire property. Cross section B -B' begins approximately 200 feet north of the southwest property corner and also extends northeasterly across the entire property. Both cross sections extend twenty-five (25) feet beyond the eastern and western property lines. For more details, please refer to the site plan and cross sections contained in the appendices. Utilizing the computer program SB- SLOPE, we have determined minimum factors of safety against slope failure under both static and pseudostatic (seismic) loading conditions. SB -SLOPE incorporates Bishop's Simplified method of slices. To mimic a severe seismic event, a horizontal inertial force equal to 0.2 times the total weight of the potential sliding mass was used in the pseudostatic analysis. This inertial value was selected based on Seattle D.C.L.U. recommended values', which are generally accepted as conservative. parameters Soil parameters were conservatively estimated based on previously obtained subsurface soils information. Six soil types were chosen that we believe accurately characterize actual soil conditions. The typical soil . strengths selected are based on our experience with similar soils in this area. The pre - construction analysis incorporated a ground water surface based on elevations ' Cascade Geotechnical Inc., May 30, 1990, Preliminary Geotechnical Investigation for Hillcrest Slade Way, South of South 160th Street, Tukwila, Washington; and addendum report dated August 27, 1990. 2 Shannon & Wilson Inc., November 23, 1992, " Geotechnical Review of Proposed LeRoy Lowe Development, Tukwila, Washington." ' Seattle Department of Construction and Land Use Director's Rule 3 -93, March, 1, 1993. 2 CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G encountered during our subsurface investigation in May and August of 1990, and springs, which were observed at the toe of the slope. The post - construction analysis incorporated the benefits of the drainage enhancements shown on the plans. The phreatic surface is assumed to be lowered to the dense sands after drainage is installed. The following table summarizes the soil parameters used in our analysis. SOIL PROPERTIES . Soil Type & Classification Approx. Depth from Surface Dry Unit Weight (pcf) Cohesion (psf) 1, (SP) dry sand, loose 0 to 12 feet: upper slope 96 0 31 2, (SP) dry sand, medium dense 12 to 25 feet: upper slope 101 0 34 3, (SP /ML), wet sand and silt, med. dense to dense 25 to 33 feet: upper slope 38 to 45 feet: lower slope 108 200 30 4, (SP), wet sand, dense 33 to 45 feet: upper slope 45 to 65 feet: lower slope 110 0 37 5, (SP /SM) wet silty sand, loose 0 to 25 feet: lower slope 96 500 31 6, (SP /SM) wet silty sand, medium dense 25 to 38 feet: lower slope 101 500 34 RESULTS Pre- construction With respect to deep failures, our results show that the slope is presently stable under static and pseudostatic (with seismic loading) conditions. With respect to shallow failures, our results 3 CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G show that the slope is stable under static conditions, but unstable during pseudostatic conditions. The following tables summarize the results of our analysis. Pre- construction Factors of Safety . Cross Sections Deep Seated Shallow Static Pseudostatic Static Pseudostatic A -A' 1.87 1.20 1.16 0.75 B-B' 1.66 1.10 1.04 0.69 The shallow, pseudostatic failure surface for section A -A` began four (4) feet west of the crest of the slope and extended approximately forty (40) feet eastward beyond the toe. The shallow, pseudostatic failure surface for section B-B' began and ended at the crest and toe of slope, respectively. Both shallow failure surfaces were, at their deepest point, only approximately three (3) feet deep. The relatively low factors of safety for shallow, pseudostatic conditions are a result of the oversteepened slope created during the past ancient landslide, which left the fine to medium grained sands on the slope face very near their angle of repose. The deep seated, pseudostatic failure surface for section A -A' began at the crest of the slope and continued approximately forty (40) feet eastward beyond the toe. The deep seated, pseudostatic failure surface for section B -B' began forty (40) feet west of the crest of the slope and terminated eastward at the toe. Both deep seated failure surfaces were, at their deepest point, approximately twenty (20) feet deep. Post - construction With respect to deep failures, our results show that the slope will be significantly improved by the planned drainage. The slope will be stable under static and pseudostatic conditions. Shallow failures may still occur on the slope due to the slope being very near the angle of repose. The following tables summarize the results of our analysis. 4 ..4 15 CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G Post - construction Factors of Safety i Cross Section Deep Seated ' Static Pseudostatic Section A -A' 2.18 1.35 Section B-B' 1.85 I 1.25 The failure surfaces occurred at approximately the same location as in the preconstruction analysis. CONCLUSIONS Under pre - construction conditions, we conclude that the slope will be stable except for shallow instability during a maximum probable seismic event that may result in mass wasting of several feet of soil directly on the slope face. This shallow mass wasting can be characterized by localized slumping and not movement of the entire slope. With post- construction improvements, we conclude that the slope will be generally stable for deep failures, under both static and pseudostatic conditions. The post construction improvements include the French drains shown in the plans. Shallow failures may still occur under strong seismic loading. We conclude that the proposed Boundary Line Adjustment for this site will not affect the suitability of the site development provided our recommendations contained in the Recommendations Section of this report are carefully followed. RECOMMENDATIONS Building Setbacks To protect property from shallow failures (localized sloughing) during a maximum probable seismic event, we recommend the following: • The exclusion of any building development within forty-five (45) feet of toe of the slope. EXPIP1:;, ;; :` CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G GENERAI. We recommend that we engaged to review the final plans when they become available to confirm that our recommendations have been properly interpreted and to provide any additional or alternate recommendations as necessary. We expect the on -site soil conditions to reflect our findings; however, some variations may occur. Should soil conditions be encountered that cause concern and/or are not discussed herein, Cascade Geotechnical Inc. should be contacted immediately to determine if additional or alternate recommendations are required. This letter has been prepared for the exclusive use of Leroy . Lowe, for specific application to the proposed Hillcrest Residential Development located at Slade Way, South of South 160th Street, Tukwila, Washington, in accordance with generally accepted soils and foundation engineering practices. No other warranty, expressed or implied, is made. Thank you for this opportunity to work with you on this project. If you have any questions, feel free to contact us at any time. Respectfully submitted, CASCADE GEOTEf 1 C. • .�. *fir • George E. Lam President • 1 Larry Jackson Project Manager CASCADE GEOTECHNICAL INC. 12016 115TH AVENUE N.E., BLDG. H (206) 821-5080 KIRKLAND, WASHINGTON 98034 FAX: (206) 820-6953 March 22, 1993 Job No. 9005 -13G Mr. Leroy C. Lowe, A.I.A. P.O. Box 1241 Seattle, Washington 98111 Reference: Slope Stability Analysis Hillcrest Residential Development Slade Way, South of South 160th Street Tukwila, Washington Dear Mr. Lowe: -- " "•'iTY ENT As requested, we have completed a slope stability analysis for the above site. The following letter summarizes our analysis and presents conclusions regarding the present and post - construction stability of the north -south trending, east facing slope. We understand that this analysis will be used for a Boundary Line Adjustment application. SCOPE Per the proposal for services letter dated February 24, 1993, our scope of work consisted of: • reviewing existing geotechnicalleports previously prepared for this project • analyzing the present stability of the north -south trending slope on your property by constructing two cross sections oriented perpendicular to the slope. • preparation of this letter stating the results of our analysis. 'PROJECT DESCRIPTION Slope profiles were derived from an undated topographic site plan titled "Hillcrest," that was prepared by Leroy C. Lowe. The site plan is scaled at 1" =30' and topography is shown in two foot intervals. Property lines and adjoining roads Slade Way and South 160th Street are clearly delineated. Based on our understanding of the project, development will consist of the construction of five (5) single family residences, with driveway access from Slade Way and South 160th Street. All five residences are to be located in the gently sloping area at the base of the 2(H):1(V) slope. Several areas on the slope are moderately steeper than 2(H) :1(V). ATTACHMENT H CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G We reviewed two geotechnical reports previously prepared for this project by Cascade Geotechnical Inc) We also reviewed a report prepared by Shannon & Wilson, Inc.2 which addresses geotechnical implications for this project, Slade Way, and Klickitat Drive. SLOPE STABILITY Methodology We analyzed the stability of two sections of the north -south trending slope, for both pre - construction and post- construction conditions. These sections were chosen based on their proximity to previously obtained subsurface information, as well as their ability to represent the most critical areas of the slope. We performed both shallow and deep seated failure analyses. Cross section A -A' begins in the southwest property corner and extends northeasterly across the entire property. Cross section B -B' begins approximately 200 feet north of the southwest property corner and also extends northeasterly across the entire property. Both cross sections extend twenty -five (25) feet beyond the eastern and western property lines. For more details, please refer to the site plan and cross sections contained in the appendices. Utilizing the computer program SB- SLOPE, we have determined minimum factors of safety against slope failure under both static and pseudostatic (seismic) loading conditions. SB -SLOPE incorporates Bishop's Simplified method of slices. To mimic a severe seismic event, a horizontal inertial force equal to 0.2 times the total weight of the potential sliding mass was used in the pseudostatic analysis. This inertial value was selected based on Seattle D.C.L.U. recommended values3, which are generally accepted as conservative. Parameters Soil parameters were conservatively estimated based on previously obtained subsurface soils information. Six soil types were chosen that we believe accurately characterize actual soil conditions. The typical soil strengths selected are based on our experience with similar soils in this area. The pre - construction analysis incorporated a ground water surface based on elevations ` Cascade Geotechnical Inc., May 30, 1990, Preliminary Geotechnical Investigation for Hillcrest Slade Way, South of South 160th Street, Tukwila, Washington; and addendum report dated August 27, 1990. 2 Shannon & Wilson Inc., November 23, 1992, "Geotechnical Review of Proposed LeRoy Lowe Development, Tukwila, Washington." 3 Seattle Department of Construction and Land Use Director's Rule 3 -93, March, 1, 1993. 2 CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G encountered during our subsurface investigation in May and August of 1990, and springs, which were observed at the toe of the slope. The post - construction analysis incorporated the benefits of the drainage enhancements shown on the plans. The phreatic surface is assumed to be lowered to the dense sands after drainage is installed. The following table summarizes the soil parameters used in our analysis. SOIL PROPERTIES Soil Type & Classification Approx. Depth from Surface Dry Unit Weight (pcf) Cohesion (psf) 4 1, (SP) dry sand, loose 0 to 12 feet: upper slope 96 0 31 2, (SP) dry sand, medium dense 12 to 25 feet: upper slope 101 0 34 3, (SP /ML), wet sand and silt, med. dense to dense 25 to 33 feet: upper slope 38 to 45 feet: lower slope 108 200 30 4, (SP), wet sand, dense 33 to 45 feet: upper slope 45 to 65 feet: lower slope 110 0 37 5, (SP /SM) wet silty sand, loose 0 to 25 feet: lower slope 96 500 31 6, (SP /SM) wet silty sand, medium dense 25 to 38 feet: lower slope 101 500 34 RESULTS Pre - construction With respect to deep failures, our results show that the slope is presently stable under static and pseudostatic (with seismic loading) conditions. With respect to shallow failures, our results 3 CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G show that the slope is stable under static conditions, but unstable during pseudostatic conditions. The following tables summarize the results of our analysis. Pre - construction Factors of Safety Cross Sections Deep Seated Shallow Static Pseudostatic Static Pseudostatic A -A' 1.87 1.20 1.16 0.75 B -B' 1.66 1.10 1.04 0.69 The shallow, pseudostatic failure surface for section A -A' began four (4) feet west of the crest of the slope and extended approximately forty (40) feet eastward beyond the toe. The shallow, pseudostatic failure surface for section B -B' began and ended at the crest and toe of slope, respectively. Both shallow failure surfaces were, at their deepest point, only approximately three (3) feet deep. The relatively low factors of safety for shallow, pseudostatic conditions are a result of the oversteepened slope created during the past ancient landslide, which left the fine to medium grained sands on the slope face very near their angle of repose. The deep seated, pseudostatic .failure surface for section A -A' began at the crest of the slope and continued approximately forty (40) feet eastward beyond the toe. The deep seated, pseudostatic failure surface for section B -B' began forty (40) feet west of the crest of the slope and terminated eastward at the toe. Both deep seated failure surfaces were, at their deepest point, approximately twenty (20) feet deep. • Post - construction With respect to deep failures, our results show that the slope will be significantly improved by the planned drainage. The slope will be stable under static and pseudostatic conditions. Shallow failures may still occur on the slope due to the slope being very near the angle of repose. The following tables summarize the results of our analysis. 4 h I CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G Post - construction Factors of Safety Cross Section Deep Seated Static Pseudostatic Section A -A' 2.18 1.35 Section B -B' 1.85 1.25 The failure surfaces occurred at approximately the same location as in the preconstruction analysis. CONCLUSIONS Under pre - construction conditions, we conclude that the slope will be stable except for shallow instability during a maximum probable seismic event. that may result in mass wasting of several feet of soil directly on the slope face. This shallow mass wasting can be characterized by localized slumping and not movement of the entire slope. With post - construction improvements, we conclude that the slope will be generally stable for deep failures, under both static and pseudostatic conditions. The post construction improvements include the French drains shown in the plans. Shallow failures may still occur under strong seismic loading. We conclude that the proposed Boundary Line Adjustment for this site will not affect the suitability of the site development provided our recommendations contained in the Recommendations Section of this report are carefully followed. RECOMMENDATIONS Building Setbacks To protect property from shallow failures (localized sloughing) during a maximum probable seismic event, we recommend the following: • The exclusion of any building development within forty -five (45) feet of toe of the slope. 5 CASCADE GEOTECHNICAL INC. March 22, 1993 Job No. 9005 -13G GENERAL We recommend that we engaged to review the final plans when they become available to confirm that our recommendations have been properly interpreted and to provide any additional or alternate recommendations as necessary. We expect the on -site soil conditions to reflect our findings; however, some variations may occur. Should soil conditions be encountered that cause concern and /or are not discussed herein, Cascade Geotechnical Inc. should be contacted immediately to determine if additional or alternate recommendations are required. This letter has been prepared for the exclusive use of Leroy Lowe, for specific application to the proposed Hillcrest Residential Development located at Slade Way, South of South 160th Street, Tukwila, Washington, in accordance . with generally accepted soils and foundation engineering practices. No other warranty, expressed or implied, is made. Thank you for this opportunity to work with you on this project. If you have any questions, feel free to contact us at any time. Respectfully submitted, CASCADE GEOTECHNICAL INC. George E. Lamb, P.E. President Y. 6 Larry Jackson Project Manager MEMORANDUM TO: DATE: RON CAMERON / JACK PACE APRIL 9, 1993 PROJECT: HILLCREST B.L.A. SUBJECT: SLOPE STABILITY ANALYSIS GENTLEMEN : ENCLOSED IS THE FOLLOWING DOCUNENT SLOPE STABILITY ANALYSIS WITH CROSS SECTIONS PREPARED BY CASCADE GEOTECH INC . THIS DOCUMENT IS PROVIDED TO YOU FOR YOUR REVIEW AND COMMENT. LEROY C . LOWE A.I A.I.A. C.C. NEIL TWELKER P.E. PHD LEROY C. LOWE A.I.A. ARCHITECT P0. •OM 1i•1 •• *TTL•. WASHINGTON ••III HILLCREST SLOPE STABILITY ANALYSIS SITE PLAN TEST PIT (05/15/90) HAND AUGER (05/23/90) X TEST BORING (07/24/90 & 07/25/90) FROM A LANDSCAPE SITE PLAN BY LEROY C. LOWE, A.I.A. ARCHITECT (03118/93) AND FROM TEST HOLE LOCATION MAP BY CASCADE GEOTECHNICAL (05/29190) I{�r CASCADE GEOTECHNICAL INC. h 17U1611SIIIAW MIENE.. " ow Y71'.1uY1 I,e� i RSt AN11, NMf1YNG70N 9N7N IAX (MLIY711 I.1'.1 106 No 9303.OSG I SCALE : 1" = 50' LOCATIONS ARE APPROXIMATE "1'04/08/93 rwo. ey HLA le0y -cw) B 350 — 320 — 290 — 0 LL 260 — 230 — 200 PROM- RTY IINC 1 2 3 4 EXISTING HILLCREST SLOPE STABILITY ANALYSIS CROSS SECTION B - B' 0 I 1 30 60 90 5 4 PROPERTY LINE 1 1 1 T E 1 I 1 120 150 180 210 240 270 300 330 Feet 360 B' — 350 — 320 — 290 — 260 — 230 200 VI CASCADE GEOTECHNICAL INC. 12016115TNAvENUE N E. 61CG N (2061631 5060 3I3c1ANOwASNINGTON96034 F', I:06i3206953 Job No. 9303-05G sc.l. 1' = 30' LOCATIONS ARE APPROXIMATE 1:4" 03/17/93 0. ILA Enu-G.ol A 330 - 300 - 270 - 240 - 210 - 180 PIIOPLIIIY I INE 0 HILLCREST SLOPE STABILITY ANALYSIS CROSS SECTION A - A' EXISTING 1 2 3 4 SURFACE EXIST/N SURFA 5 6 4 1 1 1 1 I I 1 1 1 1 30 60 90 120 150 180 210 240 270 300 Feet PROPERTY LINE A' - 330 300 - 270 - 240 - 210 330 360 180 CASCADE GEOTECHNICAL INC. 12016 1151H AVENUE N E . BRCG 11 12:6, e:1 5080 KIPKIAND. WASH NGTON 58034 /Ar 1226. E2] 6961 .lob H.. 9303-05G Sul. 1' = 30' LOCATIONS ARP APPROXIMATE 0.1. 03/17/93 Dw to Enp•G.ol LEROY C. LOWE A.I.A. ARCHITECT P.O. SOX 1241 SEATTLE. WASHINGTON 96111 RON CAMERON P.E. CITY ENGINEER CITY OF TUKWILA 6200 SOUTHCENTER BLVD TUKWILA, WA. 98188 HILLCREST BLA DEAR SIRS : RECEI\IE^1 MAR 761993 COMMUNITY JACK PACE SENIOR PLANNER MAR. 25, 1993 THANK YOU FOR YOUR CORRESPONDENCE OF MAR. 15, 1993 AND A COPY OF DAN CLAYTON'S CORRESPONDENCE OF MAR, 2. WE TAKE EXCEPTION TO SEVERAL OF MR . CLAYTON'S COMMENTS IN HIS MAR. 2No LETTER AND HAVE INCLUDED THEM WITH OUR CORRECTIONS; THEY ARE AS FOLLOWS : A meeting was held at the request of Mr. Lowe to discuss the recommendations presented in our geotechnical review of the proposed Hillcrest Development, dated November 23, 1992. The primary point of contention during this meeting was Shannon & Wilson's recommendation that additional geotechnical studies are needed to evaluate the potential for shallow and deep- seated Iandsliding on the proposed property. RESPONSE : THE JAN. 13, 1993 MEETING WAS HELD AT MY REQUEST BECAUSE THE CITY SENIOR PLANNER INSISTED THAT IT WAS MY RESPONSIBILITY TO PROVIDE ADDITIONAL GEO- TECH STUDIES ON PROPERTY OTHER THAN THE HILLCREST SITE. THIS DEMAND WAS MADE AT OUR DEC. 9, 1992 MEETING, BETWEEN PACE , FRASER, SCHULZ , AND LOWE. IT IS MY OPINION THAT I AM RESPONSIBLE FOR STUDIES AND REPORTS ON MY PROPERTY ONLY. Mr. Lowe's representatives acknowledged that there was a potential for a slope failure on the proposed Hillcrest Development property, but maintained that it was not Mr. Lowe's responsibility to evaluate this potential as a prerequisite to obtaining a Boundary Line Adjustment. Specifically, his consultants, Mr. George Lamb of Cascade Geotechnical Inc. and Mr. Neil Twelker, indicated that because a large slide that might affect the Lowe property would also involve adjoining downslope properties (owned by Puget Western and the City of Tukwila), it was someone else's responsibility to evaluate, or accept the responsibility for, the safety of Mr. Lowe's property with respect to the potential for deep- seated landsliding. RESPONSE : MR . TWELKER & MR. LAMB HAVE NEVER ACKNOWLEDGED NOR STATED THAT THERE WAS A POTENTIAL FOR A SLOPE FAILER ON THE PROPOSED HILLCREST PROPERTY. TO STATE OTHERWISE IS FALSE. THE JAN, 13, 1993 MEETING WAS TAPE RECORDED BY JACK PACE AND COPIES OF THOSE TAPES WERE MAILED TO ME AND RECEIVED ON FEB. 13, 1993 cc, KN THESE TAPES HAVE SIGNIFICANT OMISSIONS OR ERASURES SO AS TO ELIMINATE MAJOR ELEMENTS OF THE DISCUSSION DURING THAT JAN. 13, 1993 MEETING. THESE ERASURES ARE OF GREAT CONCERN TO ME AND MY CONSULTANTS. THESE TAPES CHALLENGE THE ACCURACY OF MR. CLAYTON'S COMMENTS, FOR IN TRUTH THE TAPES SPEAK FOR THEMSELVES. MR. LAMB & MR. TWELKER BOTH STATED "LOWE'S PROPERTY IS OUTSIDE THE DEEP SEATED SLIDE AREA." MR . LAMB & MR . TWELKER STATED THAT IT WAS NOT LOWES RESPONSIBILITY TO MAINTAIN SLADE WAY OR THE DOWN -SLOPE PROPERTY, BUT RATHER THE CITY AND WASH. STATE DEPT. OF TRANSPORTATION HAD TOTAL RESPONSIBILITY TO MAINTAIN THE SLOPE AND SLADE WAY FOR THE HEALTH WELWARE AND SAFETY OF THE CITIZENS OF TUKWILA. TO SIT BY AND WATCH SLADE WAY SLIDE DOWNHILL THEREBY REMOVING THE LATERAL SUPPORT FOR THE HILLCREST SITE IS "CORP. NONFEASANCE" NEIL TWELKER QUOTE THE TAPE RECORDING CLEARLY INDICATES THAT ALL PARTIES AGREE THAT DEWATERING THE LOWE PROPERTY WOULD ADD STABILITY TO THE SITES SHALLOW STRUCTURE. MR. CLAYTON WAS RELUCTANT TO STIPULATE THAT DEWATERING THE LOWE SITE WOULD ADD STABILITY TO THE HILLSIDE THO MR. LAMB & MR . TWELKER WERE NOT. SIMPLY STATED, MR. LOWE'S PROPERTY IS OUTSIDE THE DEEP SEATED SLOPE STABILITY PROBLEM & DEWATERING THE LOWE PROPERTY WILL ADD TO THE GENERAL AREAS STABILITY. AS A RESULT OF THIS MEETING WE ARE PREPARING A SLOPE STABILITY ANALYSIS FOR\YOU RR RR VIE AND COMMENT. fr_ VERY TRJ".Y YOURS •Lt OYr C. LO\ E A.I A.I.A. City of Tukwila John W. Rants, Mayor • Department of Community Development Rick Beeler, Director March 15, 1993 Leroy Lowe P.O. Box 1241 Seattle, WA 98111 Re: Soil and geotechnical evaluation of proposed Lowe Development Dear Mr. Lowe: At the January 13, 1993 meeting, you asked for clarification on the basis for Shannon E. Wilson, Inc. recommendations for additional geotechnical studies. Attached to this letter is their clarification for the additional studies. Under the Sensitive Areas Ordinance, you may appeal this decision. The ordinance states: "18.45.125 Appeals. (a) Any aggrieved party who objects to or disagrees with Department of Community Development (DCD) decisions or conditions for development in a sensitive area shall appeal to the Planning Commission. Any such appeal shall be made in writing within ten days of the interpretation, condition or decision being appealed, and shall set forth the basis for the appeal.. (b) In considering appeals of decisions or conditions, the following shall be considered: (1) The intent and purposes of the sensitive areas ordinance from which this chapter derives; (2) Technical information and 'reports considered by the Department of Community Development; and (3) Findings of the DCD Director which shall be given substantial weight." ATTACHMENT J 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431 -3670 • Fax (206) 431.3665 Leroy Lowe March 15, 1993 'Page 2 As I mentioned on the phone, another letter will be sent to you to respond to Cascade Geotechnical, Inc., proposal for slope stability analysis date' February 24, 1993. As of this date, Shannon E. Wilson, Inc., have not given the city a time frame for response. Should you have any questions, please feel free to call or write. Sincerely, JP/dg attachment . l�dti ck Pace Senior Planner cc: Ron Cameron, City Engineer =ill S HANNON ILO I T ENVIRONMENTAL CONSULTANTS March 2, 1993 City of Tukwila Department of Public Works 6300 Southcenter Boulevard, Suite 100 Tukwila, Washington 98188 Attn: Mr. Jack Pace, Senior Planner RECEIVED MAR - 41993 TUKWiLA PUBLIC WORKS RE: SOILS AND GEOTECHNICAL EVALUATION OF PROPOSED LEROY LOWE DEVELOPMENT, TUKWILA, WASHINGTON The purpose of this letter is to summarize the results of a meeting held between the City of Tukwila and Mr. LeRoy Lowe and his consultants on January 13, 1993, and to provide clarifica- tion on the basis for our recommendations for additional geotechnical studies for the subject property. Issues pertaining to wetland development or geotechnical design considerations were not discussed at depth in the meeting and are not addressed in this letter. A meeting was held at the request of Mr. Lowe to discuss the recommendations presented in our geotechnical review of the proposed Hillcrest Development, dated November 23, 1992. The primary point of contention during this meeting was Shannon & Wilson's recommendation that additional geotechnical studies are needed to evaluate the potential for shallow and deep - seated landsliding on the proposed property. Mr. Lowe's representatives acknowledged that there was a potential for a slope failure on the proposed Hillcrest Development property, but maintained that it was not Mr. Lowe's responsibility to evaluate this potential as a prerequisite to obtaining a Boundary Line Adjustment. Specifically, his consultants, Mr. George Lamb of Cascade Geotechnical Inc. and Mr. Neil Twelker, indicated that because a large slide that might affect the Lowe property would also involve adjoining downslope properties (owned by Puget Western and the City of Tukwila), it was someone else's responsibility to evaluate, or accept the responsibility for, the safety of Mr. Lowe's property with respect to the potential for deep - seated landsliding. Our report concluded that additional site - specific geotechnical evaluations, including both static and dynamic slope stability analyses, are necessary to evaluate the potential for both shallow and deep - seated landsliding that could involve the proposed Hillcrest Development. This conclusion is based on a review of the geotechnical investigations on the Hillcrest property and adjoining areas which indicated the following: 400 NORTH 34TH STREET • SUITE 100 P.O. BOX 300303 SEATTLE, WASHINGTON 93103 206.6.2.8020 FAX 2: 7.633. 6777 W- 6367 -02 136 City of Tukwila Attn: Mr. Jack Pace March 2, 1993 Page 2 SI- IANNON &WILSON, INC. ► The proposed development is situated on an old landslide deposit, with the scarp of that slide crossing the property. ► A steep slope exists on the property (greater than 25 degrees), there is some evidence of instability in the site surficial soils, and there is considerable seepage on the property. ► Surficial soils in several areas downslope from the site appear to be unstable, including an area which is affecting Slade Way to the southeast of the proposed development. ► Slope indicator data from the vicinity of Slade Way immediately downslope of the property provide questionable evidence of minor, relatively deep - seated slope movement since the 1960 landslide. ► A large deep- seated slope failure occurred in 1960 on an adjoining property a short distance downslope. Recurrent movement on this landslide, if it were to occur, might threaten the stability of the site. ► Three static and dynamic slope stability analyses conducted during the 1980s indicate that the slope stability of the area immediately downslope from the proposed Hillcrest Development is marginal at best, and likely to diminish over time if the hillside dewatering system operated by WSDOT continues to deterio- rate. The findings listed above indicate to us the need for careful evaluation of the potential for land - sliding on this property before proceeding with any development that might endanger human health or safety. This conclusion is consistent with the requirements set forth in the City of Tukwila Sensitive Areas Ordinance (SAO). Specifically, the property fits the description of a Class 4 area of potential geologic instability as described below from the Draft Codified SAO October 18, 1991, Section 3, 18.45.020E: "Class 4 areas, where landslide potential is high, which include sloping areas with mappable zones of ground water seepage, and which also include existing mappa- ble landslide deposits regardless of slope." Section 18.45.080E of the Sensitive Areas Ordinance states that prior to permitting alteration of an area of potential geologic instability, the applicant must demonstrate either: W- 6367 -02 City of Tukwila Attn: Mr. Jack Pace March 2, 1993 Page 3 SHANNON &,WILSON. INC. 1) There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no significant risk to the proposed development or surrounding properties; or 2) The area of potential geologic instability can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability. Our review indicates that the proposed development does not meet the first condition, and has not been demonstrated to meet the second condition required for permitting a development in an area of potential geologic instability. The Sensitive Areas Ordinance further states that a geotechnical report must be submitted for areas of potential geologic instability, and specifies that a detailed slope stability analysis must be done for all Class 4 areas. It is our opinion that in order to comply with the requirements and the intent of the SAO, such slope stability analyses should be accomplished for both static and dynamic (earthquake ground motion) conditions. In the case of a proposed development which would significantly modify the drainage or ground configuration, stability analyses also should be accomplished for pre -and post - construction conditions. Obviously, a stability analysis for post - construction conditions cannot be accomplished until specific development plans are completed. It is our recommendation, and the stated intent of the SAO, that the geotechnical study evaluating the current site stability be accomplished early in the design stages of the a project. This study should be performed by a geotechnical engineer and in accordance with the requirements of the SAO. We would recommend that the geotechnical engineer selected to perform this study be experienced in slope stability analysis and familiar with the geologic conditions and slope stabilization efforts that have been accomplished in the vicinity of the proposed development. Upon completion of the geotechnical report documenting the findings of this study, we recom- mend that an independent review be performed by a geotechnical engineer. This recommenda- tion is consistent with our interpretation of the requirements of the Tukwila Municipal Code 21.04.140. W- 6367 -02 1 City of Tukwila Attn: Mr. Jack Pace March 2, 1993 Page 4 Please call if there are any questions regarding this letter. Sincerely, SHANNON & WILSON, INC. Daniel N. Clayton, C.E.G. Senior Associate Reviewed by: EXPIRES 11/10/ 9 1. W. Paul Grant, P. E. Vice President DNCMPG/dnc W6367-02.LTR/W6367-1kci/eet )41 Si-LANNON &WILSON. INC. W-6367-02 -W SHANNON FJWILSON, INC. == GEOTEC- .tCAL AND ENVIRONt. EIai:.L CCNSI:LT4I.TS November 23, 1992 City of Tukwila Department of Public Works 6300 Southcenter Boulevard, Suite 100 Tukwila, Washington 98188 Attn: Mr. Phil Fraser, Senior Engineer RE: GEOTECHNICAL REVIEW OF PROPOSED LEROY LOWE DEVELOPMENT, TUKWILA, WASHINGTON Dear Mr. Fraser, Attached is our report presenting the findings of our geotechnical review of the proposed Hillcrest Development and the stability of Slade Way adjacent to it. We trust that this review will assist you in evaluating the proposed development and in assessing further investigations that will be required to evaluate the stability of the area. Sincerely, SHANNON & WILSON, INC. Daniel N. Clayton, CIG., R.G. Senior Associate DNC /eet Enclosure: 'Report W 6367 -01.TM 1 / W 6367- Ikd /eet 400 NOGTH 34TH STREET • SUITE 100 F.O. S0? 300303 SEATTLE. WASHINGTON 98103 206.632.8020 FAX 206.633.6777 A HMEIIELK W- 6367 -01 1 A'. SHANNON &WILSON, INC. TABLE OF CONTENTS Page 1.0 INTRODUC'T'ION 1 2.0 SCOPE OF WORK 1 3.0 SITE CONDITIONS 2 3.1 Surface Conditions 2 3.2 Subsurface Conditions 3 4.0 PREVIOUS GEOTECHNICAL INVESTIGATIONS AND REMEDIATION • • 4 4.1 1960 Landslide Investigations 4 4.2 Geotechnical Investigations for the Proposed Valley Vue Estates 5 4.3 Geotechnical Investigations for the Hillcrest Development 5 5.0 DISCUSSION OF FINDINGS 6 5.1 Slope Stability 6 5.1.1 Shallow Landslide Potential 6 5.1.2 Deep - Seated Landslide Potential. 8 5.2 Geotechnical Considerations for Slade Way Stability 9 5.3 Geotechnical Considerations for the Proposed Hillcrest Development. 11 5.3.1 Slope Stability Considerations 11 5.3.2 Seismic Hazard 12 5.3.3 Site Drainage 12 5.4 Alternative Considerations for Hillcrest Development 13 5.5 Stormflow Drainage 14 14 7.0 LIMITATIONS 16 6.0 CONCLUSIONS LIST OF FIGURES Figure No. 1 Site Plan 2 Generalized Geologic Profiles W- 6367 -01 SHANNON FIWILSON. INC. TABLE OF CONTENTS (Cont.) LIST OF APPENDICES APPENDIX A - SEQUENTIAL LIST OF DOCUMENTS REVIEWED APPENDIX B - IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL ENGINEERING REPORT ii W- 6367 -01 SHANNON &WILSON, INC. GEOTECHNICAL REVIEW OF PROPOSED LEROY LOWE DEVELOPMENT, TUKWILA, WASHINGTON 1.0 INTRODUCTION This report presents .the results of our geotechnical engineering review of the proposed Hillcrest residential development at South 160th Street and Slade Way, Tukwila. The purpose of the review was to provide the City of Tukwila with an independent evaluation of the stability of Slade Way and the proposed development, to evaluate the need to dewater the wetlands on the site in order to develop the site as has been proposed by the developer, and to evaluate the need for a drainage detention system. Our review focused on: a) development plans and recent geotechnical investigations done for the Hillcrest developer /owner, Mr. LeRoy Lowe; b) geotechnical investigations performed in the 1980's for the proposed Valley Vue Development downslope from the Lowe property, and c) geotechnical investigations and remedial actions that were taken in the 1960's to stabilize a large landslide that extended eastward to the vicinity of Slade Way from a borrow excavation at the toe of the slope. 2.0 SCOPE OF WORK The scope of our services is described in detail in Exhibit A to your Standard Consultant Agreement. This work was authorized by your signed Standard Consultant Agreement and notice to proceed dated October 2, 1992. Our scope consisted of a field reconnaissance of the property; a review of documents, reports and analyses prepared by others; and preparation of a report presenting the findings of our study. Our scope of services originally included: 1. Review of the existing geotechnical reports, wetland reports, and site plans and drawings for the proposed Hillcrest Development which were provided by the City. 2. Review of geotechnical reports from the site vicinity, including those prepared in conjunc- tion with the construction of Interstate Highway 5 and the proposed Valley Vue Develop- ment. 3. Estimate of site runoff under existing conditions, and combined runoff and dewatering system discharge for the proposed Hillcrest Development. 1 W- 6367 -01 1 ii SHANNON &WILSON. INC. 4 Provision of a professional opinion regarding the present stability of Slade Way in the im- mediate vicinity of the proposed Hillcrest Development, and if appropriate, discussion of alternatives that are available for improving the stability of the slope. 5. Determination whether stability analyses by others for adjacent property may be applied to assess the stability of Slade Way and provision of a professional opinion as to how these factors of safety might be affected by the proposed Hillcrest Development. 6. Evaluation of the feasibility of various alternatives for site dewatering and runoff manage- ment that might exist with respect to slope stability of the proposed development and the adjacent part of Slade Way. 7. Evaluation of the geotechnical suitability of measures that could be taken to prevent an in- crease in peak flows from the property when it is developed. Once the work was begun, it became clear that there was not sufficient site - specific data available to make a reliable estimate of site runoff, so it was agreed to drop this task from the scope of work. A list of the documents that were reviewed in the study is provided in Appendix A of this report. Appendix B provides important information about your geotechnical report. 3.0 SITE CONDITIONS 3.1. Surface Conditions The proposed Hillcrest Development (or LeRoy Lowe Development) is located•on the west side of Slade Way, just south of South 160th Street, in Tukwila, Washington. It is situated approxi- mately half -way up the steep, east- facing slope of the hill, which is located southwest of the Tukwila freeway interchange. The property consists of approximately three acres of undeveloped land bounded on the north, west, and south by single family residences and on the east by Slade Way. The site is wooded and has about 64 feet of topographic relief across it. A steep slope (2H:1V) about thirty feet high separates a relatively flat upland in the western part of the site from an adjoining lowland to,the east. The lowland area is hummocky and poorly drained, with an- overall gentle eastward • slope toward Slade Way. Much of this lowland area is boggy, and at the time of our reconnaissance, locally contained standing water. Large parts of the eastern part of the site have been classified as a Type 3 Wetland as the result of a wetland study in 1992 by Bredberg and Associates, Inc. 2 W- 6367 -01 SHANNON &WILSON. INC. The steep slope that separates the upland and lowland portions of the site is interpreted to be the southern end of the headscarp of an ancient extensive landslide. This landslide escarpment extends about 3000 feet northwest of the site and typically displays about 30 to 40 feet of topographic relief. At the base of this escarpment, the ground is typically hummocky and poorly drained and commonly is the site of springs and boggy conditions, as it is on the Lowe property. This disrupted drainage probably has contributed to the numerous smaller slides within the ancient slide mass. To the east of the site and Slade Way, the hillside slopes moderately steeply down to the Duwamish Valley. This densely wooded, vacant property below Slade Way was modified in the early 1960s by regrading and other remediation measures after a large, deep - seated landslide propagated upslope from the base of the hill in 1960. The extent of this 1960 landslide and subsequent movements through April 1961 is shown in Figure 1. 3.2 Subsurface Conditions Borings drilled on the site by Cascade Geotechnical, Inc. (1990) and downslope by others indicate that there are three main geologic deposits of concern in the site vicinity. These geologic units are portrayed in Figure 2 in generalized geologic profiles that extend eastward from SIade Way. The uppermost deposit consists of fine to medium sand and silty sand with scattered gravel. This upper unit varies from about 10 to 30 feet thick, blankets the slope, and thins eastward, downhill from Slade Way. Groundwater within this layer is perched on the underlying clayey silt and comprises the surficial aquifer at the site. The upper sand unit overlies a relatively thick layer of clayey silt and silt that ranges from about 30 to 70 feet thick. This intermediate layer of clayey silt typically is laminated, fractured, and slickensided. The clayey silt unit, as a whole, slopes eastward, roughly paralleling the slope of the hillside, as do bedding planes within it. This clayey silt unit has a low permeability and serves as an aquitard between the water table aquifer above and a confined aquifer below it. Underlying the clayey silt unit is a lower sand and gravel deposit that is typically about 10 to 40. feet thick. Although it varies in grain size, this sand and gravel is continuous throughout the area. Based on its prolific capacity as an aquifer, it appears to extend far into the hillside to the west,. where.it is recharged and from which it has considerable hydraulic head. The water table in the surficial sand unit is about 25 to 30 feet below the upland part of the proposed development and seeps out of the base of the steep slope on the property. However, on the lower, eastern part of the site, this sand is saturated essentially to the ground surface, 3 W- 6367 -01 SHANNON El WILSON, INC. as it also appears to be in lower areas to the east of Slade Way. The underlying clayey silt is an aquitard, confining the groundwater in the deeper sand and gravel deposit and restricting hydraulic communication between this deeper aquifer and the overlying water table aquifer. Hydrostatic uplift on the base of the clayey silt from confined groundwater in the underlying sand and gravel aquifer has been an important contributing factor to landsliding on the hillside. 4.0 PREVIOUS GEOTECH•ICAL INVESTIGATIONS AND REMEDIATION 4.1 1960 Landslide Investigations The slope below Slade Way has been the subject of intensive geotechnical investigations since 1960, when gravel borrow pits were developed at the base of the hill. Excavation of the natural buttress at the toe of the hillside resulted in a series of progressive landslides which extended upslope to the approximate location of Slade Way (see Figure 1). The center of this slide mass moved laterally as much as about 70 feet, with the main slide plane located about 30 to 50 feet below ground surface within the clayey silt unit. Geotechnical investigations of the 1960 and subsequent landslides revealed that the main contributing geologic factors to sliding were: 1) the pre - existing landslides on the hillside, 2) the downslope dipping structure o: the laminated clayey silt unit, 3) the presence of a natural reservoir of water in deep sand and gravel deposits uphill and behind the unstable slopes, and 4) high artesian water pressure in a confined aquifer underlying the clayey silt unit. Remedial measures to stabilize the hillside were accomplished by the Washington Department of Transportation ( WSDOT). These measures consisted of regrading, surface drainage, and installing a system of vertical and horizontal drains. These measures, mostly accomplished by the fall of 1961, were effective in lowering the artesian pressures and stabilizing the sliding at that time. Additional related geotechnical investigations by Shannon & Wilson on behalf of WSDOT continued through 1968 to stabilize other parts of the hillside further to the north and west of the 1960 landslide. The same general geologic conditions that contributed to the 1960 slide also exist in these areas. These slopes were also stabilized by various combinations of vertical and horizontal drains, regrading, soldier pile walls, and rock buttresses.- 4 W- 6367 -01 140 SHANNON F&WILSON, INC. 4.2 Geotechnical Investigations for the Proposed Valley Vue Estates Between 1982 and 1989, a multiple- family condominium complex was proposed on the site of the 1960 landslide,. directly east of the proposed Hillcrest Development. GeoEngineers, Inc. was retained to conduct . geotechnical investigations of that site. Their work involved a limited subsurface exploration program, installation of slope indicators and piezometers, slope stability analyses, and development of recommendations for measures that would be necessary, in their opinion, to successfully build and maintain the proposed complex. At the time of their investiga- tions, GeoEngineers, Inc. noted small offsets a short distance below Slade Way as well as local surficial slides within the large area below Slade Way. Dames & Moore and WSDOT provided review of GeoEngineers, Inc. investigations and findings for the Valley Vue Estates, particularly with respect to slope stability analyses and requirements for remediation and maintenance of the WSDOT drainage system. The ongoing deterioration of the WSDOT hillside drainage system, installed some 20 years earlier to stabilize the hillside, was a primary concern addressed in their reviews. Dames & Moore, WSDOT, and Geo- Engineers, Inc. all indicated that without a functional drainage system, there would be an unacceptable risk of a deep - seated slope failure on the site. The lack of achieving a feasible and acceptable means of improving and maintaining that drainage system apparently was a major consideration in the disapproval of the proposed Valley Vue Development. 4.3 Geotechnical Investigations for the Hillcrest Development. Geotechnical investigations conducted thus far for the proposed Hillcrest Development consist of: 1) two phases of site explorations and evaluations reported by Cascade Geotechnical in May and August of 1990, 2) subsequent letter reports from Cascade Geotechnical providing review of site drawings and plans, 3) a letter describing site reconnaissance observations from Mr. Stuth of Resco Mechanical and Civil Engineering, and 4) a letter from Mr. Dennis Joule, Civil Engineer, describing his site observations and conclusions from reviewing earlier geotechnical reports. A wetlands study on the property by Bredberg and Associates, Inc., dated April 1992, also offered several observations relevant to geotechnical issues, albeit from a biologist's perspective and expertise. 5 W- 6367 -01 1•0► 5.0 DISCUSSION OF FINDINGS 5.1 Slope Stability SHANNON &WILSON, INC. Slade Way and the proposed Hillcrest Development are situated on a hillside that has experienced repeated slope.movements since pre- historic time. An extensive landslide or sequence of slides in pre - historic time downdropped the eastern part of the proposed development, Slade Way, and the area to the east by at least 30 feet. The failure plane for this slide is believed to have been deep - seated, probably within the clayey silt unit. It is within the slide mass of this ancient slide that numerous subsequent slope movements have occurred. The largest known subsequent slope failure within the boundary of the ancient slide mass occurred in 1960 on the hillside below Slade Way. This landslide was initiated by excavation of the toe of the hillside about 700 feet east and 200 feet below Slade Way. The slide progressed upslope to within less than 100 feet east of the north- trending part of Slade Way and cut across the area where the road turns eastward (see Figure 1). The failure plane for the main part of this slide was also deep - seated, about 30 to 50 feet below ground surface within the clayey silt The failure was caused by the combined effects of undercutting of the toe of the hillside and high uplift pressures from the confined groundwater in the deep sand and gravel aquifer. Numerous relatively small shallow slides also have occurred on the hillside, mostly within the disturbed topography of the ancient slide mass. Evidence of shallow soil creep, including bowed and leaning tree trunks, is widespread in the area, most notably on the eastern part of the proposed Hillcrest Development and immediately to the east below Slade Way. • Based on a review of previous investigations and a site reconnaissance of the hillside, it is our opinion that Slade Way, the proposed Hillcrest Development, and the adjoining properties could be damaged by either shallow- seated or deep seated landslides. The potential for these two types of landslides to affect Slade Way and the adjoining properties is discussed below. 5.1.1 Shallow Landslide Potential, A shallow slide in the vicinity of Slade Way would likely occur within or at the base of the upper sand unit. Conditions that could contribute to such a failure include high groundwater levels..in .this.water table aquifer,. poor surface. drainage conditions which in turn contribute to a high water table, weak soils, and ground modifications that either undercut the toe or overload the head of a potential slide area. All of these conditions exist to varying degrees in the immediate vicinity of Slade Way. 6 W- 6367 -01 (so SHANNON &WILSON, INC. A short distance below the north - trending segment of Slade Way where it borders the pro- posed Hillcrest Development, the slope was oversteepened by the 1960 landslide. In September 1961, a series of horizontal drains were installed, and slope grading was accomplished in this area below the present location of Slade Way to stabilize existing slide area and to protect the road alignment and adjoining properties. The horizontal drains extended westward from a bench that was excavated at approximately elevation 240 feet on the slope east of the roadway. Although documentation of exact orientation or lengths has not been found, their locations and elevation (see Figure 1) indicate that these drains were installed in the upper sand unit to provide drainage to the shallow water table aquifer, primarily in the area immediately east of what is now the proposed Hillcrest Development. The site grading and horizontal drains were effective in 1961 in stopping the westward progression of the slide. As of 1968, the horizontal drains were flowing freely, and there was no surficial evidence of slide movement (Shannon & Wilson, 1968). However, by 1982, GeoEngineers, Inc. (1982) reported that a 200 -foot long, 12- to 20 -inch high scarp had formed just below the road. At that time the drains were flowing, although there is no indication of how well they were working. Dames & Moore concluded in 1989 that all but four of the horizontal drains within the proposed Valley Vue Development had deteriorated significantly and that groundwater levels recorded in the piezometers were significantly higher than they had been when the drainage system was installed. WSDOT also noted in 1989 that a slope indicator in this area (location F on Figure 1) indicated movement down to a depth of 24 feet. During our reconnaissance in October 1992, flow was minimal from the horizontal drains that could be found. At that time there was no surficial evidence of recent soil movement below the north - trending part of Slade Way other than widespread bowing of tree trunks caused by shallow creep. It appears from this review that, in the three decades since the horizontal drains were in- stalled, their effectiveness in draining the soils underlying Slade Way has diminished. If the water table has risen as is expected, it follows that the stability of the road has decreased correspondingly. GeoEngineers, Inc. (1985) calculated that for a 3H:1V slope (the approximate slope below Slade Way), the minimum depth to the water table required for a factor of safety of greater than unity is about 6 feet, given the soil strength they assigned to the upper soils (0 = 25 degrees) at a groundwater depth of 12 feet, the factor of safety for Slade Way would be only 1.4. Without specific information on the depth to groundwater and the depth of the upper sands in the roadway area, it is not possible to calculate a meaningful factor of safety for a shallow slope failure that would involve Slade Way. Nevertheless, the field observations over the past 7 W- 6367 -01 SHANNON &WILSON, INC. several years, as described above, suggest the north - trending section of the road is at risk of movement, particularly during the rainy season when the perched water table below the road and seepage pressure at the slope face are highest. These field observations include the very shallow groundwater levels measured on the adjoining proposed Hillcrest Development, evidence that the horizontal drains below the road have deteriorated, small slope movements that have occurred below the roadway in the past several years, and the wet conditions observed locally on the slope below Slade Way. Further to the south, where Slade Way turns eastward, there is an area of the roadway which is currently slipping, apparently within the shallow soils on the slope below the roadway. Within the last year the outside half of the outside lane of the road in this bend subsided about 10 inches. Rather than excavating or recompacting the fill under this section of the road, the City of Tukwila road maintenance crew placed an overlay to reduce the immediate hazard to traffic. Since that repair, the fill and edge of the pavement edge has continued to subside in response to shallow soil movements on the slope below the road. This sliding at the bend in Slade Way is occurring in an area that failed in the 1960 landslide (see Figure 1). Unlike the deep - seated sliding further downslope, movements here probably occurred as relatively small, shallow slips that may be related to a high groundwater table or to loose slide material or fill. It is not known if the slide material in this area was subsequently excavated and filled, or if fill material was simply placed over the downdropped slide material. Unlike the area along Slade Way immediately to the north, there apparently were no horizontal drains installed below this part of the roadway. It appears that the current instability at the bend in Slade Way can be attributed in part to the earlier movement, the lack of subsurface drainage, and, perhaps, the placement or composi- tion of the materials that were used to repair the sliding. Without remediation, this area can be expected to continue to slide. The rate or extent to which this will happen cannot be predicted without an understanding of the subsurface conditions in this area. It appears likely, though, that drainage improvements further north along the roadway would not improve the stability of this area. 5.1.2 Deep - Seated Landslide Potential, The risk of a deep - seated landslide on the hillside below Slade Way, like that which oc- curred in 1960, was evaluated by slope stability analyses conducted for WSDOT by Shannon & Wilson, Inc. in 1964 and for the proposed Valley Vue Development by GeoEngineers, Inc., and WSDOT in 1989. This latter analysis, which was based on 1989 groundwater levels, 8 W- 6367 -01 '15. SHANNON &WILSON, INC. provided computed factors of safety ranging from 1.18 to 1.24 for static conditions and 0.86 to 1.08 for dynamic loading (seismic coefficient of 0.15 g). The more conservative factors of safety correspond closely with Profile A -A' and the less conservative correspond roughly with 'rofile B -B' on Figure 2. WSDOT concluded that the maximum static factor of safety reason - ab:y achievable by groundwater drawdown would be 1.29 and, conversely, that a 20 -foot rise in groundwater in the confined aquifer would result in a static factor of safety of only 0.82. Given the deterioration of the WSDOT drainage system and the resulting rise in groundwa- ter levels that has occurred over the last 30 years, it is reasonable to conclude that the factor of safety for a deep - seated landslide will continue to decrease without remediation of the drainage system. It is not known what plans WSDOT has for such remediation. It was apparent in 1989, however, that WSDOT was not prepared to make or allow repairs or modifications to the drainage system for the benefit of a private development. 5.2 Geotechnical Considerations for Slade Way Stability Slade Way and the adjoining areas appear to be at some risk of sliding from two different mechanisms: a shallow slope failure, resulting largely from shallow groundwater in the surficial aquifer, and a deep - seated landslide that would result from buildup of artesian pressure in the underlying confined aquifer. It is likely that the risk of deep - seated sliding has increased since 1961 and will continue to increase over time if remedial measures are not taken. There is also evidence to suggest that the potential for shallow sliding has increased. However, it is not clear that this risk will continue to increase without remediation. It is our opinion that, during periods of high rainfall and a corresponding high groundwater level, the stability factor of safety would be close to unity for a shallow slide surface beneath Slade Way. Typically, factors of safety in excess of 1.25 are desired for the design of permanent slopes on highway projects. This philosophy was expressed by WSDOT in their 1989 review of the Valley Vue Estates Project. Based upon stability analyses performed for an adjoining area by GeoEngineers, Inc. (1989), it is inferred that the water table would need to be at or below a depth of 10 feet in order to provide a static factor of safety in excess of 1.25 for the shallow slope stability. Based on these considerations, it is our opinion that soil samples and groundwater levels should be obtained from the Slade Way area to better assess the stability of the road. This could be accomplished by a series of shallow monitoring wells installed in borings drilled on either side of the roadway. The wells should be installed in time for water level measurements during. the rainy season when groundwater is shallowest and most significant in reducing slope stability. 9 W- 6367 -01 SHANNON &WILSON. INC. If stability analyses based on these water levels and soil conditions indicate an unacceptable risk, several approaches are available for improving the stability of the road. The least costly would likely be an attempt to clean out existing horizontal drains below the roadway and monitor the new piezometers (discussed above) for any change in water levels. If such a cleanout is not successful in lowering the water level, additional horizontal drains could be installed. Flow from any horizontal drains should be conveyed in a lined ditch or tightline to the storm sewer. Another relatively simple measure that would likely improve conditions would be to line the ditch along the west side of Slade Way in order to reduce infiltration of surface runoff. A more elaborate method to improve the shallow stability of the roadway and adjoining proper- ties would be to collect surface water and shallow groundwater from the base of the steep slope on the Lowe property by installing a French drain system that is tightlined to the storm sewer. This would likely be the most effective passive solution for improving the hillside stability, but would require dewatering of the Type 3 wetland on the Lowe property. Alternatively, a somewhat deeper French drain could be placed along the west side of Slade Way with similar results. One possible advantage to a drain in this location is that it would probably not complete- ly eliminate the wetland, as would a drain located further to the west. In order to reduce the potential for continued slippage in the area of the bend in Slade Way, the soils underlying the pavement and shoulder should be removed and replaced with compacted fill. Depending on the subsurface conditions encountered, subdrainage measures may be required to improve this section of the road. If remediation of this section of the roadway is seriously considered, it is recommended that additional subsurface explorations (borings and test pits) be conducted in this area to define subsurface conditions, the mechanism of the soil movement, and the details of the site remediation. Depending on the subsurface conditions that are observed in both the north- trending section and in the bend in Slade Way, it is possible that more elaborate measures may be required to achieve a level of safety desired by the City. Such measures might include rock buttresses or retaining walls which would have to be designed to match site conditions. W:;.h regard to the potential for deep - seated sliding, it is our opinion that the most productive approach the .City can take to improve hillside stability would be to encourage WSDOT to rehabilitate their hillside drainage system. If measures are not taken to improve the system, it may also be appropriate for the City to develop an ongoing groundwater monitoring program to assess the effectiveness of the drainage system. In the absence of any remediation by WSDOT, the City may prefer to defer any remediation until such time that movements affect 10 W- 6367 -01 is SHANNON &WILSON, INC. Slade Way. At that time, remedial schemes could be considered ranging from fill placement in the faded sections of the roadway to installation of subsurface drains (horizontal drains and /or French drains) to improve the area. Any remedial action should be based on performance standards and requirements established by the City for this section of the hillside. 5.3 Geotechnical Considerations for the Proposed Hillcrest Development. 5.3.1 Slope Stability Considerations Based on our field reconnaissance and review of previous geotechnical reports for the site and vicinity, the proposed Hillcrest Development is interpreted to be situated on the uppermost part of an ancient landslide. This ancient slide mass has been reactivated in the form of smaller slides within the debris from the large ancient slide. The most significant of these slides occurred in 1960, when the reactivation of the slide mass extended nearly to the eastern boundary of the proposed development. Given this history of sliding, it is our opinion that the subject property should be considered an area of potential geologic instability, as defined in the City's document entitled " Areas of Potential Geologic Instability Development and Report Criteria." Specifically, we would consider it to be a Class 4 area, where landslide potential should be considered high unless a sufficient geotechnical evaluation, including site - specific slope stability analyses, demonstrates otherwise. We recommend that slope stability analyses be performed for both shallow and deep sliding and for both pre- and post- development conditions. Such analyses will require additional geotechnical studies to obtain the necessary subsurface and groundwater data. We anticipate that a slope stability analysis for a deep - seated slope failure will require deeper monitoring well installations than currently exist on the property and rainy season water level measurements from such wells. The stability analyses would also require information on the seasonal fluctuation of the near - surface water table. This may require the installation of additional shallow monitor- ing wells. This work should be accomplished soon if the results are to be available in time for construction during the next dry season. We anticipate that the slope stability analysis for a deep - seated landslide may indicate similar results to those obtained by. WSDOT for. the hillside below Slade Way. We also believe it is likely that this analysis will indicate that construction of the development would have little effect on reducing the potential for deep - seated sliding. If the stability analyses for a deep- seated slope failure indicate a significant risk, either considerable work and cooperation from WSDOT 11 W- 6367 -01 SHANNON &WILSON, INC. in rehabilitating the existing drainage system or installation of deep on -site dewatering wells would be required to reduce the risk. Either approach may not be economically feasible for this development. With respect to the results of a stability analysis for shallow sliding, the site development could have significant effects, either increasing or decreasing stability depending on the construc- tion methods and site design. However, it is likely that the potential for shallow sliding on site and adjacent to the property can be minimized by proper drainage, construction, and maintenance of the site. 5.3.2 Seismic Hazard The developer of the property will need to address the potential for seismically induced geological hazards, such as liquefaction potential and the potential for earthquake- induced slides as a result of liquefaction in underlying soils. Earthquake- induced landsliding should be addressed by dynamic stability analyses for both deep and shallow sliding and for both existing and post - development conditions. The developer also should consider potential amplification of ground motions in the areas underlain by peat and the minimal lateral resistance the peaty soils may provide to any pile supported structures. 5.3.3 Site Drainage We understand that the owner /developer of the proposed Hillcrest Development has pro- posed to drain the wetlands and capture both surface and groundwater in a French drain system along the base of the steep slope on the property. We also understand that the Tukwila Sensitive Areas Ordinance (SAO) specifies that filling of a Type 3 wetland requires either on -site or off- site mitigation to include a 50 percent increase in total wetland area. As we are not aware of any plans the developer may have for off-site wetlands mitigation, we have assumed that this ordinance will effectively require that, for each acre of wetland which is filled or otherwise removed, construction of 1 -1/2 acres of on -site wetlands would be required to comply with the SAO Based on our site reconnaissance, review of the wetlands study, and geotechnical reports of the site - and vicinity, it is our opinion that the .complete drainage of the wetlands, as proposed by the developer; would be necessary. forsuccessful development of the property as it currently is planned. Relocation of wetlands within the property does not appear to be feasible or advisable given the proposed layout of five residences in the lowland area of the property. This 12 W- 6367 -01 tg SHANNON iWILSON. INC. is particularly the case because of the SAO requirement to increase by 50 percent any wetlands that are relocated. From theperspective of constructibility, it is our opinion that dewatering of the site would be required for foundation-preparation, construction; access roads, and utilities associated with the five proposed residences as they are currently planned. Consequently, it does not appear to be feasible to develop the site as planned without elimination of much of the existing wetlands. Also, the drainage systems that would be required for the foundations, and surface structures would likely continue to affect surface water to the detriment of any wetland area that could be retained or re- established. With respect to the slope stability of the site and adjacent roadway, removal of the surface water and lowering of the water table on the property would improve the shallow stability of both the site and the adjacent north - trending section of Slade Way. Conversely, a partial relocation of the wetlands within the property may do little, if anything, to improve the long -term stability of the hillside and Slade Way. Until site - specific slope stability analyses are accom- plished, the importance of this drainage issue remains unknown. However, we believe it could be a significant factor in the feasibility of the project. 5.4 Alternative Considerations for Hillcrest Development In lieu of the proposed development, which would require the elimination of the wetlands, it is our opinion that a reduced -scale development may be accomplished outside of the wetlands which would then save this ecological area. Such a reduced -scale development could include building on the uplands portion of the site, with access provided through easements on existing property to the west or possibly along a roadway off Slade Way at the south end of the property. In our opinion, such alternative development schemes could both maintain the existing wetlands and allow development of the property. Regardless of the final proposed development plans, it will be imperative for the owner's geotechnical consultant to perform slope stability analyses for pre- and post- development conditions for both shallow and deep landslide potential. The geotechnical consultant also will have to provide recommendations for drainage, foundations, and cuts and fills and retaining walls that are based on the stability analyses. 13 W- 6367 -01 SHANNON &WILSON, INC. 5.5 Sformflow Drainage We understand from the City of Tukwila that the existing storm sewer systems available to service the proposed Hillcrest Development are operating at capacity. Therefore, as a part of the development it will be necessary to provide sufficient drainage retention to prevent an increase in peak flows to the storm drains. As proposed by the developer, the permanent drainage of the site would involve collection of both the surface runoff and the shallow groundwater. Surface runoff from the property flows eastward to Slade Way via an 18 -inch culvert on the northern end of the site and along a few minor stream paths further south. This runoff, along with shallow seepage from the surficial soils, enters an unlined ditch along the west side of Slade Way. This ditch drains northward to the storm sewer at the intersection of South 160th Street and Slade Way. This runoff is fed by precipitation directly on the property and, more significantly, by a series of springs at the base of the steep slope that divides an eastern lowland and western upland. Perched groundwater in the lowland area of the property is very shallow. Over much of the area, the water table intersects the ground surface, resulting in shallow ponds and bogs. In this area the shallow groundwater flows eastward under Slade Way where the groundwater is partially drained by horizontal drains that extend below the road and partially surfaces as springs further down the hillside. A large percentage of this perched water is probably intercepted by the WSDOT surface and subsurface drainage system on the hillside below Slade Way. Runoff and spring flow data are not available from the site to use in developing reliable, quanti- fiable estimates of the amount of runoff under either existing or developed conditions. However, it is clear that development of the site will lead to an increase in surface and subsurface water flows into the existing storm /sanitary sewer system and roadway culverts. Therefore, construc- tion of the proposed project will require construction of a runoff detention system to reduce peak storm flows. The developer will need to perform additional subsurface hydrogeologic and geotechnical studies to determine the volume of flow that a detention system will have to accommodate. 6.0 CONCLUSIONS ► Slade Way and the proposed Hillcrest Development are situated within and near the head of an ancient landslide, and immediately upslope from the head of a deep - seated landslide that failed in 1960. Based on our reconnaissance of the area and review of existing data, it is our opinion that there is a potential for both deep- seated and 14 W- 6367 -01 IS a SHANNON &WILSON. INC. surficial landsliding that could involve Slade Way and the proposed Hillcrest Develop- ment. ► . The, deep - seated slope stability of the hillside below Slade Way is believed to be decreasing because of deterioration of an extensive drainage network that was installed in the 1960's to relieve artesian pressure within the hillside. As of 1989, the factor of safety for a deep - seated landslide downslope from Slade Way apparently was unacceptably low for a proposed residential development in that area. ► Additional site - specific geotechnical evaluations, including slope stability analyses, are necessary to evaluate the potential for deep - seated landsliding that could involve the proposed Hillcrest Development. Both static and dynamic stability analyses for existing and post- development site conditions are needed. These analyses will require additional geotechnical and hydrologic data from on -site monitoring wells installed in the deep confined aquifer. ► If slope stability analyses indicate an unacceptably low factor of safety for the proposed Hillcrest Development, remedial measures might include rehabilitation and maintenance of the WSDOT drainage system, or installation of a new dewatering system to achieve drawdown of the confined artesian aquifer. In either case, such remediation may not be feasible for the proposed Hillcrest Development. ► The stability of the shallow soils on the hillside is controlled largely by the level of perched water in the surficial soils and by the angle of the slope. It is likely that groundwater below Slade Way has risen over the past 30 years as a result of clogging of horizontal drains installed below the roadway. Such a rise in the water level would reduce the stability of this section of Slade Way and adjoining properties. ► Site - specific slope stability analyses are also needed to assess the static and dynamic stability of the site and the adjoining section of Slade Way with respect to shallow landsliding. Stability analyses should be performed for both existing conditions and for the proposed development conditions, including cuts, fills, and drainage measures. Shallow piezometer installations will be needed along Slade Way to provide water level information during the rainy season for use in the shallow slope stability analyses. ► If the slope stability analyses for shallow landsliding indicate a risk of slope failure for Slade Way that is unacceptable to the City, a variety of methods are available to improve .drainage conditions. and lower the risk of a slope failure. These methods include: a) rehabilitating the existing horizontal drains or installing new horizontal drains below the road; b) installing a deep French drain along the west side of Slade Way; c) paving the ditch along the west side of Slade Way; and d) installing a French drain at the toe of the slope as has been proposed by Mr Lowe for the Hillcrest Development. Assuming one or more of these measures are taken, their impact on 15 W- 6367 -01 SHANNO slope stability should be evaluated by water level monitoring installed along the roadway. ► Slade Way. has recently. experienced minor slippage or settlemen the road turns eastward. This movement, which occurred in sliding, can be expected to continue if corrective action is not tak reconstruction of the fill material underlying the roadway would be this area. If remediation is desired, explorations are recommen the cause of sliding and to develop an appropriate remedial sche area does not appear to be related to groundwater or surface water property. &WILSON. INC. n the piezometers in the area where area of previous n. At a minimum, equired to stabilize ed to better assess e. Sliding in this n the LeRoy Lowe ► In our opinion, the proposed Hillcrest Development, as show on the site plans submitted for the Boundary Line Adjustment (received by City of kwila on Septem- ber 9, 1991), is not feasible from a geotechnical perspective unless the wetlands are drained and a subsurface drainage system is installed to substanti ly lower the water table on the site. ► Alternative approaches to site development, involving relocation number of residences, appear feasible without significant impact stabilization, of Slade Way adjacent to the wetlands area is warran this could be accomplished by off -site drainage enhancement whi impact on the wetlands. • Development of the site will result in an increase in peak discharg runoff and from the groundwater collection system. A runoff d be required within the drainage system to accommodate the of this system will require shallow geotechnical and groundwat determine the detention capacity that will be required to handle t the subsurface drains and surface runoff from the development. 7.0 LIMITATIONS This report has been prepared for the City of Tukwila in accordance with professional engineering principles and 'practices for use by the City of the proposed LeRoy Lowe Development, also known as the Hillcrest Deve warranty, either expressed or implied, is made as to the nature of the conc 16 d reduction of the o the wetlands. If ed, it is likely that h would minimize from both surface tension system will flows. The design ✓ investigations to e added flow from generally accepted kwila in review of opment. No other usions and recom W- 6367 -01 l lDO SHANNON &WILSON. INC. mendations. The content of this report is not intended for the use of other parties or for other purposes. It may or may not contain sufficient information for other uses. SHANNON & WILSON, INC. Daniel N. Clayton, RG, CEG Senior Associate DNC:WPG /dnc W6367 -01.RPT/W6367- lkd /eet 17 W- 6367 -01 HP I irTdro,n, 77j • , '.N •/ r\ t (Rii \ • i 1 \.� \ • �''✓ \ \ i tat Drive,�� ;� .•'�•/ — ti • • r\ : • O e► 11 1. 4."V may . -' \ 1 *, ' 4-4 ��\ �/ , -HILLCREST -'•a`� ��� \DEVELOPMENT '\`�` 'SITES 0 • '`‘'' Approximate Extent `Ancient of Slide 4 -20 -61 Landslide ' ♦ ..�� -•'... " \�_ Scarp • :•••••• •r • • • , • Drains e' • 1960 -1968 SLOPE IMPROVEMENTS o Original Test Drains 0---- Recommended Drain (Grade: 1 -3 %) =, Cylinder Pile Wall Existing or Proposed .Right of Way - . Additional Vertical Drains (6 -9 -in. dia.) t• Large Diameter (5') Deep Well -1:1-3- Final Excavated Slope (horizontal:vertical) • Piezometer Installed During 1966 Investigation F• Slope Indicator Profile Location 0 190 200 400 Scale in Feet NOTE Modified from Shannon & Wilson, Inc., 1966. Geotechnical Review LeRoy Lowe Development Tukwila, Washington 1960 LANDSLIDE LOCATION AND REMEDIATION October 1992 W- 6367 -01 SHANNON & WILSON, INC. Geot.dnical and Environmental Consultants FIG. 1 o Sz P U) 0 0 11 L) N R G) g z m W pr Dm DO vo_ m r W * 0 E T rn 1 m O (1 0 8 8 Elevation in Feet Elevation in Feel l 1 : z i Iz� r , • 'v W_ 1 z LIUr I 1 ( g Y F. Elevation in Feet SHANNON &WILSON, INC. APPENDIX A SEQUENTIAL LIST OF DOCUMENTS REVIEWED W- 6367 -01 SHANNON iWILSON, INC. APPENDIX A SEQUENTIAL LIST OF DOCUMENTS REVIEWED 1960 Landslide Investigations Documents Dames & Moore, September 29, 1960, Letter to John Graham & Company, re: Borrow Area Slide, South 162nd Street near 54th Avenue South, Tukwila, Washington, 2 p. Dames & Moore, October 14, 1960, Letter to John Graham & Company, re: Borrow Area Slide, South 162nd Street near 54th Avenue South, Tukwila, Washington, 2 p. Dames & Moore, October 19, 1960, Letter to John Graham & Company, re: Borrow Area Slide, South 162nd Street near 54th Avenue South, Tukwila, Washington, 3 p. and map. Dames & Moore, November 1, 1960, Letter to John Graham & Company, re: Borrow Area Slide, South 162nd Street near 54th Avenue South, Tukwila, Washington , 2 p. Dames & Moore, December 1, 1960, "Report of Stability Investigation, Borrow Area Slide, 54th Avenue South and South 162nd Street, Tukwila, Washington," report to John Graham & Company for Puget Western, Inc, 10 p. Dames & Moore, May 19, 1961, Letter to Puget Western, Inc., re. Earthslide South 162nd Street near 53rd Avenue South, 1 p. Dames & Moore, June 27, 1961, "Report of Soils Investigation, Earth Slide, South 162nd Street near 53rd Avenue South, Tukwila, Washington," report and drawings and 2 appendices. Dames & Moore, August 8, 1961, Letter to Puget Western, Inc., re: Slide'Control Measures, Earth Slide, South 162nd Street near 53rd Avenue South, Tukwila, Washington, 2 p. Dames & Moore, September 14, 1961, Letter to John Graham & Company re: Slide Area Grading and Drainage , 2 p. Dames & Moore, September 18, 1961, Letter to Puget Western, Inc., re: technical comments, 1 P. Dames & Moore, September 30, 1961, Letter to Puget Western, Inc., re: technical comments, 2 p. Dames & Moore, October 6, 1961, Letter to Puget Western, Inc., re: technical comments, 1 p. (appears to include 2 poor reproductions of maps). A -1 W- 6367 -01 t 1.A SHANNON FiWILSON. INC. Shannon & Wilson, Inc., June 12, 1964, "Report on Foundation Investigation, Existing Slide Area, Tukwila Interchange, PSH 1 (SR5)," for Washington State Highway Commission, Department of Highways, District No. 1, 14 p. Shannon & Wilson, Inc., July 14, 1964, "Report on Foundation Investigation, North of Existing Slide Area, Tukwila Interchange, PSH 1 (SR5), Supplement No. 2" for Washington State Highway Commission Department of Highways District No. 1, 9 p. and figures. Shannon & Wilson, Inc., March 1, 1965, "Report on Foundation Conditions, Certain Parcels of Land in Tukwila," for Washington State Highway Department District No. 1, 7 p. and figures. Dames & Moore, March 10, 1965, Letter to Puget Western, Inc., re: comments on borrow area stability, 1 p. Shannon & Wilson, Inc., April 30, 1966, "Slope Stability Investigation, Tukwila Interchange, PSH 1 (SR5)," for Washington State Highway Commission, Department of Highways, District No. 1, 116 p. Shannon & Wilson, Inc., June 21, 1968, "Summary Report, Soil Conditions and Earth Move- ments, Vicinity of the Tukwila Interchange," prepared for Washington State Highway Commission, District No. 1, 8 p. Shannon & Wilson, Inc., February 6, 1973, Letter report to Don Koll Company, Inc., re: Slade Way Property, Tukwila, Washington, 3 p. and map. Proposed Valley Vue Development Documents GeoEngineers, Inc., May 3, 1982, "Report, Geotechnical Design Consultation, Proposed Condominium Development, Slade Way and 53rd Avenue South, Tukwila, Washington ", report to The Mithun Associates, 9 p. GeoEngineers, Inc. Undated 1983 ?, Geotechnical and Hydrological Studies, Proposed Valley View Estates, Slade Way and 53rd Avenue South, Tukwila, Washington," Report for Dr. H. M. Allenbach, 17 p. and 2 appendices (Report included as an Appendix C to Tukwila Department of Community Development, 1984 ?). Tukwila Department of Community Development, undated 1984 ?, Valley View Estates Working Draft Environmental Impact Statement, Part III. "Existing Conditions, Impacts and Mitigating Measures," pp. 24 -31, Including Appendix C (see GeoEngineers, Inc. undated 1983 ?), report prepared by R. W. Thorp and Associates, Inc. A -2 W- 6367 -01 1 6,4 SHANNON &WILSON, INC. Dames & Moore, January 24, 1985, "Review of Draft Environmental Impact Statement, Proposed Valley View Estates" draft report of engineering consultation for the City of Tukwila, 10 p. Dames & Moore, March 10, 1985, Letter to Puget Western, Inc., re: technical comments,1 p. "Engineers, Inc. April 25, 1985, Geotechnical and Hydrological Studies, Proposed Valley View Estates, Slade Way and 53rd Avenue South, Tukwila, Washington," Report to Stepan & Associates, Inc. (and Puget Western), 32 p. and 3 appendices (Report included as an Appendix A to Tukwila Department of Community Development, January 1986). Stepan & Associates, Inc. May 1, 1985, "Valley View Estates Utility Report," in Tukwila Department of Community Development, January 1986, Appendix B, 4 p. Tukwila Department of Community Development, January 1986, Valley View Estates Final Environmental Impact Statement, Part III. "Existing Conditions, Impacts and Mitigating Measures," pp. 50 -63, Including Appendix A (see GeoEngineers, Inc., April 25,1985) and Appendix B (see Stepan & Associates, May 1, 1985). Haggard, Joel E. (Attorney), January 27, 1989, Letter to Washington State Department of Transportation transmitting "Instrumentation Plan" and "Monitoring Action Plans" for the Valley View Estates Development, 2 p. and attachments prepared by GeoEngineers, Inc. Washington State Department of Transportation, April 19, 1989, Letter to City of Tukwila re: Valley View Estates, including attachment of intradepartmental letter dated April 19, 1989, reviewing GeoEngineers, Inc., geotechnical analysis of the proposed Valley View Estates, 5 p. and drawings. Attorney General of Washington, February 7, 1989, Letter to Joel E. Haggard re: Valley View Estates Instrumentation Plan and Monitoring Action Plan, 2 p. Dames & Moore, March 6, 1989, "Review of Plans, Proposed Valley View Estates," report to City of Tukwila, 8 p. Tukwila Department of Community Development, March 10, 1989, Letter to Puget Western, Inc., re: Valley View Estates Development, Request for Resubmittal of Plans per Initial Geotechnical /Hydrological Report from Dames & Moore, 6 p. Washington State Department of Transportation, March 23, 1989, Letter to GeoEngineers, Inc., re:. Valley.. View Estates,. review. of stability analysis, 3 p. and drawings. Tukwila Department of Community Development, April 6, 1989, Letter to Puget Western, Inc., re: Valley View Estates building permit application, 6 p. A -3 W- 6367 -01. .1105 SHANNON FIWILSON. INC. GeoEngineers, Inc., April 10, 1989, Letter to Washington State Department of Transportation re: Valley View Estates, Tukwila, Washington, slope stability analysis review, 4 p. Dames & Moore, April 20, 1989, "Valley View Estates Geotechnical/ Hydrological Review," letter report to City .of. Tukwila, 8 p. Proposed Hillcrest Development Documents Cascade Geotechnical Inc., May 30, 1990, "Preliminary Geotechnical Investigation for Hillcrest, Slade Way, South of South 160th Street, Tukwila, Washington," prepared for LeRoy C. Lowe, 10 p. and Appendices A, B, and C. Cascade Geotechnical Inc., August 27, 1990, untitled - additional subsurface site investigations for Hillcrest, Slade Way, South of South 160th Street, Tukwila, Washington," prepared for LeRoy C. Lowe, 10 p. and Appendices A, B, C and D. Cascade Geotechnical Inc., April 24, 1991, Letter to LeRoy Lowe, re: Plan Review, Hillcrest, Slade Way, Tukwila, Washington, 3 p. LeRoy C. Lowe, A.I.A. Architect, September 3, 1991, Application for Planned Residential Development for Hillcrest. Bredberg and Associates, Inc., January 8, 1992, Letter to LeRoy Lowe, A.I.A. Architect, re: Hillcrest Boundary Line Adjustment, 2 p. Joule, Dennis, P.E., January 15, 1992, Letter to LeRoy Lowe, A.I.A., re: Hillcrest Dewatering /Stability of Slade Way, 2 p. Resco Mechanical & Civil Engineering, February 17, 1992, Letter re: Hillcrest Boundary Line Adjustment, Dewatering and Stability, 1 p. LeRoy C. Lowe, A.I.A. Architect, February 18, 1992, Letter to Tukwila Planning Commission, re: Hillcrest, 1 p. • aRoy C. Lowe, A.I.A. Architect, February 18, 1992, Letter to Tukwila Planning Department re: Hillcrest, 2 p. Bredberg and Associates, Inc., April 16, 1992, "Wetlands Study, City of Tukwila, Hillcrest," report prepared. for LeRoy Lowe, A. I. A, Architect; 17- p. Bredberg and Associates, Inc., May 15, 1992, Letter to LeRoy Lowe, A.I.A., re: Slade Way and S 160th St., Tukwila, 1 p. and attached map. • A -4 - W- 6367 -01 SHANNON &WILSON, INC. Cascade Geotechnical Inc., June 27, 1992, Letter to LeRoy Lowe, A.I.A., re: Supplemental Letter, Hillcrest, Tukwila, Washington, 3 p. LeRoy C. Lowe, A.I.A. Architect, June 29, 1992, Letter to Tukwila Department of Community Development, re: Hillcrest B.L.A.,.2 p. Tukwila Department of Community Development, July 17, 1992, Letter to LeRoy Lowe A. I. A Architect, re: Hillcrest Boundary Line Adjustment/Administrative Planned Residential Development, 5 p. LeRoy C. Lowe, A.I.A. Architect, July 18, 1992, Letter to Tukwila Planning Commission, re: Hillcrest Boundary Line Adjustment, Reasonable Use Exception, 2 p. with attachment. LeRoy C. Lowe, A.I.A. Architect, July 27, 1992, Letter to Tukwila Planning Commission, re: Hillcrest Boundary Line Adjustment, 5 p. Tukwila Department of Community Development, August 19, 1992, Letter to LeRoy Lowe, re: Hillcrest Boundary Line Adjustment/Administrative Planned Residential Development, 1 p. Tukwila Department of Community Development, undated, "Wetland and Watercourse Special Studies Report Criteria," 2 p. Tukwila Department of Community Development, undated, "Areas of Potential Geologic Instability Development and Report Criteria," 3 p. 11- 23- 92/APPENDDC.A/W6367- Ikd /eet LLD? A -5 W- 6367 -01 SHANNON bWILSON. INC. APPENDIX B IMPORTANT INFORMATION ABOUT YOUR SUBSURFACE WASTE MANAGEMENT (REMEDIATION) REPORT W- 6367 -01 41:e �'l J SHANNON & WILSON, INC. ® Geotechnical and Environmental Consultants w— bib / —U1 Attachment to Letter Report Page 1 of 2 Dated: November 23, 1992 To: City of Tukwila Attn: Mr. Phil Fraser Important Information About Your Geotechnical Engineering/ Subsurface. Waste Management (Remediation) Report AN ENGINEERING REPORT IS BASED ON PROJECT - SPECIFIC FACTORS. A geotechnical engineering /subsurface waste management (remediation) report is based on a subsurface exploration plan designed to incorporate a unique set of project - specific factors. These typically include: the general nature of the structure and property involved, its size and configuration; historical use and practice; the location of the structure on the site and its orientation; physical concomitants such as access roads, parking lots, and underground utilities;and the level of additional risk which the client assumed by virtue of limitations imposed upon the exploratory program. To help avoid costly problems, have the consulting engineer/ scientists determine how any factors (which change subsequent to the date of the report) may affect the recommendations. Unless your consulting geotechnical /civil engineer and /or scientist indicates otherwise, your report should not be used: ❑ when the nature of the proposed project is changed; for example, if an office building will be erected instead of a parking garage, or if a refrigerated warehouse will be built instead of an unrefrigerated one; or chemicals are discovered on or near the site; ❑ when the size or configuration of the proposed'project is altered; ❑ when the location or orientation of the proposed project is modified; ❑ when there is a change of ownership; or ❑ for application to an adjacent site. Geotechnical /civil engineers and /or scientists cannot accept responsibility for problems which may develop if they are not consulted after factors considered in their reports have changed. MOST GEOTECHNICAL AND SUBSURFACE WASTE MANAGEMENT "FINDINGS" ARE PROFESSIONAL ESTIMATES. Site exploration identifies subsurface conditions only at those points where samples are taken and when they are taken, but the physical means of obtaining subsurface data precludes the determination of precise conditions. Consequently, the information obtained is intended to be sufficiently accurate for design, but is subject to interpretation. Additionally, data derived through sampling and subsequent laboratory testing are extrapolated by the geotechnical /civil engineer and/or scientist who then renders an opinion about overall subsurface conditions, their likely reaction to proposed construction activity, and/or appropriate design. Even under optimal circumstances actual conditions may differ from those opined to exist, because no geotechnical /civil engineer and /or scientist, no matter how qualified, and no subsurface exploration program, no matter how comprehensive, can reveal what is hidden by earth, rock and time. For example, the actual interface between materials and /or chemicals may be far more gradual or abrupt than the report indicates, and actual conditions in areas not sampled may differ from predictions. Nothing can be done to prevent the unanticipated, but steps can be taken to help minimize their impact. For this reason, most experienced owners retain their geotechnical /waste management consultant through the construction stage to identify variances, conduct additional tests which may be needed, and to . recommend solutions to problems encountered on site. Prudent owners establish contingencies to account for such variations in subsurface conditions as exposed during construction. SUBSURFACE CONDITIONS CAN CHANGE. Subsurface conditions maybe affected as a result of natural changes orhumaninfluence. Because a geotechnical/ waste management engineering report is based on conditions which existed at the time of subsurface exploration, construction decisions should not be Page 2 of 2 based on an engineering report whose adequacy may have been affected by time. Speak with the geotechnical /wastemanagement consultant to learn if additional tests are advisable before construction starts. For example, groundwater conditions commonly vary seasonally. Construction operations at or adjacent to the site and natural events such as floods, earthquakes or groundwater fluctuations may also affect subsurface conditions and, thus, the continuing adequacy of a geotechnical /waste management report. The geotechnical/ civil engineer and /or scientist should be kept apprised of any such events, and should be consulted to determine if additional tests are necessary. THE GEOTECHNICAL ENGINEERING /SUBSURFACE WASTE MANAGEMENT (REMEDIATION) REPORT IS SUBJECT TO MISINTERT RETATION. Costly problems can occur when other design professionals develop their plans based on misinterpretation of a geotechnical engineering /subsurface management (remediation) report. To help avoid these problems, the geotechnical /civil engineer and /or scientist should be retained to work with other appropriate design professionals to explain relevant geotechnical, geological, hydrogeological and waste management findings and to review the adequacy of their plans and specifications relative to these issues. BORING LOGS SHOULD NOT BE SEPARATED FROM THE ENGINEERING/WASTE MANAGEMENT RE- PORT. Final boring logs are developed by the geotechnical /civil engineer and /or scientist based upon interpretation of field logs (assembled by site personnel) and laboratory evaluation of field samples. Only final boring logs customarily are included in geotechnical engineering /waste management reports. These logs should not, under any circumstances, be redrawn for inclusion in architectural or other design drawings, because drafters may commit errors or omissions in the transfer process. Although photographic reproduction eliminates this problem, it does nothing to minimize the possibility of contractors misinterpreting the logs during bid preparation. When this occurs, delays, disputes and unanticipated costs are the all- too-frequent result. To minimize t.ie likelihood of boring log misinterpretation, contractors should be given ready access to the complete geotechnical engineering /waste management report. Those who do not provide such access may proceed under the mistaken impression that simply disclaiming responsibility for the accuracy of subsurface information always insulates them from attendant liability. Providing the best available information to contractors helps prevent costly construction problems and the adversarial attitudes which aggravate them to a disproportionate scale. READ RESPONSIBILITY CLAUSES CLOSELY. Because geotechnical engineering /subsurface waste mangement (remediation) is based extensively on judgment and opinion, it is far less exact than other design disciplines. This situation has resulted in wholly unwarranted claims being lodged against geotechnical /waste management consultants. To help prevent this problem, geotechnical /civil engineers and /or scientists have developed model clauses for use in written transmittals. These are not exculpatory clauses designed to foist the engineer's or scientist's liabilities onto someone else. Rather, they are definitive clauses which identify where the engineer's or scientist's responsibilities begin and end. Their use helps all parties involved recognize their individual responsibilities and take appropriate action. Some of these definitive clauses are likely to appear in your report, and you are encouraged to read them closely. Your engineer /scientist will be pleased to give full and frank answers to your questions. OTHER STEPS YOU CAN TAKE TO REDUCE RISK. Your consulting engineer /scientist will be pleased to discuss other techniques which can be employed to mitigate risk and to provide a variety of materials which may be beneficial. ContaL your engineer /scientist for further information. The preceding paragraphs are based on information provided by the Association of Soil and Foundation Engineers, Silver Spring, Maryland 1/92 170 ••.. • 'l'lkCf of'• ....� �. • . • _.•S_... _• • 1 /It • • 4. / I •l•• • 411 . • r t -. Ai• € 030. t• • • • i !C: Li.:t t..! 3 • • • i. • •Mr 4. Kaie • 1~ 4 .+...• *I*41/41J ! • r jt1�'�i • Ji - at • 1'6%1'1 • 11. •ti t. - _ -. •.wry r. .nwti.. • A 2 1 3 °r, Av EN O E WEST • SEATTLE 99. WAS r.' N OTO N • ASSOCIATE. JOSEPH LAMONT. JR. Puget Western, Inc. woo Washington Building Seattle, Washington kttentiona BO. Wells McCurdy Gentlemen r .35 •N3E.C5 S••. rq•NCI5C3 PDOTL•ND 5E• "_C 5•, .A•C Cif- NEw •Cq4 A T W A T E R 4-8160 September 30, 1961 We have recently reviewed the possibility of excavating additional soil whish had been previously left in place on the former alignment of South 162nd Street adjount to the slide area on yew property at Tukwila. la our letter to you at Miy 23, 1961, we suggested that this material remain until tbs slide movement had stepped, or until the slide material had been removed. made in the During area. This has consisted off (1)sexearat� material fres the upper portion of the slide sale, and (2) installing a series of bsrisontal drains at several levels in the hillside. She installation of these drains is still in progress, and furtbar .zsavatiea mar yet be made within the slide area this fall, depending on weather co diticss. We are advised by Morrison - Knudsen Company that no slide movement of this area has been Observed srbs.qu.nt to the installation of Slade Way. In vier of the above fasters, it is our opinion that the nnexcavat.d portion of 162nd Street cast of 54th Avenue South may new be removed. The ant fans should be established at a maximum slaps oft to 1, and the excava- tion should be graded into the previously r000mmeaded system of slopes and terrines being developed is the slide area. t marked espy of Jahn Graham Aram 31-10 aa.emponies this letter, acid outlines in red the limits of the excavation aver rs.e■msalded la this area. is a protective measure in drain- ing water which sight be trapped is the hillside on the flank of the slide area, ve also r.oaemend the Installation of one to three additional herisental drains at the approximate l.satisos shorn en Drraring 31 -10. We would plan to review the need for the s..ssd and third drains after the first one is installed. John Or'ahamrs officio has indicated on Drawing 3I -10 the ultimate grades to which the area below the slide should be azsavat.d. It is our reesememdati.n that the axsavatian not preo..d to those limits until all of CABLE ADDRESS!DAMEMJRE September 20,�1061� Pap -4 the wt s1 sy soil have bees removed bees the slide area. Fir the present, Me recommend that the exesystisn not proceed beyond the limits shown by the rid lines on Drams SL-10 which have been extended into this area. /ours very truly, fit Attachment ec $ Jobe Graeae, « Compaq 1426 Fifth Li *s Seattle 1, Washington Attentive* Mr. A. P. Sureenski es, ' 6211 Seats 1S4t�e Street Ls. lestsn, Washiadten Attentions lie. Sill Miller MISS k MOM LOS ANGC.CS SAN r4•NCISCO ^ DAMES Pi M O O R E SEATLANO SEATTLE SALT AE CITY SOIL MECHANICS ENGINEERS c+•.cAGO NEw rOaA A -LANTA • Puget Western, Inc. 1100 Washington Building Seattle, Washington Attention: Mr. F. W. Kimball Gentlemen: Late last week we submitted to John Graham & Company a letter and a marked reproducible print of the area below Slade Way in the vicinity of South 162nd Street on your property at Tukwila, Washington. The information presented in the letter and on the print outline a program whisk includes the excavation of soil and the installation of a series of horizontal drains below Slade Way. The excavation consists of a series of 2 to 1 outs separated by benches 12 feet in width with the vertical height of each cat not greater than 20 feet. The installation of the horizontal drains is to proceed con- currently with the excavation of the slope face in this area. It is our opinion that the installation of the drains will provide the necessary pro- tection to preserve the stability of Slade Way, and that the program of excavation and drainage which is outlined is the best course of action to follow at this time. JL :fm 3 copies submitted nos Puget Weiiern, lac. 1400 Washington Building Seattle, Washington Attention: Mr. Wells MsCurdy oat John graham & Compaq, 1426 Fifth Avenue Seattle 1, Washington Attention: Mr. A. P. Bnrsenaki CABLE ADDRESS +DAMEMORE Sours very truly, DAMES & MOORE By Joseph Lamont, r. DAMES 8 MOORS .._ t. ;er John Graham & Company, Architects and engineers 32426 Fifth Avenue Seattle 1, . ;ashington Attention: 14r. A. Burzens:•:i Gentlemen: Slide Area Grading and ::rainage Tukwila, Washlrton We have recently forwarded to you under separate cover a marked reproducible print showing the next step in the proposed regrading of the slide area at South 162nd Street near 53rd Avenue South in Tukwila. It is our understanding that you will prepare prints of this reproducible for submission to the various interested parties and will also send us a copy for our files. The suggested regrading involves a series of cut slopes and benches accomplished concurrently with the instailatici: of a series of horizontal drains. Evaluations of stability indicate that . a maximum slope of 2 to 1 will be stable in limited heights provided that satisfactory drainage is obtained. We have therefore developed a grading plan us__.: this maximum siop•e with a vertical distance between benches of 20 feet. ..,P have recommended benches 12 feet in width, anticipating that these benches wculd ee 7ermanent. : ;a under- stand from discussions with Bill miller of : :orrisor.- iinudsen Company that a substantially wider bench area will be required 'or the ecui :vient which they have obtained to install the horizontal drains. The wider temporary benches may be cut back to the 12 -foot width after installation of the drains has been accomplished. We have shown benches at - 1ev=:ticne 221, 201, 1E1, and 161. Below Elevation 161 the cut may be sloped at 2 to 1 to :,ha layer of firm sandy loam which underlies the permeable and wet stratum of sand. The upper limit of the earthwork has been indicated on the drawi:_2; at : feet from Slade 'gay. We understand that some excavation already started in this ::r= a has extended closer to Slade Way. e do not feel this is parti rul xr "1-y critical, provided the upper bench is installed at Elevation 221 at the location shown on the drawing. The result will simply be a flattening of the slope in this area. In order to relieve pressures within the slide area which would contribute to the tendency for further movement, we recommend that the excava- tion begin below Slade Way and work in a downhill direction. The drains should be installed as the excavation proceeds in order to prevent the exposure of substantial undrained areas. DAMES 8 MOORE SOIL MECHANICS ENGINEERS LOS ♦NDELES S•N ,w•NCISCO fOO•LAND SE•, -LE S•IT ••■E 421 3.•• AVENUE WEST • SEATTLE 99. WA S, I N GTO N ATWAT E4 4. 8.00 ASSOCIATE. JOSEPH LAMONT. JR. John Graham & Company, Arehitects and Engineers 1426 Fifth Avenue Seattle 1, Washington Attention* Mr. A. P. Buraenski Gentlemen* September 14, 1961 Slide Area Grading and Drainage Tukwila, Washington We have reesatly forwarded to you under separate Dover a marked reproducible print showing the next step in the proposed regrading elf the slide area at South 162nd Street near 53rd Avenue South in Tukwila. It is war understanding that you will prepare prints of this reproducible for submission to the various interested parties and will also send us a oepy for our files. The suggested regrading involves a series of out slopes and benches accomplished concurrently with the installation of a series of horizontal drains. Evaluations of stability indicate that a maximum slope of 2 to 1 will be stable in limited heights provided that satisfactory drainage is obtained. We have therefore developed a grading plan using this maxim= slope with a vertical distance between benches of 20 feet. We have recommended benches 12 feet in width, anticipating that these benches would be permanent. We under- stand from discussions with Hill Miller of Morrison - Knudsen Company that a substantially wider bench area will be required for the equipment which they have obtained to install the horizontal drains. The wider temporary benches aey ii out bank to the 12-feet width after installation of the drains has been accomplished. We have shown benches at Elevations 221, 201, 181, and 161. Below Elevation 161 the out maybe sloped at 2 to 1 to the layer of firm sandy low whisk underlies the permeable and wet stratum of sand. The upper limit elf this earthwork has been indicated on the drawing at 50 feet from Slade Way. We understand that some excavation already started in this area has extended closer to Slade Way. We do not feel this is particularly critical, provided the upper bench is installed at Elevation 221 at the location shown an the drag ng. The result will simply be a flattening of the slope in this area. In order to relieve pressures within the slide area which would contribute to the tendency for further movement, we recommend that the excava- tion begin below Slade Way and work in a downhill direction. The drains should be installed as the excavation prooesds in order to prevent the exposure of substantial =drained areas. CABLE A:OAESS■OAMEMORE '�`"a =sir Page -t- asrisemtal drains should be installed above the banshee at Elevations 221, 181, and 161. The horizontal drains at Elevation 221 aher<ld penetrate beneath the roadway to pick up water draining through the upper sand stratum whieh occurs in this area. This water will largely be infiltration from sur- face runoff or from springs farther uphill. The horisontal drains above the benches at Elevations 181 and 161 should penetrate sufficiently far into the hillside to intersect the underlying water bearing sand layer in which the artesian pressure was observed in the borings pr.vietuly drilled in this area. The length of the drains will vary from point t• point within the slide area. The drains should also be located to tap observed areas of significant seepage. In general, we anticipate that they would be on the ardor of 50 feet apart, but the astual spacing should be a matter of field deeisisms as the work progresses. The drains should, of course, be on a suffioiont gradient so that the water will be fr,s to pass from the sand stratum through the drain lines to the terraces. Each bench er terrace should be sloped so that the drainage can be carried laterally away from the slide area. In addition to the drains which would be established in the embankments from the constructed terrains, w else reed m end that additional drains be installed an needed in unemsaveted areas in the hillside eel the west side ef the slide area underneath Slab Wq in order to intercept any signifisant areas of seepage. L a means of dstestion of any passible Were mev meemt, we reese meard that a systole ef refsrrnes hUbs be installed ea the uphill side of Slade Way. Prevision should be made for periodic measurements of the leeatssns and slew- time of each ef these hobs. The measurements should be rsfsreneed to a fined point sufflciently removed from the slide area so that it world net be Wasted by any further movement in this so=re. JL -MSiI r th 3 espies srbmittsd se s Puget Western, ins. Washington Building Seattle, Washington Attentions Mr. 1. W. Kimball ens Pegst Western, Inc. Waskiagtsa Iailding Seattle, Washington Attentions Mr. Wells M.Curd, so s Merrise.m- Lnwdson Co■pany 6211 South 154th Street ]teantea, Washington Attentions Mr. Sill Miller Yours vary truly, LAMS & MOCKS BT /Joseph Lamont Jr. •-LANtw mONOLUL'J DAMES e MOORE LOUSTCN LC_ .NGE_ES CONSULTANTS IN APPLIED EARTH SCIENCES NC. •oar SOIL ME:, HANICS SNG.NE =RING 3EOLOG+ 3Eo aHr Sl:S Sw Lt E _ ■- 42I THIRD AVENUE WEST • SEATTLE 99. WASHINGTON • A TWAT ER 4 -8160 ASSOCIATE: JOSEPH LAMONT. JR. Puget Western, Inc. Washington Building Seattle 1, Washington Attention: Mr. F. W. Kimball Gentlemen: August 8, 1961 Slide Control Measures Earth Slide South 162nd Street near 53rd Avenue South Tukwila, Washington Pursuant to your meeting with Mr. J. K. Tuttle of our office on August 1, 1961, we submit herewith our comments concerning supplemental measures to be taken in the stabilization of the earth slide in the vicinity of South 162nd Street and 53rd Avenue South in Tukwila, Washington. Recently an excavation has been made at the toe of the hill across the former align- ment of 57th Avenue South to sufficient depth to permit drainage of sub- surface water from the water - bearing sand layer which underlies the slide area. Considerable water has been draining from this excavation, and water level observations in Boring 5, which is located a short distance upslope from the excavation, has indicated that the ground water level has dropped approximately 13 feet as the result of this drainage. However, in general, the ground water level in the major portion of the slide area has not shown appreciable change to date. In view of the relatively short time left in the summer construc- tion season, we believe that it is desirable to take further action to dewater the general slide area before the fall and winter rains begin. The most economical means of accomplishing this within the remainder of the summer season would be to excavate a portion of the material near the top of the slide area to a sufficient depth to intercept the water - bearing sand stratum. This would reduce the weight of material acting on the upper portion of the slide mass, and once the drainage pattern is established, would relieve the hydrostatic pressure beneath the lower portion of the slide mass. At the present time we understand that Morrison - Knudsen Company is clearing the vegetation, trees, and fallen debris out of the slide area in order to deter- mine what surface drainage should be installed, and to prepare the site for excavation. It is planned that all surface runoff which now has access to the slide will be diverted from the area. CABLE ADDRESS' DAMENORE Puget Western, Inc. August 8, 1961 Page -2- In order to maintain as much of the support of the higher ground to the southwest as possible, we believe that any excavation in the upper por- tion of the slide mass should be oriented in a northeast - southwest direction, and should be cut to as narrow a width as practical. The initial excavation should be confined to the area between Boring 4 and South 162nd Street. We believe that the cutting of such a slot is the most feasible means of drain- ing the main portion of the slide area in the time which remains before the wet season begins. Prior to the time the excavation is cut deep enough to intercept the water - bearing stratum, and to permit drainage of the overlying saturated soils, there may be some possibility that earth movements will be experienced along the sides of the newly cut slopes. We believe that, in general, this slippage would be minor, and that once the water pressure is relieved the possibility of any extensive slippage will be minimized. As a part of the completion of the roadway relocation work, we recommend that a series of horizontal drains extending back a distance of approximately 100 feet from the upper sides and end of the cut be installed in the water - bearing sand layer. If this is accomplished and the excessive hydrostatic head on the lower portion of the slide area is relieved, then consideration can be given to the further excavation of soils from within the slide area. During and following the completion of such an excavation due consideration must be given to the adequate diversion of water from the slide area, and water should not be allowed to pond in any area within the existing slide. We suggest that a series of survey hubs be established across portions of the slide mass adjacent to the excavation so that any movement of these areas can be detected promptly. Yours very truly, DAMES & MOORE By -72L JL- JKT:fm Joseph am nt, Jr.= 2 copies submitted cc: Puget Western, Inc. (2) Attention: Mr. Welts McCurdy cc: John Graham & Company (2) Attention: Mr. A. P. Burzenski c_• DAMES 8 MOORE CONSULTANTS IN APRLIED EARTH SCIENCES SOIL MECHANICS • ENGINEERING GEOLOGY • GEOPHYSICS ke Lf.5 ANGELES SAN FRANCISCO PORTLAND SEATTLE SALT LAKE CITY C•IICAGO NEW YORK ATLANTA 421 THIRD AVENUE WEST • SEATTLE 99, WASHINGTON • ATWATER 4•9150 ASSOCIATE' JOSEPH LAMONT. J R. June 6, 1961 John Graham & Company, Architects and Engineers 1426 Fifth Avenue Seattle 1, Washington Attention: Mr. A. P. Bursienski Gentleman: Enclosed herewith are two copies of the draft of our report of investigation of the earthslide in Tukwila, Washington. This draft is submitted for your review and comment before the report is presented is final form. Inasmuch as the loss of our borings end other pertinent information have been forwarded to you during the course of our work, we are not enclosing copies of the various plates which will be included in our final report. At such a time as all parties concerned have had an opportunity to review the report we would be pleased to meet with you to discuss any questions which may arise. JiT: f* Enclosures cc: Puget Western, Inc. Attentions Mr. F. W. Kimball cc: Paget Western, Inc. Attentions Mr. Wells McCurdy Tows very truly, MIMES & By Jack E. Tuttle 19/a/4( nt'el-tj Ch.; 14, Ylt-• CABLE AODRESSIOAMEMORE LOS ANOCLCS SAN ,AANC,SCO DAMES [ + a MOORE '° °'LAN° SttLC SALT LAAE CiT+ NEN •OAn •TL +r, t• 421 3 °0 AVENUE WEST • SEATTLE 99, WAS -•ING TON • ATWAr�TER 4.6160 ASSOCIATE. JOSEPH LAMONT. JR. May 23, 1961 SOIL MECHANICS ENGINEERS Puget Western, Inc. Washington Building Seattle, Washington Attention: Mr. Wells McCurdy Gentlemen: Excavation Program Andover Industrial Park Tukwila, Washington Pursuant to our recent discussions we submit herewith our comments concerning the excavation of borrow material downhill from the slide area which exists on the hillside to the west of the Andover Industrial Park. Preliminary logs of Borings 1 to 4, drilled in and above the slide area, have already been submitted. We enclose herewith the logs of Borings 5 through 8. Boring 5 was drilled near the toe of the slide and Borings 6, 7, and 8 were drilled farther downhill at the locations which were marked on your aerial photograph of this site. We have reviewed the proposed excavation program which has been prepared by John Graham & Company on their Drawing No. C -2. With due consideration to the slide area, we have made certain revisions in the area downhill from the slide location. Our recommen- dations for further excavation are indicated by the red contour lines on the attached drawing. The recommended excavation would include work in the area of the present alignment of 57th Avenue South and further work in the area south of South 162nd Street. As indicated to you, we consider it desirable to leave a portion of the 162nd Street alignment in place to act as a buttress until the slide movement has stopped or until the slide material has been removed. It is our opinion that the excavation procedures which are outlined will not cause the slide to extend beyond the approximate limit shown by the red line on the attachment to our letter to Puget Western, Inc. dated May 19, 1961. CABLE AOO, "SiS I DAMEMOAE LOS ANGELES SAN FRANCISCO DAMES 8 MOORE PORTLAND SEATTLE CONSULTANTS IN APPLIED EARTH SCIENCES SALT LAKE CITY SOIL MECHANICS • ENGINEEPING GEOLOGY • GEOPHYSICS CHICAGO NEW rORK ATLANTA continue farther up the hillside, affecting adjacent properties, ` -- ..it•.is not otherwise relieved. In doing so, we would also expect that it might widen sufficiently far to the west to include a portion L 0,..1.40A7,..k., ■ of 53rd Avenue South and the private property to the west. As pointed :�' =_�� out yesterday, the movement of such a slide is based on a number of -C.,./ tst,'c „_7 variable and complex factors, and it is therefore impossible to predict with accuracy the extent of such future movement. �.- The information presented on the map is based on our best judgment resulting from information gained in our current investigation, but at best should be construed only as highly approximate. JL:nn Attachment Yours very truly, DAMES & MOORE B `Joseph Lamont, CABLE ADDRESS, DAMEMORE DAMES e MOORE CONSULTANTS IN APPLIED EARTH SCIENCES SAIL ME :•LN :- • E11G11'EEPIN i Ci LO i'' 3_.:P.+vi1CS `.▪ 0.40.._1 . 5 -C v I )!1 +NOE.CS +:YH _Oww ▪ ....A` tC... 5..1,.. tE :17+ 5•.■ L'...1.7, 5 =0 421 THIRD AVENUE '+‘ EST • �cATTLE 99. WAS -'Nf_TCN • tWATER 4 -8160 >ssocl. ∎ -E L4MONT. J R John Graham & Company, Architects and Engineers 1426 Fifth Avenue Seattle 1, Washington Attention: Mr, A. P. Burzenski Gentlemen: May 10, 1961 Copy Ad Corr 041 M.o.o. � ,�i arcr. .0 0 c.t,_v � �ARCH. 11-j I 4y i 9[[ f.r . f ISTR1Ci _ .r' C:.A.H..M AND COMPANY Proposal Supplemental Explorations Earth Slide Investigation South 162nd Street near 53rd Avenue South Tukwila, Washington Pursuant to our recent discussions, we present herein our proposal for drilling one additional boring below the toe of the slide area in our current investigation of the earth slide at South 162nd Street near 53rd Avenue South in Tukwila, Washington. At your request, we have reviewed the charges to date for the investigation of the slide which you have already authorized. At this point it is impossible to tell if the maximum limitation will be reached; how- ever, the cost of clearing and grading the access road to the boring location on Mr. Robinsonls property has entailed a substantially greater expenditure than had been estimated. We therefore believe that it is doubtful that any additional exploratory work could be accomplished within the present authoriza- tion. Data obtained from our explorations indicate that strata of sandy soils containing water under artesian pressure underlie the general slide area. It appears that these strata may underlie the surface of the borrow area at rather shallow depth below the toe of the slide, and that a drainage system installed from the toe may be practical to assist in arresting the slide movement. The boring proposed would be extremely valuable in this regard. To explore the subsurface and ground water conditions near the toe of the slide, we propose to drill the boring to sufficient depth to determine the extent of the water - bearing granular soils. We estimate that the boring would be on the order of 40 feet in depth. The boring would be logged and representa- tive samples obtained by our field engineer. Following the completion of drill- ing, ground water observations would be made, and if artesian flow is encountered the boring would be backfilled with pea gravel to permit the flow to continue. CABLE ADDRESS' DAMEMORE John Graham & Company May 10, 1961 Page -2- We estimate that our fee for drilling this additional boring would be approximately $350 to $400. This is broken down as follows: Grading eroded areas in borrow area to provide an access road to the boring location $ 75 - $100 Drilling, sampling, and backfilling boring, estimated depth of 40 feet $275 - $300 Total $350 - $400 The upper limit of this fee may be taken as a guaranteed maximum for the work. We would undertake this further exploration immediately after receiv- ing your authorization in order to be able to utilize the data obtained in the studies which are presently being made. Acceptance of this proposal may be indicated by the owner's representative signing in the space below and return- ing one signed copy to this office. Yours very truly, I14MES & MOORE By JL- JKT:fm (/ Joseph Lamont, Jr. 3 copies submitted The above proposal is herewith accepted and authorization is granted for Dames & Moore to commence work in accordance with the terms outlined above. OWNED?. BY DATE DAMES 8 MOORE CONSULTANTS IN APPLIED EARTH SCIENCES SOIL MECHANICS • ENGINEERING GEOLOGY • GEOPHYSICS LOS ANGELES SAN FRANCISCO PORTLAND SEATTLE SALT LAKE CITY CHICAGO NEW •OR• jATLANTA 421 THIRD AVENUE WEST • SEATTLE ?O WA5-i N3TDN • A-wA'E : .1'9150 ASSOCIATE JOSEPH LAMONT. J John Graham & Canpaay, Arohitosts and Engineers 1426 Fifth Avenue Seattle 1, Washington Attentions Hr. 1. P. lurasashd tientls+oenw November 1, 1960 Copy 4e1 M.O.O. -7 w T e' ru t' =:N - _' le .. AFCH. 1960 P. I STFJC7 E"' Clvn 771 TRAFNe J� JOHN GRAf:;=.m F: CO. Borrow Area Slide South Wad Street near 5hth Swum South 114 cC44A% f' Ibis letter is "rifle* in sonfiroatian of your telephone dismission. today with our Ir. J. hi. Tuttle saemesrniug stabilisatia of the barrow area slide along Beath 162nd Street near 54th iwase Sawttli in lrinwdia, i ooiinetloa. Oa October 31 we :matted from you eross- seotisn data which had been obtained in the soathiwset earner of the borrow area in early Octabes bT hterrisan- Zandsan Sartpesy. the reading were taken prior to re- shaping the means is the too area of the slide in mid-October, and do net acearately show the mist. ing ground svrteen' in this area at the present tins. Isms er, these data, when compared with the original topographic information, indicate approximately the amount of excavation which has been aosanplished. l drawing showing the tope - graphi,before and after excavation has been made and a copy is attached to this letter. We have also received tsdgy survey data from Merriam-[=4m Company 'hewing lateral namemeuts thigh have occurred at hobs set along South 162nd Street sines Oeteber 20, 1160, and a plot of those data is also inslwded with this latter. Ton will notice that the slide movement is continuing at a sisol- fisemnt rats. l't is evident that this slide is amt stabilising after limited movement as did some of the more lssalised movements observed in other ad3aoeemt borrow areas. Oseenidering the progressive nature and areal extent of the slide, it is apparent that stabilisation of the slide will be an extensive and costly operation. As previously resaomeuded and further discussed today., w believe that a websarfaas drainage program should be initiated promptly. 'me have previously indicated that slosh • program should tooleds deep drainage both in the tee of the slid area awed around the top of the slide some. 1a our letter of October 19 ve suggested that the eater- bsariui atirstun found in our lacing h be drained as CABLE AOORESS. DAMEMORE i mere r i, 1940 Paige .2. the first step is • slisss'tsas drainage program. we believe that the scot practical nesss of effecting drainage in this sane is to drill vertical holes into the san4y strata and pump from the 1.v!sr portion of this layer. The holes ay be nix inches in diameter and 40 to 60 feet in depth. The pattern of cftaina would have to be dstaridined in the fielJ. `°e would suggest starting in the vicinity of Daring 4 and progressing in each direction across the lower portion of the slide. Based on our review of the topographic data, aid consider- lag that the proud surfaos in the vicinity of Daring 4 is ass at approeiaately Elevation 1.0 as a result of recent excavation in the street area, we de not eee0alsider it prettiest to excavate a trench approzinately 30 feet deep to intar- sept the sandy soils encountered at this location. Draining this area by beri- soital drains also has dreebaska since it would be necessary to start drilliat some 200 to 230 feet da ma1ope from Boring 4 in order to develop an adequate gradient. Drain well around the top of the slide scarp vovld also be required to eftsctiw],y redoes the hydrostatic fordss vhick are meting as the overall slide nuns. As provisus]a disguised, it is expected that Beene drainage vslls meld be rmlative/y deep. Ia cedar to facilitate pia sung for lartsllaVma of salsa drafts, at least one deep exploratory boring should be drilled above the slide warp. The drainage program diaaeeseed herein 'rill aid !a wag the feriae widen are s*s.iag tts earth awimante. fir, ewe adequate drainage seer ash, eel' itself, be suffisiont to halt the sliding. In visor eft tie large amount et mantis eebb& eras ands at the toe area of the slide, it nay sloe be asesssaay to re- istsbliwa a portion of the toe resistance chick was weeded by filling part of the ersavatsd area. As soon as you have had opportunity tee review this letter, we weld lib to arrange for farther personal discussion se that prompt attention ogee be given to time remedial measures. Tours wry truly, t*KL3 & MORE SY JI..JLT t 5 /,/ Joseph toast, Jr. Attaolaeents ttj ✓ 6 copies seiedtted / • • ( • /i • • T / MES 8 INI10014E •TANTS IN APPLIED EARTH SCIENCES 4ANiCS • ENGINEERING GEOLOGN• • GEOPHYSICS LOS ANGELES SAN •RANCIRCO PORTLAND SEATTLE SALT LASE CITY CHICAGO EH «ORK •7 LA NiA ,T.•. SEATTLE 99. WAS H iNGTON • ATWATER 4-8160 LA M ONT. JR rseasHsi October 19, 1960 • . ' : ; • • e.,-.311=e1-1011diec... A • !M4- IVT:17 .03.411460 AVaitilas Ale Present Air, elevate, stessent ill11011801111V- iks* Critalfah"AgAirollilltIMMICOM •Osisler15, -17, t1 l�} tow* .1ilabliasorat.4* Ider,elbie as..14.* ass saittaa. that•tat ". vont eavasatins _trig Al "101.0104sh gun ass ranplated, additiassitM \..?4 PIMP$111144ainersjtas *del_ ',t •_' .elids Ares. C•1• 11..1C.C14! •. 1, • Allibsidemee t& eseurred in South 162ad Street near 53rd Avenue south between Oeteber 12 and 15 exposed a slide scarp extending to the north and south of 1625d Sr: This-oaavtraesoi amiss sow impartial" -et Ootobor 3.7, mut * Karl Irma Amid to mild* approdastely 150 feet to seek side -of the *treat Paotesiitawiff.i_ .2110 ORM ACM. t poweent spar limit at the slide woosb7 To the..aeuth 44.442e/1trapt,thsliar.tisal displaamost of the ground sorines hal:, aal AVIM4...11 Ailtni)1109 114-.1•0•!1111eations...:. The vortisal displitoemat .to .41.114.._L*4110 SAM Moirksimmt443,7 loose ow taw giant grade/. late loom)".461410011140. 40411114. Mar "Imauder tie slide areas .-Ta. is • 41041.1. Allerbaa. an*: LWOW entirety because el the heavy growth et *Ti 111 elanditakswing thecspromilaste area of the aids moil 1.5. ..,1%e dimensions shown ars based on field esti- mates and ihmald he eensidered *ppm:bats. iraa wttiefiset iimprament sew. appears to be ognaiderabIT larger. tams *Tits: --aliadapAismsporestian 01. prllW6 e1111111-11111111111shad. to date ars . isitia13a, , tia,,,s4eaeramehee distanee hack antes-adjaeset _:**111-0404014/164.01seirlatiOtabililaing •the area where the major. .• .1041. tAttlaar, liosireed, At- t. espesteCtitat thrr. mill have only a somewhat limited effort is stabilising the larger area of movement which hem been Observed. CABLE ADDRESS.OAMEMORE DAMES 8 MOORE CONSULTANTS IN APPLIED EARTH SCIENCES SOIL MECHANICS • ENGINEEAING GEOLOGY • GEOR1. ••51 _S 1.03 ANGELES SAN BRA n.,:5_� °�R"LANO 5 CATTLE SAL, _A,.L • 421 THIRD AVENUE WEST • SEAT-LE 99 WAS ASSOCIATE. ..'CSERH �yti +_ IT _ John QrSham & Company, Architects and Engineers 1126 Fifth Avenue Seattle 1, Washington Attentionz Mr. 1. P. Burz3enski Gentlemen: N='cV • -LOA' c- ='5 October 19, 1960 Borrow Aroma Slide South 162nd Street near 54th Avenue Swath Tukwil , Washington This letter is written to present the results of reeenht inspections of the borrow area slide by oar engineers on October 15, 17, and 18, and to summarise our opinions with respect to the slide as it now exists. During these inspections, observations were made of the excavation and baokfilling operations on South 162nd Street which are now completed, and additional reconnaissance was made of the general slide area. Subsidence which occurred in South 162nd Street near 53rd Avenue South between October 12 and 15 exposed a slide scarp extending to the north and south of 162nd Street. This was traced daring our inspection on October 17, and a pronounced scarp was found to extend approximately 150 feet to each side of the street right- of -vsy. The scarp for the present upper limit of the slide mass. To the south of 162nd Street the vertical displacement of the ground surface has been as much as six feet in some locations. The vertical displacement to the north of 162nd Street is substantially less. The larger scarp grades into a tension crack which extends around the remainder of the slide area. It is difficult to follow the tension crack in its entirety because of the heavy growth of vegetation in some areas. L sketch showing the approximate area of the slide mass is attached to this letter. The dimensiane shown are based on field esti- mates and should be considered approximate. The area of major aazrement now appears to be considerably larger than WAS initially apparent, and encroaches a substantial distance back onto adjacent private properties. While the correction proeedures accomplished to date are expected to hams a beneficial effect in stabilising the area where the major aoraunt was initially observed, it is expected that they will have only a somewhat limited effect in stabilising the larger area of movement which has been observed. CABLE ADDRESS DAMEMORE John Graham & Oemp sty 0stcher 1!, 1960 Page -2_ As a result of our inspections, we believe that certain additional steps should be taken. As discussed with you on October 18, we recommend that a line of hubs be installed on the alignment of South 162nd Street. The end hubs should be placed well outside the indicated limits of the elide zone. Frequent observations should be made to check both the alignment and elevation of these hubs. It would also be beneficial to install a eeries of hubs across the location of the slide scarp and make periodic observations of the hub elevations and distance between the hubs. In our letter of October Ili, we recommended that water encountered in the zone excavated an South 162nd Street near 53r0 Avenue South should be removed from the slide area and not be permitted to enter the soil where it would be apt to cause further difficulty. In our inspections on October 17 and 18 appreciable water was noted coming from this area and flowing into the adja- cent soil. We recommend that this water be drained out of this area. The slops below the location of the rock fill has been flattened and reshaped. This is the general area in which we had previously suggested the installation of same gravel- filled trenches in order to drain additional water from the toe of the slope. In our inspection on October 18 water was observed flowing at several points from the toe of the rock fill. As sn alternative to the installation of a series of trenches previously described, we would suggest that a single interceptor trench be installed along the to of the rook fill. We suggest that this ditch be at least five feet deep, and that it be backfilled with coarse sand or pea gravel. The gradient of the ditch should be as steep as possible, and preferably should drain from west to east. The eater should be carried is a ditch or culvert extending downalope. The water should be carried out of the area and should not be allowed to become ponded or seep into the soil farther downhill. The procedure just described will tend to remove some of the subsurface water fro* the slide areas however, it is probable that additional water exists at greater depth and may still canoe difficulty with respect to further movement. In order to remove such water before it can gain access to the weakened area where movement his occurred, deep subsurface drainage would be needed on the uphill side of the slide scarp which defines the upper licit of the slide area. Pumping frma deep vertical drains would be most feasible. Before attempting to install any vertical drains, it is our opinion that at least one exploration boring should be made near the head of the slide area in order to determine the depth of the water - bearing strata. A significant water - bearing stratum was also encountered at a depth of 40 feet in Boring 4 located north of South 162nd Street in the southwest corner of the borrow area. Another means of removing water from the slide area would be to drill into the water - bearing layer at this location and pump water from it. This procedure would be a means of removing some subsurfaoe water at less cost, and is considered a reasonable first step in subsurface drainage. John Graben & Company October 19, 1960 Pogo -3- Surface water was also observed to be seeping into tension cracks and into the soil at certain locations south of 162nd Street. Such surface water should be intercepted and removed from the slide area. This can be accomplished in some areas by extruding the drain pipes which are now in operation, or by the placement of additional pipes. To effectively prevent shallow subsurface flow free gaining entrance into the slide area, a french drain would be needed around the upper limit of the slide mass. However, the cost and installation problems for such a trench around the slide mass would be substantial. It is our opinion that every effort should be made to remove surface water and sub- surface water at shallow depths by other procedures within the slide area, if at all possible. During discussions an October 18, a question was raised concerning the excavation of the area presently occupied by 57th Avenue South. It is our opinion that the portions of the right -of -way on either side of the intersection with South 162nd Street maybe safely excavated to the level of the adjacent excavation to the west. We do not believe that extensive excavation an the vacated portion an South 162nd Street should be made at this time. Tours wry truiy, DOM & MOORE By JL- JXTetfa Joseph Lament, Jr. v Attachment 3 copies submitted 3 8 ra 8 #42..0.54 t34.6 D/Ree-'71O.a Cr E 67-R t TEA% 54t rd A v c. S. Si or- -54z / •c'sPieg7/4 • ce= 53- •Li-- • orl A ,Alc. _mc , 4) BL= 7A: .4 Poizo L./.5 TA Arc e c.e.4 g Az -iE 'b/47 1...y 4 envd C(/ 7. 4 5 0 .edee, lea Ax="meo) AR GA AREA AS '-I. &Y EX G W.4 764) /^101C.47E-5. AR E-A .5 v./Ali._ 14 At/A YE BEFA/ ■CY c... v4 Al ho e c F/LL Pen TW t • ARA ‘Pi ,eo C SLIDE CO/VD/WA/5 OCT. /7,•/'7a0 SCA L. = Zoo' IMISIO1111 • 11114.011111111 SOIL •CICIIMIC 111 RI GI 1112■A 111f1 E l G 7 I Q I 01 ATV 1 _.. , J OS ANOE.i JI 5"N FgANC�5CO DAMES 8 MOORS /r-/_,I�/ "Oq'`ANO SEATTLE CONSULTANTS IN APPLIED EARTH SCIENCES SALT LASF CITY SOIL MECHANICS• F,NGINEEPING GEOLOGv• GEOPHYSICS = GO Ew voAK _ AT IA NSA 421 THIP ^v AVENUE WEST • SEAT .i bb. WAS H I NCi-C N • A "h A'_= •- ASSOCIATE JOSEPH LAMONT. JR. October 1)4, 1960 John Graham h Company, Architects and Engineers 1126 Fifth Avenue Seattle 1, Washington Attentions Mr. A. P. Bursenski Gentlemen: Borrow Area Slide South 162nd Street near 54th Avenue South Tukwila, Washington On October 10 and 12 engineers !roe this office inspected the slide area in the southwest portion of the borrow area owned by Puget Western, Inc. The purpose of these inspections was to observe the excavation and baokfill- ing now in progress along South 162nd Street, and to inspect the general conditions in the remainder of the slide area. These observations revealed that a portion of South 162nd Street approximately 30 feet wide located near the intersection with 53rd Avenue South has continued to subside. Aeditional cracks have opened on the Garen property south of South 162nd Street, on the terrace below the rook baekfill being placed adjacent to South 162nd Strut, and along the west side of the borrow area. Some seepage has continued at the toe of the slope along the west side of the proposed Slade Way right -of- way, and the surface in this area is very soft and wet. On the basis of these observations, we recommend that pertain addi- tional measures be taken to alleviate the conditions outlined above. We recommend that the subsiding sone in South 162nd Street be excavated to fire soils and baskfilled with quarry rook, as is presently being accomplished at the east and of the slide area. Provisions should be made to drain water which may accumulate in this sone. 1 trench could be excavated between this sone and the rook backfill to the east and baokfilled with gravel or small quarry rook. The large cracks which have opened along the top of the terrace below South 162nd Street extend to a depth of several feet. We recommend that this slope be out back as flat as space will allow to remove the sone of fractured soil. Flattening of the slope above Slade Way along the west side of the borrow area should also be accomplished. The wet, mucky soils along the pro- posed alignment of Slade Way should be stripped out and the drainage ditches CABLE AOORES.i. JAMEMORE Jelin Graham & Cempa' y October Ili, 1960 Pap -2 recommended in our letter of September 29 should be installed. We understand that it may be possible to re- locate the right -of -way of Slade Way further to the east. We believe that this would be desirable as it would allow further flattening of the slopes along the west side of the borrow area. Fil�irg of the cracks on the south side of South 162nd Street should be accomplished as soon as the excavation and backfilling in this area is completed. Two means of backfilling might be considered. One means would be to pump a bentonite slurry into the cracks. While this could be accomplished without extensive clearing in the arms, there would be no assurance that the cracks would be continuous enough so that would be properly filled. It is possible that cracks hidden beneath surface vegetation would be left open. Another means of closing the cracks would be to clear the area of surface vegetation and debris, plow or otherwise loosen the upper 18 to 30 inohss of soil, and recompact the surface. The stripping should be accomplished with- out removing any of the trees, and reseeding of the area should be accomp- lished immediate/7 after the compaction is completed to prevent erosion of the surface during the coming winter season. It may also be desirable to backfill, by ane of these mama, the cracks which extend on to private prop - erty north of South 162nd Street and west of the borrow area. Tours very truly, DAMES & MOORE By JL- JIT:1* Joseph an , Jr. )11 3 copies submitted ..:- • 2 AVCNUE WEST • SEATI-LE 9i; WAt vIINC.iTON ATwATir q eil€30 ASSOCIATX. JOS IC PH LA.MONT ..• cf 401h4 Or 46mm & OmpAgy .. ••••, .. riaiiires are net expected to be eafficient eamplately 110110.inaoa, me believe that they in reducing Us rate et ;Atli is presently sacxubring, and say:reduce the possibility Of 1. *Asi'..the priest* prepprwtt.,Ahe teeth sledebei of t.. *.ltrentserst■ "ire a1 :',417.`.' • '? • ." ' ''',,- -'; 1) ray r" Sio-tviti on In th. Ito rrow- arti West'. at.-57teit': • -4:44‘4.-ilte4:ttP." 2) X order Lo re tilee vac-Ant of water which Das scos.32 - to th slide .ae& fro tear f ace • Doti:roes reit clik along •';'. the south side of South 167r.:d Street should ha air • grado4, and paved wi t s;.-,h 11 tie conweit40:'-trow :motion of 53rd A vt ntm to the culvert which a athe • • ' • . - the water under 162nd Stree t . Pro vial on should • ba lefeda • carat wtsr rur.nin g off the !lined+, to the 3(*th to • pia- soca as to the d1tc. In the procaas of sco(apli.shLni • the, ditch correct i .ois are found *are watsi le es tting undo r 162nd St such flat! eivit1411.A. ck.4 a4. • the water • should be div•rtod 3) Vet. flowing tbr ptigh the c • he storied liki-4415.00,41111Pati6IV ,•. ,•',. . • Jchn Graham & 3ompany September 20, 1960 ?age -2- Representative undisturbed samples obtained from the borings will be subjected to shear tests in our laboratory. Stability analyses utilizing t }:e resu_ts of t1 ese siren" t^ tests will he ner:or -,e '. o evaluate the stability of the slope an:i the nrecedures .f7hic'• i.'.ay e fec -'vel' restrain further :love-lent. 1 -_1 - cc%centrete cr v i.s;:.ec° o_' t-e : ore - ::er. i' ._C'.. _._rer:;" occu a cC ;; -at ;,e ;:a-- a: - se ;; ^u of a:-.7 n ti.a1. which : :ay e tal"en to 7:-event a .'.o='': : :as3ive rovene:i t of t.ie area involve ".. Our conclusion in this re :.'ar-_, and also with res ect to the more permanent stabilization of tLis inclu :.ed in a Tina: written report. `Lscussel ' :i.4: you, we pro7,ose "etermine our fee for this '.'_lv°_st•i- gat_on on the basis of our standard 3c`:e-+u:.e of ar -es, a cony of which is ::tte.ched. ?or an investi-ation of the scope outline' above, we estimate tha'. our fee will ran4 ;e Jetwee: 25'20 and 3200. The upper limit of this ranp:e may be considered a .TUaranteed maximum which will not be exceec'e ' without your prior aut: o='ization. '.re wish to than:: you for er.trusting this work to us. We look forward to serving you on this project. Yours very truly, :AkM7R & MOORE Rp JL:fm Attachment 3 copies submitted Jchn Graham & Company September 20, 1960 Page -2- Representative undisturbed samples obtained from the borings will be sub;ected to shear tests in our laboratory. Stability analyse= utilizinr t"'e results of tLese strenrt:: tests will he nerfor' e-, to evaluate the 3ta:'_ 't-r of the slope anti the nroce_ures which may effectively restrain further 7love cc ?:cent." =te r;r. - a 3.:"e ^.. of t ' `'o le.. ^? a ... _. " +^ 0" rent 32t:i ch has _.i reap;; occur 'e sb that ie :'a'" a:'v $e you of a:-.y :hi ch nay be ta'_:en to - :•:'e' :e:: t a :,ore ::.assive -:ovement of the area involve-k. Our conclusions in this re ?ar :, and also with respect to the more permanent stabilization of this area will `3e ircluied in a final written report. discussed :•:it'.. you, we propose to ietermine our fee for thi ;es; _- gat_on on the basis of our standard 3che:iu'_.e of :ar•-es, a cony of which is attached. For an investi;-.ation of the scope outline ? above, we estimate that our fee will range between X25:0 and .2;'C0. The upper 1'_;rit of this range may be considered a guaranteed naxi.:-:ur, which will not be exceeds -2 . without your prior authorization. :;e wish to than you for entrusting this work to us. We look forwar to serving you an this project. JL:fm Attachment 3 conies submitted Yours very truly, DAMES & MOORE By DAMES 8 MOORE E. _ c- = _ • _ • John Graham & Company, Architects and Engineers 1112 Fifth Avenue Seattle 1, . :ashington Attention: !•'r. A. P. Burzenski Gentlemen: :,er...tember 2r; • • { • Proposal - Confirmation Stability Investigation Borrow Area Slide 54th Avenue South and South 162nd Street Tukwila, Washington, for Puget Western, Inc. We confirm herein our proposal discussed with you last week for a stability investigation at the location of the slide in your borrow area on the north side of South 162nd Street near 54th Avenue South at Tukwila, `•Tahington. At the time we inspected the slide area movement of the soils under South 1C2nd Street had occurred, and the roadway had been backfilled with grave.. A large tension crack was observed in the borrow area, and a number of additional tension cracks were evident on 162nd Street some distance to the vest of the zone of principal movement. The purpose of the stability investigation is (1) to evaluate the cause of the movement which has occurred, (2) to Jeter:rine the zone in which the principal movement is taking place, (3) to recommend corrective action for the repair of the movement which has occurred thus far, and (►) to recommend such stabilization as is deemed necessary to provide adequate protection for the roadway which will ultimately be constructed through the borrow area. To accomplish this, we propose to drill six borings to an average depth of 35 feet each in the area in which movement has occurred. Three of these borings will be drilled on the alignment of South 162nd Street, and the regaining three will be drilled to the north of South 162nd Street within the area of movement. Undisturbed samples will be obtained at intervals of five feet throughout the depth of each boring, and in some areas of the boring continuous or near continu- ous samples would be obtained. The drilling and sampling operations will be accomplished under the direction of one of our field engineers who will classify the soils and maintain a complete log of each boring. AB.°_ A: HESS LI A,+('M_li LEROY C. LOWE A.I.A. ARCHITECT P.O. BOX 1241 SEATTLE. WASHINGTON S8111 40110 • EDUCATION: UNIVERSITY OF MINNESOTA - ARCHITECTURE UNIVERSITY OF COLORADO - ARCHITECTURE CERTIFICATE OF QUALIFICATIONS; NATIONAL COUNCIL OF ARCHITECTURAL REGISTRATION BOARDS, WASHINGTON, D.C. MEMBER; AMERICAN INSTITUTE OF ARCHITECTS INSTITUTE FOR URBAN DESIGN ARCHITECTURAL EXPERIENCE; 25 YEARS INCLUDING 10 YEARS AS PRINCIPAL AND 11 YEARS AS PROJECT ARCHITECT PROJECTS; HOTELS, SCHOOLS, CONDOMINIUMS, HOSPITALS, MILITARY INSTALLATIONS, OFFICE OFFICE MANAGEMENT; SUPERV IS ION PROFESSIONAL LICENSES; BUILDINGS, FACTORIES AND RESIDENCES OF 23 MAN DESIGN DRAFTING GROUP ALABAMA CALIFORNIA KANSAS MONTANA NORTH DAKOTA TEXAS WYOMING INVENTIONS; LIFE SUPPORT CONSOLE - RECOVERY ROOM; UTILITIES CARRIER MED GASES; CHARTIG SHELF - ACCESSORIES HOLDER - TIMING DEVICE; CONSTRUCTION COST ACCOUNTING SYSTEM CONTINUING EDUCATION; BUS. ADMIN.; HISTORY; RESEARCH ARCTIC CONST.; LICENSED PILOT; PHOTOGRAPHER ALASKA COLORADO LOUISIANA NEBRASKA OH IO UTAH ARIZONA 1DAHO MINNESOTA NEVADA OKLAHOMA WASH INGTON ARKANSAS IOWA MISSOURI NEW MEXICO OREGON WISCONSIN '"I wfiA.1 LEROY C. LOWE A.I.A. ARCHITECT P.O. BOX 1241 SEATTLE. WASHINGTON 98111 DEAR NEIGHBOR NOV. 14, 1991 TO THOSE OF YOU WHO WERE KIND ENOUGH TO WRITE OR CALL ABOUT THE "HILLCREST PROJECT", I THANK YOU, YOUR SUPPORT HAS BEEN MOST GRATIFING. SHOULD YOU WISH MORE INFORMATION PLEASE DO NOT HESITATE TO CALL AND 1 WILL MAIL YOU A COMPLETE SET OF THE SITE PLANS FOR THE "HILLCREST PROJECT". PHONE: 454-4423 VERY TRULY YOURS LEROY C . LOWE A.1 .A. ARCHITECT CITY OF TUKWILA Wetland and Watercourse Special Studies Report Criteria A development proposal that is within 50 feet of a sensitive area will submit appropriate studies to adequately identify and evaluate the sensitive area and it's buffer. Projects proposing sensitive area impacts will require specific studies to assess the impacts and propose mitigating measures. Professional Qualifications Wetland and stream specialists performing work for City review will, upon request, submit professional qualification statements. A project list with references should be included to verify work history and performance. Wetland and Watercourse Analysis The exact location of wetland and watercourse boundaries will be determined by the applicant's consultant. Wetland delineations, performed by wetland specialists, will apply the wetland definition in TMC 18.06.938 and the methodology in the "Federal Manual for Identifying and Delineating Jurisdictional Wetlands" (1989). Watercourse analysis will be performed by qualified stream or wetland specialists to characterize and classify the watercourse according to the watercourse definition in TMC 18.06.395 and the City's Water Resource study (1990). All buffers will be measured from the ordinary high water mark (OHWM), if field delineation is possible, or from the top of bank. Wetland and watercourse reports submitted to the City should contain the following: 1. A plant species list or description with scientific names (nomenclature), relative abundance and distribution of species, and the major habitat types of vegetation. 2. Data plot forms, according to the Federal Manual method, to substantiate wetland study findings. 3. Report site maps should include: a. Vicinity map b. Public resource document maps including City's Sensitive Area inventory mapping, if applicable. c. Accurate topographic mapping, if required, showing contours at the smallest available interval. d. Field delineated and professionally surveyed wetland and /or watercourse boundary mapping. 1011\ bik E•• 4. The written report should discuss the following: a. Site description and general observations of habitat value related to wildlife use. b. Study methodology. c. Soil types mapped on the site including on -site verification and analysis. d. Vegetation description according to the classification system outlined in "Classification of Wetlands and Deepwater Habitats of the United States ", Fish and Wildlife Service, U.S. Department of the Interior, 1979 (FWS /OBS- 79/31). e. Wetland or watercourse rating and associated buffer width according to the Sensitive Areas Ordinance of the Zoning Code. Mitigation Proposals A mitigation proposal of wetland or watercourse relocation and /or buffer reduction should include the standard report format plus the following: 1. Conceptual mitigation or enhancement plan to describe and illustrate what impacts and compensatory actions are proposed. a. Include hydrology aspects, vegetation composition, and wildlife habitat details. b. Describe how water quality and flood storage potential would be improved. 2. Upon approval of conceptual plan, a final mitigation or enhancement plan will be required to include the following components: a. Detailed planting and grading plan including species to be used for revegetation. b. Performance standards. c. Construction management. d. Monitoring program to ensure success of the plan. e. Contingency plan to correct performance standards or unanticipated impacts. f. Performance security in the form of a monetary bond or other means to guarantee the successful completion of the plan. CITY OF TUKWILA Areas of Potential Geologic Instability Development and Report Criteria All development applications on property having slopes greater than or equal to 15% are subject to the Zoning Code - Sensitive Areas Ordinance. Three primary factors that influence slope stability were incorporated into the geologic classification system. Important characteristics of the site are topographic relief, stratigraphy of subsurface soils, and local ground or surface water environment related to potential slippage and massive soil movement. Areas of Potential Geologic Instability are considered sensitive areas and classified as follows: - Class 2 areas, where landslide potential is moderate, which slope between 15 and 40 percent and which are underlain by relatively permeable soils. - Class 3 areas, where landslide potential is high, which include areas sloping between 15 to 40 percent and which are underlain by relatively impermeable soils or bedrock, and which also include areas sloping more than 40 percent. - Class 4 areas, where landslide potential is very high, which include sloping areas with mappable zones of ground water seepage, and which also include existing mappable landslide deposits regardless of slope. - Areas of potential seismic instability, with soft soils, loose sand and a shallow groundwater table. - Areas of potential coal mine hazard, ie. subsidence from subsurface excavation and tunneling. In order to identify the extent of sensitive slopes, the applicant must submit a survey of existing topography, drawn in two -foot contour intervals accurate to within one foot of elevation. The topographic survey must be stamped by a professional,land surveyor licensed in the State of Washington. Mapped slope areas exceeding 15 percent should be designated on the site plan for potential geotechnical site review. Professional Qualifications The applicant is required to submit a geotechnical report appropriate to both the site conditions and the proposed development. A geotechnical investigation will generally not be required for development of Class 2 slopes when: 1) any portion of the site is a minimum of 200 feet from a Class 3 or 4 area and 2) the proposed alteration remains outside the sloping area. Development of Class 3 and 4 areas and any identified seismic or coal mine hazard areas requires a geotechnical investigation and associated report. All geotechnical studies must be conducted by a geotechnical engineer. TMC 18.06.323 defines this individual as a professional civil engineer licensed with the State of Washington who has at least four years of professional employment as a geotechnical engineer with experience in landslide evaluation. Geotechnical engineers performing work within the City must submit professional qualification statements in addition to work history and references. Geotechnical Analysis The geotechnical report analyzes the site for overall stability and makes recommendations on the need for and width of buffer setbacks necessary to protect post- development site stability. The investigated geologic, hydrologic, and topographic conditions of the site will be used to confirm or revise the City's geologic classification. The scope of the investigation should comply with the specific requirements presented below. 1. Landslide Hazards Class 2: Geotechnical reports for Class 2 areas are required to have, at a minimum, a review of available geologic site data and a surface reconnaissance of the site and adjacent areas. Subsurface exploration of the site is at the discretion of the geotechnical consultant and the City. Class 3: Geotechnical reports for Class 3 areas are required to have a review of the available geologic site data, a surface reconnaissance of the site and adjacent areas, and a subsurface exploration program suitable to the site conditions and the proposed development. Class 4: Geotechnical reports for Class 4 areas are required to perform the tasks listed for Class 3 areas. In addition, detailed slope stability analysis should be performed based on the information obtained during the field investigation. 2. Erosion Hazards Class 2, 3, and 4 landslide hazard areas are also potential erosion hazard areas. Geotechnical reports regarding proposed development in these areas will include erosion and sediment control recommendations that are appropriate to the site conditions and the proposed development. 3. Seismic Hazards Proposed development within areas o should include an evaluation of si potential relative to the proposed story single - family dwellings, this performance of similar structure conditions. For proposed devel structures, this evaluation should exploration to provide a site coeffic lateral force procedure described in 4. Coal Mine Hazards significant seismic hazards e response and liquefaction evelopment. For one or two valuation may be based on the under similar foundation pments of other occupied nclude sufficient subsurface ient (S) for use in the static the Uniform Building Code. Proposed development within area of historical coal mine activities or mapped subsurface co .1 formations will require a detailed site reconnaissance by a geologist or a geotechnical engineer. Site specific information i egarding the presence of mine entrances or workings is needed prior to permitting new construction in these areas. Permitting Requirements Prior to permitting any developme geologic instability, the applicant following: 1. There is no past or present evi quantitative analysis indicate proposed development or surrou OR 2. The potentially instable area can be designed so that prop surrounding properties are elim decreased, and the increase i sedimentation will not affect t of an area of potential must demonstrate one of the ence of slope instability and no significant risk to the ding properties. an be modified or the project sed impacts to the site and ;mated, slope stability is not surface water discharge or lope stability. City of Tukwila John W. Rants, Mayor Department of Community Development Rick Beeler, Director City of Tukwila PUBLIC HEARING NOTICE Notice is hereby given that the City of Tukwila Planning Commission will be holding a work session at 6:00 p.m. and a public hearing at 8:00 p.m. on March 26, 1992 in the City Hall Council Chambers located at 6200 Southcenter Blvd. to discuss the following: PLANNING COMMISSION WORK SESSION CASE NUMBER: 90 -3 -CA: Multi- Family Design Standards APPLICANT: City of Tukwila Department of Community Development REQUEST: Complete review of draft changes to Title 18, Tukwila Municipal Code, which affect the minimum development standards for future multi - family improvements, approve public distribution and set a public hearing date. LOCATION: City-wide BOARD OF ARCHITECTURAL REVIEW PUBLIC HEARING OLD BUSINESS CASE NUMBER: 91 -9 -DR: Tukwila Dental Center APPLICANT: Torjan Ronhovdc REQUEST: To construct a 5,000 sq. ft., 2 -story dental office. LOCATION: Between 13985 & 13995 Interurban Ave.: Lot "A ", Tax Parcel #336590- 0229 -09. PLANNING COMMISSION PUBLIC HEARING NEW BUSINESS CASE NUMBER: P92 -0023: Non - conforming Setback Amendments. APPLICANT: City of Tukwila Department of Community Development REQUEST: Amend TMC 18.70.050 non - conforming single family structures to allow limited expansion along building line setback areas. LOCATION: City-wide CASE NUMBER: 92 -01 -APP: Hillcrest (90- 13 -BLA, 91 -3 -APRD) APPLICANT: LeRoy Lowe REQUEST: Appealing an administrative decision by the Department of Community Development for a wetland delineation plan. LOCATION: Slade Way & S. 160th St., Sec. 26, Twn. 23, Rng. 4E. CASE NUMBER: L92 -0014: Toys -R -Us APPLICANT: The WBDC Group REQUEST: Replace existing non - conforming sign of 256 sq. ft., which exceeds sign code limit of 150 sq. ft., with a sign of the same size and content. Per TMC 19.32.140, Planning Commission approval is required. LOCATION: 16700 Southcenter Pkwy., Tukwila, WA, Sec. NW26, 'Tutt. 23, Rge. 4. 6300 Southcenter Boulevard, Suite /1100 9 Tukwila, Washington 98188 • (206) 431 -3670 • Fax. (206) 431 -3665 Page 2 Persons wishing to comment on the above items may do so by written statement or by appearing at the public hearing. Information on•the above cases may be obtained at the Tukwila Planning Department. The City encourages you to notify your neighbors and other persons you believe would be affected by the above items. Published: Valley Daily News Distribution: Mayor, City Clerk, Property Owners /applicants, Adjacent property owners, File. LEROY C. LOWE A.I.A. ARCHITECT P.O. SOX 1241 •<ATTI_t. WA•HINOTON omit• FEB 18 1992 CITY OF TUKWILA PLANNING DEPT. L. RICK BEELER DIRECTOR, DEPT. OF COMMUNITY DEVELOPMENT 6300 SOUTHCENTER BLVD. SUITE 100 TUKWILA , WA. 98166 DEAR MR. BEELER FEB. 10, 1992 WE HAVE BEEN INVOLVED IN INVESTIGATION AND STUDIES FOR A RESIDENTIAL DEVELOPMENT OF PROPERTY ADJACENT TO AND WEST OF SLADE WAY. OUR WORK HAS, IN PART, RESULTED IN BECOMING AWARE OF CONCERNS OVER SLADE WAY. APPARENTLY THE CITY'S DESIGN AND CONSTRUCTION OF SLADE WAY HAS NOT RESULTED IN A STABLE SITUATION. AS THE CITY IS AWARE, THERE HAS BEEN SINKING OF THE ROADWAY AND ASSOCIATED DOWN HILL.. SLIDES. AND AS THE CITY IS AWARE THIS SITUATION HAS LIKELY RESULTED FROM INADEQUATE CONTROL OF SUBTERRANEAN WATER FLOW. APPARENTLY YOUR STAFF AND/OR CONSULTANTS RECOGNIZE THAT THE INSTALLATION OF A FRENCH DRAIN SYSTEM IS AT LEAST ONE THING THAT CAN BE DONE TO STOP THIS PROBLEM. FEB 181992 CITY OF TUKWILA PLANNING DEPT. OUR CONSULTANTS WOULD SUPPORT YOUR EFFORT IN THIS REGARD. SINCE A FRENCH DRAIN INSTALLATION MAY REQUIRE ENTRY AND WORK ON OUR PROPERTY, WE WOULD BE GLAD TO WORK OUT THE APPROPRIATE PERMISSION. VERY TRULY YOURS LEROY C . LOWE A.I .A. ARC CC : JOHN McFARLAND, CITY ADMINISTRATOR GARY SCHULZ , URBAN ENVIRONMENTALIST JACK PACE, SENIOR PLANNER PHIL FRASER, CITY ENGINEER DARREN WILSON, ASSISTANT PLANNER tI1LLCQJ T r. iL DENNIS JOULE, P.E. CIVIL ENGINEER 32729 S.E. 44th Street Fall City, WA 98024 (206) 392 -1108 Leroy C. Lowe, A.I.A. P.O. Box 3972 Bellevue, Washington 98009 January 15, 1992 Project 1442 Re: Hillcrest Dewatering / Stability of Slade Way Ground & Surface Water Hydraulics Geoteehaieal Engineering V.30 _,�' FEE 18 1992 CITY OF TUKWILA PLANNING DEPT. At your request I have reviewed the geotechnical reports regarding Hillcrest Subdivision in Tukwila, Washington. The reports reviewed included; Preliminary Geotechnical Investigation for Hillcrest Slade Way, dated May 30, 1990, prepared by Cascade Geotechnical, and a follow -up report titled Hillcrest Way, dated August 27, 1990, also prepared by Cascade C eotechnlcal. I have also performed a surface inspection of the site. You have informed me that Slade way, adjacent to the proposed Hillside development, has slope stability problems and has undergone land movement. This report discusses the relationship between dewatering the Hillcrest site and the slope stability of Slade Way. Soil logs from the geotechnical reports show the site to be underlain by slightly silty sands with some interbedded lenses of silt. The Hillcrest site involves a lower, relatively flat bench, with a steep slope to the west. Four springs (or seeps) were observed at the toe of the steep slope. Soil logs show a perched groundwater table within the benched area. Slade Way is downslope and just east of the site. The road bed was constructed cross - slope by cutting upslope and filling downslope. Slope stability is simply the ratio of the magnitude of forces acting to hold the soil in place (shear strength) divided by magnitude of forces acting to drive the soil downslope (soil weight). When water is added to soil, the shear strength goes down and the weight goes up. From this, it can be seen that adding water to a slope lowers slope stability, dewatering a slope improves slope stability. Since the Hillcrest site is adjacent to, and upslope, from Slade Way, dewatering the lower portion of the Hillside site will improve the stability of both the Hillside site, and Slade Way. The degree to which the slope stability is improved depends on the degree to which the slope is de- watered. This depends on the location, configuration, and depth of the dewatering facilities. The lower portion of the Hillcrest site has been designated a wetland. If you de -water a wetland, it is no longer a wetland. Thus, you have a problem. De -water the site, remove the wetland, and improve the slope stability; or not. Page January 15, 1992 • Project 1442 If Slade Way fails (is involved in a slope failure) the utilities within the road will likely fail also. These utilities involve gas, water, power, sanitary sewer, and storm sewer. The environmental damage downslope from Slade Way that would be caused by the failure of the utility pipes within Slade Way would certainly overshadow the Loss of any small wetland that may be present within the Hillcrest site. If Slade Way is currently moving downslope, then Slade Way is involved in a slope failure. As stated above, this report simply discusses the relationship between dewatering the Hillcrest site and the slope stability of Slade Way. It must be the decision of the governing agency as to whether the site should be de- watered, improving the slope stability, or not. --Mum FEB 1 8 1992 CIT i OF TUKWILA PLANNING DEPT. 32729 S.E. 44th Street Fall City, WA 98024 (206) 392 -1108 =ci.edberg Associates, Inc. Post Office Box 1337 Gig Harbor, WA 98335 (206) 858 -7055 January 8, 1992 MOE FEB I8 1992 CITY OF TUKWILA PLANNING DEPT. Leroy C. Lowe, A.I.A. Architect P.O. Box 3972 Bellevue, WA 98009 RE: Hillcrest Boundary Line Adjustment Tukwila, WA Dear Leroy: This letter is in reference to the wetlands on the above referenced site. A level 1 site walk was performed in August of 1991. The accompanying map shows the approximate edge of wetlands as defined in the three parameter unified methodology. This delineation is based on the topography and spring locations. Additional facts to be considered are: 1) The wetland is not on the City of Tukwila wetland inventory. 2) The wetland is estimated at 7,500 square feet, it is isolated with no connection to any other wetland or stream, it is surrounded by urban area, developed land, and roads. 3) The wetland source is from a series of springs at the base of a the slope. 4) The runoff from the springs and area is collected in the storm water system of Slade Way and tightlined to the, presently under construction, Klickitat stormwater system. 5) A sewer line serving McMicken Heights runs under Slade Way. 6) Slade Way is the eastern border of the property. 7) The geotechnical report and information from several engineers (ENTRANCO, RESCO, Professional Surveyors Inc., Cascade Geotechnical) indicates that Slade Way and the sewer are slipping at the rate of inches per year. 8) The water originating on the subject property saturates the soils under Slade Way and contributes substantially to the slippage problem. 9) If Slade Way has a major slip, then the sanitary sewer is likely to break, creating a major environmental health hazard, and denying sewer service to a large population. page 2 10) If the road slips greatly this could be an immediate hazard to traffic and passersby. 11) Recommended remedial action is to intercept the ground water, dewatering the hillside, and pipe the water into the stormwater system. 12) The dewatering process will drain the wetland. Several scenarios of recreating the wetland have been considered, but there is no practical way of maintaining a wetland and dewatering the hill. The two activities are contradictory. 13) Deep excavations, 10 + feet in depth, have been considered for placing water tight barriers to redirect the water and save the wetland. This is hazardous as the stratified soils would allow the water to go to a greater depth and continually destabilize the hill; the stability problem of the road would not be solved. 14) Any attempt to preserve.the wetland will jeopardize Slade Way and the sewer. 15) The City of Tukwila has provisions in its ordinance for dealing with emergencies and public safety. 16) Exempting this area from the wetland ordinance on the basis of the public safety issue is valid and recommend. 17) A wetland study is unnecessary as the dewatering of the hill is the only reasonable alternative and will result in lowering the water table of the area such that a wetland can not be supported. In summary, a wetland is present and the primary cause of slippage and lack of stability of Slade Way. Slade Way is slipping and it is a matter of time before the road and sewer cause a major environmental mishap. The remedial action for preventing further slippage involves dewatering the hill by lowering the water table. Lowering the water table will drain the wetland. There is no practical solution to the loss of the wetland. It is recommended, in the interest of public safety and to save time, that the wetland be exempted from further study and remedial action undertaken. It is safe to say that Slade Way will go, someday. Further study and delay enhances the probability that an expensive cleanup will ensue. I hope that common sense prevails to prevent an environmental disaster. If I can be of further assistance, Please contact me. ra�cy�o��o FEB 18 1992 CITY o - TUKWILA PLANNING DEPT. Mechanical & Civil Engineering 17815 S.E. 146th Renton, WA 98059 (206) 228-4244 FAX (206) 228-4292 Leroy C. Lowe, R.I.A. P.O. Box 3972 Bellevue, Wa. 98009 February 17, 1992 RE: Hilicrest Boundary Line Adjustment Dewatering and stability Dear Leroy: NI-.3LOAM tFEslsl99aJ CITY CF T UKWILA PLANNING DEPT. At your request I once again reviewed your proposed above referenced project. My review included a site visit and review of the information supplied by other consultants. There can be no doubt in anyone's mind that Slade Way is threatened by a potential landslide downslope of your property if the present conditions are not corrected. Your property and that portion of Slade Way East of it are saturated by seeps. The saturated soil is very heavy and viscous and hence, at present, lead to a very unstable situation. Slade Way is presently serving as an earthen dam holding the heavy saturated soil in place. Slade Way was clearly not built or designed to withstand the potential lateral loads due to the upland saturated soils. The most elementary Geotechnical calculations would demonstrate that Slade Way is not stable within reasonable safety limits. The other consideration is the presence of a wetland on your property. While I am not a sensitive areas or wetlands expert, it is clear from Mr. Bredberg's report and my observations, that there are some wetlands on the property. It appears to me that there is clearly a question of balancing the stability of Slade Way and the utilities in it along with the corresponding public health and safety against the value of the wetland. It would seem obvious to me that the logical and most expedient solution to the Slade Way stability problem would be the "controlled responsible" development of your property. This would include street frontage, and stormdrain improvements to Slade Way along with on -site drainage, dewatering and stability measures that would assure stability over the entire affected area. While I realize that I have not said anything here that you do not already know, I hope that my comments will be of some help to you. Please call me if you have any questions or need additional information. Sincerely, / 4i Richard E. Stu h, P.E. Washington State registered mechanical and civil engineerin^ corporation CITY OF TUKWILA 6200 SOUTKCENTER BOULEVARD. TUKWILA. WASHINGTON 98188 MEMORANDUM DARREN WILSON PHIL FRASE SEPTEMBER 16, TO: FROM: DATE: SUBJECT: PHONE # G ENVIRONMENTAL BOUNDARY LINE 1991) 1991 0614.3., NUT 1(0.1 Cary L FEB 18 1992 CITY OF TUKWILA PLANNING DEPT. COMMENTS FOR HILLCREST (LEROY LOWE) ADJUSTMENT (CHECKLIST DATED SEPT. 16, Va n Damn. Mayor 1. #9 - Name the two soils reports by author and date produced (Include reports in documentation) - Add: "City to conduct peer review of geotech /hydrological studies via independent consultant in order to determine final impacts /mitigations." 2. Earth, d. - Add: "Slippage of Slade Way has also occurred at least twice over past 10 years." 3. Under Item No. 2 Water - Comments are written in such a manner as to not address proposed use of lots, but rather treat the proposal as the act of changing lot lines only. Question: Is this environmental document for proposed 5 lot configuration for single family use or not? Impacts and mitigations for intended use need to be fully defined in responses based on intended construction for 5 single family dwellings, not just status quo undeveloped use. I request this portion of environmental checklist be redone to include impacts /mitigations with any proposed 5 unit single family development, not just under assumption that only lot line boundaries are being relocated. Second, I request that the assumptions made by the preparer of the environmental checklist identify which soils report they are relying upon in the preparation of the checklist responses so it is clear by what authority the impacts /mitigations have been determined. 4. No. 7 - Utilities - Water Utility requires "loop system" be developed. Noted is that WD #75 states in availability letter, "Water system must be looped to...." indicating that impacts of this development require mitigations for a water main extension looping the system. Public Works requests this loop be in public R /W. How it is located needs to be shown in order to determine if easements are required on plan to mitigate utilities across private lots. 5. No. 7 - Utilities - Storm drainage availability is also identified under Checklist Response No. 7. This is not the case. A storm main extension will be needed to S. 160th Street for the proposed use of property and so needs to be identified to mitigate impacts of said lot line configuration at time of first single family lot construction. Also, detention will be required in the new public line constructed in Slade Way prior to discharge into new 53rd Ave. S. storm line (now under construction). 6. No. 15 Transportation - a. This answer to be changed from n/a to Slade Way /53rd Ave. S. for four single family lots and S. 160th Street for the fifth lot. ,F 'O .7 : Z 4* ?,,2GS/ W/G.s e,k1 Nog • 2 G , 177/ �. G. imowE MEMORANDUM Darren Wilson Sept. 16, 1991 page two munTA FEB 18 1992 CITY OF T UKWILA PLANNING DEPT. No. 15 Transportation - a. - Applicant to identify nearest METRO bus stop No. 15 Transportation - e. through g. - Applicant to provide responses (n /a incorrect). 7. No. 16. Public Services - a. Identify water main loop required and extension of public storm drainage system. Also, identify extension of ped path and sidewalk systems linking Slade way to S. 160th Street. 8. On plan submittal identify how utilities services including storm side drains, water services and sanitary side sewers will be located relative to need to provide any utility easements serving one property across other properties. Do same for franchised utilities (power, telephone, TV cable, WNG). 9. On plan identify maximum elopes of private drives to allow Public Works to verify accesses demonstrate 15% max. requirement can mitigated under proposed lot configurations. Also, on plan identify proposed realignment of South 160th and how proposed access could configured /mitigated to open access (contact Brian Shelton, 433 -0179 for copy of South 160th Street plans). PRF /cd xc: Brian Shelton file: Hillcrest Development CD.D18.LEROY.PRF LEROY C. LOWE A.I.A. ARCHITECT P.O. BOX 1241 BCATTLE. WASHINGTON 0011, 0 1 rn z 0 DARREN WILSON ASSISTANT PLANNER 6200 SOUTHCENTER BLVD. TUKWILA, WA. 98166 DEAR MR . WILSON : I FEB 21 1992 CITY OF TUYWILA PLANNING DEPT. IIILLCZ6T FEB. 18, 1992 WE HAVE ATTEMPTED TO DISCUSS THE "REAL AND PRESENT DANGER" OF THE SLADE WAY SLIPPAGE PROBLEM WITH YOU ON SEVERAL OCCATIONS. YOUR LETTER OF JAN. 30, 1992 WAS RECIEVED BY ME THE EVENING OF FEB. 8, 1992, NEARLY A HALF YEAR AFTER WE, HAD FILED OUR BOUNDARY LINE ADJUSTMENT APPLICATION. ( SEPT. 9, 1992 ) 1 APPEAL YOUR LETTER DIRECTING ME TO PRESERVE AND PROTECT THE SEEPS ON MY PROPERTY FOR THAT ACTION WOULD PERPETUATE A DANGEROUS SITUATION PLACING SLADE WAY IN PERIL. THE EXPENDITURE FOR WET LAND DELINIATION IS UNREASON- ABLE IN LIGHT OF THE CITYS DEFICIENCY IN HANDLEING THE SLADE WAY CONCERNS. liILGCZcST PAGE 2 WE APPEAL TO YOU THE STAFF AND TO THE CITY TO WORK WITH US TO PROVIDE FOR REASONABLE DEVELOPMENT OF OUR PROPERTY AND THE CITYS PROTECTION OF SLADE WAY, THE SANITARY SEWER , AND THE UTILITIES THEREIN AS DISCUSSED IN THE ATTACHED LETTERS FROM : RESCO, BREDBERG , AND JOULE , YOUR IMMEDIATE ATTENTION TO SLADE WAY AND. CONTRIBUTING OFF SITE CONDITIONS IS CRITICAL TO AVOID A REAL AND PRESENT DANGER TO THE HEALTH , WELFARE, AND SAFETY OF THE CITIZENS OF TUKWILA AND KING CO. ATTACHED IS AN INTERNAL MEMO FROM PHIL FRASER TO DARREN WILSON CITING THE SLADE WAY SLIPPAGE PROBLEM. I ALSO 1-IAVE HAD DISCUSSIONS WITH RALPH HEITT AN INSPECTOR FOR ENTRANCO WHO HAS A CONTRACT WITH THE CITY OF TUKWILA TO SUPERVISE THE CONSTRUCTION OF KLICKITAT DRIVE & S, 160m ST. MR. HEITT HAS INDICATED THAT SLADE WAY HAS REAL STABILITY PROBLEMS AND HAS SETTLED SOME TEN INCHES IN THE RECENT PAST. PLEASE LET ME KNOW WHAT IT IS YOU WANT TO DO ON MY PROPERTY TO R SOLVE-THI PROBLEM. SINCERELY YOURS LEROY C. LOWE A.I.A. AR HITECT CC : MAYER, CITY COUNCIL TUKWILA BLDG. STAFF • CEB 21 1992 III PLANNING DEPT ...._---__--- FEE 211992 CITY OF TUKWILA CITY OF TUKWILA 6200 SOUTHCENTER BOULEV,4RD, TUKWILA. WASHINGTON 98188 MEMORANDUM TO: DARREN WILSON FROM: PHIL FRASE DATE: SUBJECT: PHONEY (206) 433.1800 Cary L. VanDusen, Mayor SEPTEMBER 16, 1991 ENVIRONMENTAL COMMENTS FOR HILLCREST (LEROY LOWE) BOUNDARY LINE ADJUSTMENT (CHECKLIST DATED SEPT. 16, 1991) 1. #9 - Name the two soils reports by author and date produced (Include reports in documentation) - Add: "City to ,conduct peer review of geotech /hydrological studies via independent consultant in order to determine final impacts /mitigations." Earth, d. - Add: "Slippage of Slade Way has also occurred at least twice over past 10 years." 3. Under Item No. 2 Water - Comments are written in such a manner as to not address proposed use of lots, but rather treat the proposal as the act of changing lot lines only. Question: Is this environmental document for proposed 5 lot configuration for single family use or not? Impacts and mitigations for intended use need to be fully defined in responses based on intended construction for 5 single family dwellings, not just status quo undeveloped use. I request this portion of environmental checklist be redone to include impacts /mitigations with any proposed 5 unit single family development, not just under assumption that only lot line boundaries are being relocated. Second, I request that the assumptions made by the preparer of the environmental checklist identify which soils report they are relying upon in the preparation of the checklist responses so it is clear by what authority the impacts /mitigations have been determined. 4. No. 7 - Utilities - Water Utility requires "loop system" be developed. Noted is that WD #75 states in availability letter, "Water system must be looped to...." indicating that impacts of this development require mitigations for a water main extension looping the system. Public Works requests this loop be in public R /W. How it is located needs to be shown in order to determine if easements are required on plan to mitigate utilities across private lots. 5. No. 7 - Utilities - Storm drainage availability is also identified under Checklist Response No. 7. This-is not the case. A storm.main extension will be needed to S. 160th Street for the proposed use of property and so needs to be identified to mitigate impacts of said lot line configuration at•time of first single family'lot construction. Also, detention will be required in the new public line constructed in Slade Way prior to discharge into .new 53rd Ave. S. storm line (now under construction). 6. No. 15 Transportation - a. This answer to be changed from n/a to Slade Way /53rd Ave. S. for four single family lots and S. 160th Street for the fifth lot. /tea ,oecre,,2Gh/ w /C,s o,) /ygiz 24.1 /9'/ G. 2454,1 � • MEMORANDUM Darren Wilson Sept. 16, 1991 page two No. 15 Transportation - a. - Applicant to identify nearest METRO bus stop No. 15 Transportation - e. through g. - Applicant to provide responses ('n /a incorrect). 7. No. 16. Public Services - a. Identify water main loop required and extension of public storm drainage system. Also, identify extension of ped path and sidewalk systems linking Slade way to S. 160th Street. 8. On plan submittal identify how utilities services including storm side drains, water services and sanitary side sewers will be located relative to need to provide any utility easements serving one property across other properties. Do same for franchised utilities (power, telephone, TV cable, WNG). 9. On plan identify maximum slopes, of private drives to allow Public Works to verify accesses demonstrate 15% max. requirement can mitigated under proposed lot configurations. Also, on plan identify proposed realignment of South 160th and how proposed access could configured /mitigated to open access (contact Brian Shelton, 433 -0179 for copy of South 160th Street plans). PRF /cd xc: Brian Shelton file: Hillcrest Development CD.D18.LEROY.PRF Pir lig-IMRE) I FE$ z 1992 f CITY OFTUKW,LA t P LANNING DEPT DENNIS JOULE, P.E. CIVIL ENGINEER 729 S.E. 44th Street II City, WA 98024 Leroy C. Lowe, A.I.A. P.O. Box 3972 Bellevue, Washington 98009 January 15, 1992 Project 1442 Re: Hillcrest Dewatering / Stability of Slade Way Ground & Surface Water Hydraulics Geotechnical Engineering At your request I have reviewed the geotechnical reports regarding Hillcrest Subdivision in Tukwila, Washington. The reports reviewed included; Preliminary Geotechnical Investigarion for Hillcrest Slade Way, dated May 30, 1990, prepared by Cascade Geotechnical, and a follow -up report titled Hillcrest Way, dated August 27, 1990, also prepared by Cascade Geotechnical. I have also performed a surface inspection of the site. You have informed me that Slade way, adjacent to the proposed Hillside development, has slope stability problems and has undergone land movement. Tnis report discusses the relationship between dewatering the Hillcrest site and the slope stability of Slade Way. Soil logs from the geotechnical reports show the site to be underlain by slightly silty sands with some interbedded lenses of silt. The Hillcrest site involves a lower, relatively flat bench, with a steep slope to the west. Four springs (or seeps) were observed at the toe of the steep slope. Soil logs show a perched groundwater table within the benched area. Slade Way is downslope and just east of the site. The road bed was constructed cross - slope by cutting upslope and filling downslope. Slope stability is simply the ratio of the magnitude of forces acting to hold the soil in place (shear strength) divided by magnitude of forces acting to drive the soil downslope (soil weight). When water is added to soil, the shear strength goes down and the weight goes up. ' From this, it can be seen that adding water to a slope lowers slope stability, dewatering a slope improves slope stability. Since the Hillcrest site is adjacent to, and upslope, from Slade Way, .dewatering the lower portion of the Hillside site will improve the stability of both the Hillcrest site, and Slade Way. The degree to which the slope stability is improved depends on the degree to which the slope is de- watered. This depends on the location, configuration, and depth of the dewatering facilities. Page I January 15, 1992 Project 1442 If Slade Way fails (is involved in a slope failure) the utilities within the road will likely fail also. These utilities involve gas, water, power, sanitary sewer, and storm sewer. The environmental, damage downslope from Slade Way that would be caused by the failure of the utility pipes within Slade Way would certainly overshadow the loss of any small wetland that may be present within the Hillcrest site. If Slade Way is currently moving downslope, then Slade Way is involved in.a slope failure. As stated above, this report simply discusses the relationship between dewatering the Hillcrest site and the slope stability of Slade Way. It must be the decision of the governing agency as to whether the site should be de- watered, improving the slope stability, or not. S 0 OAFS cT�.o Q = �S /0NAI. c'z,` E B F 21 1992. ...J N CITY Gi= 'i UK'NILA PLANNING DEPT. 32729 S. E. 44th Street Fall City, WA 98024 (206) 392 -1108 .edberg Associates, Inc. Post Office Box 1337 Gig Harbor, WA 98335 (206) 858 -7055 January 8, 1992 Leroy C. Lowe, A.I.A. Architect P.O. Box 3972 Bellevue, WA 98009 RE: Hillcrest Boundary'Line'Adjust:nent Tukwila, WA Dear Leroy: This letter is in reference to the wetlands on the above referenced site. A level 1 site walk was performed in August of 1991. The accompanying map shows the approximate edge of wetlands as defined in the three parameter unified methodology. This delineation is based on the topography and spring locations. Additional facts to be considered are: 1) The wetland is not on the City of Tukwila wetland inventory. 2) The wetland is estimated at 7,500 square feet, it is isolated with no connection to any other wetland or stream, it is surrounded by urban area, developed land, and roads. 3) The wetland source is from a series of springs at the base of a the slope. 4) The runoff from the springs and area is collected in the storm water system of Slade Way and tightlined to the, presently under construction, Kiickitat stormwater system. 5) A sewer line serving McMicker. Heights runs under Slade Way. 6) Slade Way is the eastern border of the property. 7) The geotechnical report and information from several engineers (ENTRANCO, RESCO, Professional Surveyors Inc., Cascade Geotechnical) indicates that Slade Way and the sewer are slipping at the rate of inches per year. 8) The water originating on the subject property saturates the soils under Slade Way and contributes substantially to the slippage problem. 9) If Slade Way has a major slip, then the sanitary sewer is likely to break, creating a major environmental health __ hazard, and denying sewer service to a 1�r- ge- =pcgul ionlT — G r:;, \ "' C F 1 11 IQN I LA page 2 10) If the road slips greatly this could be an immediate hazard to traffic and passersby. 11) Recommended remedial action is to intercept the ground water, dewatering the hillside, and pipe the water into the stormwater system. 12) The dewatering process will drain the wetland. Several scenarios of recreating the wetland have been considered, but there is no practical way of maintaining a wetland and dewatering the hill. The two activities are contradictory. 13) Deep excavations, 10 + feet in depth, have been considered for placing water tight barriers to redirect the water and save the wetland. This is hazardous as the stratified soils would allow the water to go to a greater depth and continually destabilize the hill; the stability problem of the road would not be solved. 14) Any attempt to preserve the wetland will jeopardize Slade Way and the sewer. 15) The City of Tukwila has provisions in its ordinance for dealing with emergencies and public safety. 16) Exempting this area from the wetland ordinance on the basis of the public safety issue is valid and recommend.• 17) A wetland study is unnecessary as the dewatering of the hill is the only reasonable alternative and will result in lowering the water table of the area such that a wetland can not be supported. In summary, a wetland is present and the primary cause of slippage and lack of stability of Slade Way. Slade Way is slipping and it is a matter of time before the road and sewer cause a major environmental mishap. The remedial action for preventing further slippage involves dewatering the hill by lowering the water table. Lowering the water table will drain the wetland. There is no •practical solution to the•loss of the wetland. It is recommended, in the interest of public safety and to save time, that the wetland be exempted from further study and remedial action undertaken. It is safe to say that Slade Way will go, someday. Further study and delay enhances the probability that an expensive cleanup will ensue. I hope that common sense prevails to prevent an environmental disaster. If I can be of further assistance, Please contact me. i' FEE 2 1 1992 1l •c, ; ; OF TuKWILA PLANNING DEPT. Mechanical & Civil Engineering 17815 S.E. 146th Renton, WA 98059 (206) 228-4244 FAX (206) 228 -4292 Leroy C. Lowe, A.I.A. P_O. Box 3972 Bellevue, Wa. 98009 February 17, 1992 RE: Hilicrest Boundary Line Adjustment Dewatering and stability ., . _ Dear Leroy: At your request I once again reviewed your proposed above referenced project. mv review included a site visit and review of the information supplied by other consultants. There can be no doubt in anyone's mind that Slade Wav is threatened by a potential landslide downslope of your property if the present conditions are not corrected. Your property and that portion of Slade Wav East of it are saturated by seeps. The saturated soil is very heavy and viscous and hence, at present, lead to a very unstable situation. Slade Wav is presently' serving as an earthen dam holding the heavy saturated soil in place. Slade Wav was clearly not built or designed to withstand the potential lateral loads due to the upland saturated soils. The most elementary Geotechnical calculations would demonstrate that Slade Wav is not stable within reasonable safety limits. The other consideration is the presence of a wetland on your property. While I am not a sensitive areas or wetlands expert, it is clear from Mr. Bredberg's report and my observations, that there are some wetlands on the property. It appears to me. that there is clearly a question of balancing the stability of Slade Way and the utilities in it along with the corresponding public health and safety against the value of the wetland. It would seem obvious to me that the logical and most expedient solution to the Slade Way stability problem would be the "controlled responsible" development of your property. This would include street frontage, and stormdrain improvements to Slade Way along with on -site drainage, dewatering and stability measures that would assure stability over the entire affected area. While i realize that I have not said anything here that you do not already know, I hope that my comments will be of some help to you. Please call me if you have any questions or need additional information. Sincerely, Richar• E. Stuch, P.E. Washington State registered mechanical and civil engineering corporation [E2l'199aj Cii':' OF TUKNILA 7'+ �lii)�+lG rc�; City of Tukwila 6200 Southcenter Boulevard • Tukwila, Washington 98188 John W. Rants, Mayor January 30, 1992 LeRoy Lowe P.O. Box 3972 . Bellevue, Wa. 98009 RE: HILLCREST 90 -13- BLA Dear Mr. Lowe: The purpose of this letter is to explain the sequence of events in order to complete your Boundary Line Adjustment /Administrative Planned Residential Development. Please excuse the delay in producing this letter. I was ill for one week. The comments listed below are based on our meeting of Tuesday, January 21, 1992. The conditions are divided into requirements that need to be met prior to final approval of your Boundary Line Adjustment, Administrative Planned Residential Development and Sensitive Area Ordinance. BOUNDARY LINE ADJUSTMENT After reviewing your property DCD Staff believes that a Type 2 wetland exists on the subject property. The following additional information needs to be submitted prior to staff decision on your application. 1. A wetland delineation performed by a qualified biologist with a professional boundary survey is required. The Sensitive Areas Ordinance allows Type 1 and 2 wetlands to be altered under (TMC 18.45.080 A,B & H) certain conditions i.e. maintenance, repair of existing uses and facilities construction of new essential streets, right -of -way, and utilities. If the SAO would deny all reasonable use of the property or development may be allowed (Section 18.45.115 C. 3) wetland mitigation are required in all wetlands. ATTACHMENT A Phone: (206) 433 -1800 • City Hall Fax (206) 433 -1833 LEROY LOWE PAGE TWO 90- 13 -BLA 2. All the wetland issues need to be resolve prior to approval for the Boundary Line Adjustment /Administrative Planned Residential Development. Attached are the methods needed to complete the wetland /watercourse study. 3. An additional geotechnical study will be required if you, submit the letter, you read to us during the meeting from the third engineer. The City will request a peer review of this document for the content of the report. ORIGINAL LOT PATTERNS /CONSTRUCTION If wish not to proceed with the Boundary Line Adjustment/ Administrative Planned Residential Development, you must complete the following. 4. You may submit individual building permits for lots as they are presently. An additional geotechnical report will be required for the structural approvals from the Building Division; and; 5. A wetland delineation performed by a qualified biologist with a professional boundary survey is required. Based on Section 18.45.125, if you object or disagree with DCD decision or conditions for development in a sensitive area, you may appeal to the Planning Commission. Any such appeal shall be made in writing within ten days from the date of this letter on the interpretation, condition or decision being appealed, and shall set forth the basis for the appeal. The Planning Commission meets the fourth Thursday of each month. If you wish to appeal the decision by staff, your written request must be-in our office thirty days before the Public Hearing. Should you have additional questions regarding this matter, please contact our office at 431 -3670. Thank You Darren Wilson, Assistant Planner cc: John McFarland, City Administrator Gary Schulz, Urban Environmentalist Jack Pace, Senior Planner Phil Fraser, City Engineer REPORT GEOTECHNICAL DESIGN CONSULTATION PROPOSED CONDOMINIUM DEVELOPMENT SLADE WAY AND 53RD AVENUE SOUTH TUKWILA, WASHINGTON FOR THE MITHUN ASSOCIATES G3r/3 —003 (R) /10/. yt6 _ r, 'U,i,.1 l 1 �� 90005562 GeoEngineers, Inc. Consulting Geotechnical Engineers and Geologists GeoEngineers, inc. Consulting Geotechnical Engineers and Geologists May 3, 1982 The Mithun Associates 2000 - 112th Avenue Northeast Bellevue, Washington 98004 Attention: Mr. Vincent J. Ferrese, AIA Gentlemen: Report Geotechnical Design Consultation Proposed Condominium Development Slade Way and 53rd Avenue South Tukwila, Washington File No. 340 -03 INTRODUCTION We present, in this letter, the results of our design consultation for geotechnical aspects of site development and foundation support for a proposed condominium development in Tukwila, Washington. The site is located immediately west of the right -of -way for Interstate Highway No. 5. It is bounded on the south by Slade Way and on the west by Slade Way and 53rd Avenue South. The proposed development includes construction of 18 six -unit condominium structures together with associated roadways, parking areas and utilities. The units will be stepped into the hillside, requiring varying depths of cut. Some cutting and filling will also be required to construct the roadways and parking areas as well as to establish reasonable grades around the various buildings. Wood frame construction is contemplated. Relatively light foundation loads are expected. SCOPE The purpose of our services is to develop recommendations for design of the components of the project which are affected by soil and ground water condi- 01 tions. The scope of services includes: 1. Evaluating probable subsurface conditions from information obtained from available soils reports, together with a reconnaissance of the property. P.O. Box 6325 • 2020 - 124th Ave N.E. - Suite 201 • Bellevue, Washington 98007 • (206) 881 -7900 The Mithun Associates May 3, 1982 Page Two 2. Recommending the type of foundation system to be used for building support and providing design criteria. 3. Assessing the impacts of proposed grading on site stability. 4. Providing design criteria for retaining walls in the condominium units and retaining structures required for general site grading. 5. Recommending drainage facilities to collect surface and near - surface water. 6. Commenting on general earthwork criteria. The basic information from which our conclusions and recommendations have been made are those data contained in reports dated December 1, 1960 and June 27, 1961, by Dames & Moore. While our experience on this site indicates that the conditions depicted by the available borings are representative, we recommend • that additional explorations be made to verify conditions at locations intermedi- ate to those borings. SITE HISTORY A major portion of this site was involved in a large earth slide in 1960 -61. The slide resulted primarily from the removal of toe support in a borrow area immediately to the northeast. The hillside is underlain by a deposit of generally clean sand in which water was present under artesian pressure. As the toe support was removed from the hillside the uplift pressures from this artesian zone contri- buted to the instability of the slope. A system of horizontal drains was installed in late 1961 to reduce the artesian pressure beneath the hillside. Subsequently, an extensive subsurface drainage system was installed in connection with the construction of Interstate Highway No. 5 and the Tukwila Interchange. The installation of these drainage systems has apparently been effective in stabilizing the former deep- seated movements of the area. u1 to We have made a surficial inspection of the site as part of this consultation. We have found some evidence of near - surface movement, mostly in the form of shallow creep or flow slides. There is a scarp located a short distance east GeoEngineers, Inc. The Mithun Associates May 3, 1982 Page Three of Slade Way which is about 200 feet long and varies from about 12 to 20 inches in height. The downslope extent of this movement appears to be limited to the upper portion of the property in which no construction is planned. The time of this past movement cannot be precisely determined, but it appears to have occurred a year or more ago. SOIL CONDITIONS Soil conditions on this site have been defined from the borings which were drilled in 1960 and 1961 as part of the earth slide investigations. Some minor modifications to the ground surface have been made since that time. The upper soils on the site consist of silty sands and sandy silts which possess moderate strengths and are generally waterbearing. The prevailing evi- dence of wet surface conditions and small springs noted during our recent site visit confirm the continued presence of a near - surface saturated condition. The silts and sands are underlain,by a relatively impermeable deposit of fine - grained soil which varies from clayey silt to silty clay. The shear zone associated with the large movements in 1960 -61 was located in this deposit. The silts and clays are underlain by what appears to be a continuous stratum of sand. This is the permeable zone in which high artesian pressures were ob- served at the time of the earth slide. This sand layer is, in turn, underlain by dense glacial soils. . CONCLUSIONS AND RECOMMENDATIONS GENERAL We are of the opinion that development of the site as planned is feasible providing that careful attention is given to near - surface drainage and that the extent of cutting and filling does not markedly vary from that presently antici- pated. The history of site stability for the past 20 years indicates that the drainage facilities have been effective in preventing further deep- seated move- ments. It is of prime importance, however, that these facilities remain opera- tional. Should the drainage facilities fail to operate so that artesian pressures increase beneath the hillside, the risk of possible earth movements would become significantly greater. P0005565 GeoEngineers, Inc. The Mithun Associates May 3, 1982 Page Four SITE GRADING AND DRAINAGE A key element of site grading will be the collection and control of surface runoff and near - surface seepage. We recommend that temporary drainage ditches be installed to collect these flows immediately following site clearing and grubbing. We recommend that permanent french drains be installed along the upslope sides of the parking areas and between the easterly property line and the condo- minium units. These drains should be at least five feet below finished grades and should consist of perforated pipe fully bedded in a graded• sand filter or in clean gravel which is, in turn, enclosed in a filter fabric envelope. The remain- ing trench backfill should consist of clean, free - draining sand and gravel. The drain line along the east side of the condominium units can be incorporated with a tight line drain which will collect downspout flow and flow from footing drains for the various structures. Use of the on -site soils for fill may be limited. We anticipate that the soils in their natural state will be above optimum moisture for effective com- paction. In order to use these soils, drying will be necessary. Also, we rec- ommend that only clean granular soils be used for fill behind retaining structures. While careful segregation of the natural soils during excavation may provide a source of suitable backfill, we anticipate that much of the backfill behind walls will have to be imported. We recommend that temporary cut slopes be made no steeper than approximately 11 (horizontal) to 1 (vertical). The stability of these cut slopes will be rela- tively low. (Our analyses indicate that the factor of safety under static conditions/ will be on the order of 1.2 to 1.3 -(a factor of safety of less than 1.0 indicates an unstable condition). Some sloughing or localized sliding should be anticipated particularly where zones of seepage are encountered. When the excavations are made for the various buildings, we recommend that the construction be sequenced so that alternate building sites are excavated, retaining walls built and ade- quately braced, and backfill placed before beginning excavation for the intermediate buildings. P0005566 GeoEngineers, Inc. The Mithun Associates May 3, 1982 Page Five Permanent cut or fill slopes should be established no steeper than 21 :1. Slopes should be seeded as soon as practical. Localized seeps which are not intercepted by the general drainage facilities may have to be drained and /or excavated and backfilled with gravel or crushed rock to resist damage to the slope face. STRUCTURAL FILLS AND PAVING All structural fill should be placed in relatively thin (8 to 10 inches) layers and uniformly compacted. Backfill against retaining walls, particularly those in the structures, should not be overcompacted. A percent compaction of 90 to 92 percent.of the maximum dry density determined by ASTM D -1557 should be achieved. Fill beneath paved areas or forming slopes should be compacted to 95 percent of maximum density. We understand that access roadways and parking areas will be paved with asphaltic concrete. Prior to placing base course material, we recommend that the exposed subgrade be thoroughly proofrolled. Any soft, loose or wet areas should be excavated and replaced with clean granular fill if the natural soils cannot be satisfactorily recompacted. All fill placed below subgrade level should be compacted to at least 92 percent of maximum dry density (ASTM D- 1557). A pavement section of 41 inches of clean pit run subbase, 11 inches of crushed rock base and 2 inches of asphaltic concrete should be used. RETAINING STRUCTURES Retaining structures will be required in all of the buildings because of the sloping hillside. The units will be stepped into the hillside so that each section of retaining wall will be only one story high. The setback between walls will be about 15 feet or more. In addition, retention of cut slopes and fill embankments will be required along portions of the parking areas and near Unit Nos. 1 and 2. We recommend that lateral pressures on the basement walls be determined using an equivalent fluid pressure of 35 times the height of the wall. This value applies only if clean, lightly compacted granular backfill is used against the walls and that a perforated drain is installed along the base of the wall P0005567 GeoEngineers, Inc. The Mithun Associates May 3, 1982 Page Six to prevent the buildup of hydrostatic pressure. This value also applies only where the ground surface behind the wall is level for a distance equal to the height of the wall. If a sloping ground surface will exist closer to the wall, this value may have to be increased. Retaining structures along the parking area and elsewhere outside of the buildings could consist of gabion basket walls, reinforced earth structures, or conventional poured concrete walls. We recommend against the use of rockeries except possibly for protection of low (less than 5 feet) cuts into natural soils. Rockeries should not be used to retain fill embankments. Where backfill behind the walls is level, lateral pressures may be determined using an equiva- lent fluid pressure of 35 times the height of the wall. Where the surface slopes upward behind the wall , an increased lateral pressure must be used. If a slope of 21:1 exists, we recommend that the lateral pressures be determined using 60 times the height of the wall. We should review design criteria for other backslope configurations. Drainage of the backfill as well as cut slopes behind these retaining struc- tures is important. The lateral pressure design criteria given above is based on drained conditions. For solid structures, such as a poured wall, a zone of clean backfill and a perforated drain should be installed. If gabion basket walls are used, we recommend that filter fabric be placed beneath and behind the baskets to'prevent the retained soils from washing into the relatively open work gabion rock. A perforated base drain located immediately behind the baskets is also recommended for gabion installations. Resistance to lateral loads on retaining structures may be developed by passive pressures and base friction. We recommend that passive pressures be determined using an equivalent fluid density of 200 pounds per cubic foot. Base frictional resistance can be determined using a factor of 0.3 times the vertical downward component of long -term loading. These values incorporate a factor of safety of about 1.3. FOUNDATION DESIGN We recommend that the condominium structures be supported on shallow spread foundations. All footings should bear on natural soils below the topsoil layer. GeoEngineers, Inc. The Mithun Associates May 3, 1982 Page Seven We recommend that footings be proportioned using a bearing value of 2,500 pounds per square foot. This value applies to the total of all design loads including wind or seismic, but excluding the weight of the footing and any overlying backfill. Footings should have a minimum width of two feet and be founded at least 18 inches below the lowest adjacent finished grade. Care must be taken in preparing the footing excavations to avoid distur- bance to the bearing surface. Any seepage should be collected and diverted away from the footing area. The prepared surfaces should be observed by one of our staff to determine if they are suitably prepared. It may be necessary to remove and replace soft, wet or otherwise disturbed soils at the base of these excavations. Backfill should consist of clean sand or sand and gravel compacted to at least 95 percent of maximum density (ASTM D- 1557). We recommend that footing drains be installed along all continuous wall footings. These drains, together with the basement wall drains, should be con- nected to the tight drain line along the east property line and conducted off - site for disposal in a manner which will not adversely affect the stability of the hillside. Settlements of footings designed and installed as recommended are expected to be small, probably on the order of one -half to three - fourths of an inch. Some differential settlement may occur along the length of the units because of the differences in relief of overburden pressure resulting from the varying depths of excavation into the hillside. FURTHER STUDIES We recommend that supplemental explorations be made prior to beginning site work. Several borings should be drilled to confirm soil conditions in the areas planned for building construction and in the areas of cutting or filling. Piezo- meters should be installed to monitor changes in near - surface ground water condi- tions. Some supplemental analyses will be appropriate, particularly if there are modifications in the depths of cut or fill to be made. We will be pleased to discuss the scope of services which we feel is appropriate. P0005569 GeoEngineers, Inc. The Mithun Associates May 3, 1982 Page Eight LIMITATIONS AND USE OF REPORT This report has been prepared for use by The Mithun Associates and other members of the design team. This report should be provided to prospective con- tractors for bidding and estimating purposes. Our report, conclusions and interpretations should not be construed as a warranty of subsurface conditions. As noted previously, we have used data developed by others at widely spaced locations. Additional site explorations are considered desirable before the project goes to bid. If there are changes in the loads, grades, location, configuration or type of construction planned from that which we have been provided, the conclusions and recommendations presented may not be applicable. If design changes are made, we request that we be given the opportunity to review our conclusions and recom- mendations and to provide a written modification or verification. When the de- sign has been completed, we recommend that the appropriate construction drawings and specifications be reviewed by our firm to see that our recommendations have been interpreted and implemented as intended. The scope of work does not include services related to construction safety precautions and our recommendations are not intended to direct the contractor's methods, techniques, sequences or procedures, except as specifically described in our report for consideration in design. There are possible variations in subsurface conditions between the explora- tions and also with time. A contingency for unanticipated conditions should be included in the budget and schedule. Sufficient monitoring, testing and consul- tation by our firm should be provided during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities are completed according to the intent of contract plans and specifications. Within the limitations of scope, schedule and budget for our work, we war- rant that our work has been done in accordance with generally accepted practices followed in this area at the time the report was prepared. No other warranty, express or implied, is made, OLSS000d GeoEngineers, Inc. . ' �« ���; ��'. The conclusions and recommendations in this report should be applied in their entirety. We are available to review the final design and specifications to see that our recommendations are properly interpreted. If there are any questions concerning this report or if we can provide additional services, please call. JKT:wd Ten copies submitted Respectfully submitted, Geo ngineers, Inc. k K. Tuttle resident • P0005571 GeoEngineers, Inc. •• • ,-. ATTACHMENT E CASCADE GEOTECHNICAL April 24, 1991 Mr. Leroy Lowe Job No. 9005 -13G Page 2 should review the final building plans in detail. We understand from Sheet #8 (Drainage site plan) that groups of six (6), two (2) inch diameter horizontal drains, separated from each other by ten (10) feet O.C., will be installed along the toe of the existing slope, in areas where the slope is to be regraded. No specific details for these horizontal drains were provided on the plan we reviewed. These horizontal drains will exit into a north- easterly trending French Drain to the west and extend westwards approximately twenty (20) feet horizontally. We recommend that specific details for the horizontal drains be reviewed by us and incorporated into the final plans. We understand from the Drainage Site Plan that proposed French Drains will be a minimum of six (6) feet deep and two (2) feet wide. The French Drain detail provided on the plan shows visqueen lining across the bottom and along the downslope side of the French Drain trench, with electric, telephone, and cable lines located within the French Drain trench. In our soils report we recommended that the French Drain trench be lined with a permeable geotextile fabric to avoid future clogging from siltation. We recommend that this be included in your final plans for the uphill face not covered by visqueen liner. We recommend against placing any utilities within the French Drain trench, since access to the utilities would compromise the integrity of the French Drain. With the exception of the above stated discrepancies, we believe that the Drainage Site Plan reflects our recommendations. We understand from the Grading Site Plan that you propose regrading some parts of the toe of the existing slope. The grading plans show that the slope will be regraded at 1.5(H):1(V) or steeper grade behind the southwestern corner of the house proposed on Lot #3. In our soils report dated August 27th, 1990 we had recommended against making any cuts into the toe of the slope, and that all cuts steeper than a 2(H):1(V) grade be supported with a structural retaining wall. We understand from conversations with you that you anticipate placing a four (4) foot high retaining wall in the area of the steep cuts. We recommend that we be engaged to calculate the lateral forces for the retaining wall and review the retaining wall design. The houses proposed on Lots #3, #4 and #5 appear buildable, provided the recommendations in our soils reports and this review are followed carefully, and adequate drainage .of. the site is Amjad I. Khan Geologist N Cr) 00 W Lt. U Z W 0 ° J = Z z U U • y.Q Er Q J Q x 0 0 a DARREN WILSON ASSISTANT PLANNER C3 J H z W 0 N 0 0 N FEB. 18, 1992 DEAR MR. WILSON : N 1 W p I- !}} .. W JQF I-Z>- O a. Q WataaO°�QWOF -? = ZO Z �I"ZN IA uW. J H d V F- W w w in o Q 1 F -<CLZ0 J JQOpUU Z 3 .7 =< N Z 0. J W° IL NN► aWN< .Z0w f.m�WO °t Q° Z 3= to Ce Z p "= Q o N 2 W J Q W 0 2 III W I J M el Z z Z Z x W Z Q C O (A g 0 O} Z H O W Q p J a23 12 U Z W }J Q Z u . z> N W '1 O QceFJZ < <UZF. Zo a' U F-- x Q I- W • > S p J CC N U W }_ Q"' •CC >0 J IL ,- W FO- W Z Q }_- F- U N Q O °re w w V !L W CC Ji:: �Q�F- �waOQ CIO - ,JppN w a x Q w Q O> W >. W W U O In > W x p O Q p U F U W Z m N O Q Z Z H N N ? W O U J Z � I - F.w J >C.OW m- CO .. Q ra a J i Z Wi N W 0 Q W> I- W Z m W Q a z W p J= Q x W N N Q Z }- J O w QF-3VUcc= 1(L -00I u re < �' 3 0_ ..1 m Q 1 . Q u. U J U U RCHITECT _ ATTACHMENT B Geo„ `JEnginee>ls Mr. Alan P. Kilian, P.E. Chief Geotechnical Engineer Washington State Department of Transportation Transportation Building KF -01 Olympia, Washington 98504 -5201 Dear Mr. Kilian: NOTED APR 1 2 1989 PUGET WESTERN INC. April 10, 1989 Re: Valley View Estates Tukwila, Washington Consulting Geotechnical Engineers and Geologists We have reviewed the slope stability analyses which the Materials Laboratory has performed in reviewing the studies made for the proposed Valley View Estates project by GeoEngineers, Inc. As you have pointed out, an error was made in the input of coordinates of one of the search boxes used in analyzing the stability of the slope• for the 1985 topo- graphic conditions. The net result of this error was to overstate the computed factor of safety by about 0.3; i.e., 1.55 rather than 1.25. You have indicated that the stability of the hillside., both in its present state and when developed, is and would be below the "industry standard" of 1.5. We believe there are sound technical reasons backed by historic precedent which demonstrate that a factor of safety lower than 1.5 is acceptable for a project such as this. After review of the WSDOT analyses, as well as our own, we conclude that the lower factor of safety of 1.25 is acceptable, providing the site drainage requirements which we have previously set forth are met. Our rationale for this conclusion is described below. The "factor of safety" concept in engineering analysis has..-as its basis the risk associated with our knowledge of the soil and ground water parameters and the correctness of the analytical method. A factor of safety of 1.5 has historically been found to satisfy the inherent risk in the analysis of a slope where the slide mechanism is not known and the soil and ground water conditions are.typically established by testing of soil samples on a relatively limited basis and measurement or modeling of the ground water regime. However, in the case of an existing slide, a significant amount of the risk is mitigated by improved knowledge of the parameters used in -the analysis and confidence in the analytical method GeoEngineers, Inc, 2405 140th Ave. NE, Suite 105 Bellevue, WA 98005 Telephone (206) 746 -5200 Fax. (206) 746 -5068 Washington State Department of Transportation April 10, 1989 Page 2 itself. This has, in our experience, resulted in the ability to success- fully complete projects on sites where the calculated factor of safety is on the order of 1.2 to 1.3. One of the primary parameters in the stability analysis is the strength of the soil along the slide plane. The soil strengths along the identified zone of sliding on the Valley View Estates site have been evaluated by three separate investigators (Dames & Moore, Shannon & Wilson and GeoEngineers). Each firm reached very similar conclusions on the residual strength parameters. (This in itself is testimony to our assertion that there is reduced risk in selection of parameters in an existing slide condition.) It should be pointed out that the strength parameters selected for the analyses represent the low end of the range of probable soil strengths calculated. Thus, the level of risk associated with the results of analyses using this parameter should be regarded as being small. A similar argument may be made with regard to the correctness of the assumed failure mechanism in the slope stability analysis. In this case, information is available on the slide plane from a number of inclinometer measurements. Hence, the confidence in the assumed failure mechanism is increased and the risk of incorrect modeling in the analysis is reduced. In the back analysis of an existing failure, there is a certain degree of dependence between the assumed failure plane and the shear strength yielded by the analysis. This dependence also contributes to an increased confidence level in the analysis itself and, accordingly, a lower risk level. There are a number of successful projects in known slide areas in the Puget Sound area for which lower factors of safety have been determined from analyses of the known conditions. These projects have performed satisfactorily. The following are some examples: 1) The Meadowdale slide in the City of Edmonds represents a zone of extensive earth movement. Following studies of the mechanism of sliding and appropriate remedial action, the city has lifted their building moratorium and residential construction in this area is now allowed. The factor of safety of the slide mass in this area is considered to be between 1.2 and 1.3; 2) A slide along 44th Avenue Southwest in the City of Seattle encompassed a section of parkland as well as a number of residences at both ends of the slide area. Stabilization measures consisting of subsurface drainage and a reinforcing toe berm were installed to stabilize the area. The factor of safety achieved is calculated to be about 1.2. Other examples such as the stabilization of the Sahalee Way slide near Redmond and stabilization of a segment of I -90 a short distance west of 76th Avenue Southeast on Mercer Island for which factors of safety of 1.2 to 1.3 were achieved can be cited. Washington State Department of Transportation April 10, 1989 Page 3 We have reviewed the alternate section analyzed by WSDOT. It is our opinion that this section is slightly more stable than the section selected by GeoEngineers. The reason for our disagreement with your analysis results is that the locations of the search boxes in the central portion of the slide mass do not correlate well with either the known configuration of the 1960 -61 slide mass or the section developed by Shannon & Wilson in their 1968 studies. Specifically, the search box coordinates used by WSDOT are some 20 to 25 feet lower than the slip plane defined by the slope indicator in Dames & Moore Boring 3 in the 1960 -61 studies. Further, Remedial Section A (see Figure 3, Summary Report, Soil Conditions and Earth Movements, Vicinity of the Tukwila Interchange, by Shannon & Wilson dated June 21, 1968) shows the lower limit of the silty clay stratum along which the slide has been demonstrated to occur as intersecting the reshaped slope at about the top of the wall. By contrast, the slip plane assumed in the WSDOT analyses is shown to pass below the wall. We understand that the slip plane selected for your analyses is based on data in a 1964 study by Shannon & Wilson. We do not know the basis for the information which they presented in that report. However, the information in the 1968 Shannon & Wilson summary report appears to better represent the known slip plane. Reanalysis of this section using a slip plane compatible with that defined by the Dames & Moore and the 1968 Shannon & Wilson studies together with the water level data recently obtained by both WSDOT and GeoEngineers yields a factor of safety of 1.29. It also appears that the analyses performed to evaluate slope stability for earthquake conditions were conducted using the same drained residual strength values rather than using undrained strength parameters which include a cohesive value and which are considered to be more representative for this loading condition. In doing this, we obtain a factor of safety of about 1.05. While low by "conventional" standards, we consider this acceptable for a situation where we have a good definition of the failure mechanism. We conclude that a factor of safety of 1.25 is acceptable for the Valley View Estates project and that the corrected analyses show this to be realistically achievable if the WSDOT drainage system is kept function- ing in a condition comparable to that which has existed through 1986. We say 1986 because there is some indication that portions of the system have deteriorated since that time. Readings of the water levels in the deep wells obtained on March 27, 1989 indicate that some of the wells and /or horizontal drains are not functioning as well as about two years ago. The water levels in several of the wells have risen some 4 to 5 feet and, in two instances, over 15 feet. • • _ Washington State Department of Transportation April 10, 1989 Page 4 There is not a regular pattern in the way the water levels have risen with the exception that the two wells which have risen the most are located in the northernmost well group. This suggests to us that it would be prudent to consider doing more than just flushing the horizontal drains during the next maintenance activity. We note that the northernmost group of wells is adjacent to the right —of —way in which a major water line and the Olympic Pipeline to SeaTac Airport are located. Slope movements in this area could have serious consequences. In summary, we believe that the analyses done by WSDOT and GeoEngineers are in general agreement when taken on the same basis. Further, it is our opinion, based on the results of our analyses and our experience with stabilization of many similar landslides, that a factor of safety of 1.25 is acceptable for this project. We do not, in any way, want to diminish the importance of maintaining satisfactory drainage of the deep aquifer with regard to sustaining an adequate factor of safety against deep— seated failure. Also, the improvement of near — surface drainage is critical to arresting the more shallow, creep —type movements which have been evidenced on the site. Obviously, the matter of how these drainage systems are kept in good repair goes beyond a purely technical issue which we cannot address. We thank you for the technical review made by your staff and hope that this information is helpful to you. Please call, if there are any points which you wish to discuss. Yours Very Truly, GeoEngineers, Inc. 1,4 K T Jack K. Tuttle GMD :JKT :wd Principal cc:V/Puget Western, Inc. Attn: Mr. Richard Causey Mr. Joel Haggard Haggard Law Offices Mr. Phil Fraser City of Tukwila Mr. Larry Hard File No. 0340 -09 -1 em=su-9 K1L T E S L 1111 a 4Z E CC.N&,L L'!•.KTE. IN r.rr. LIE? EARTH S.CIENCL: !: LC• ••••if.: [a.., •.L L'G�•.: i L:.v4` 6f.C•••• •.0 4Z i : 1.tC: L . .. .Li r. •• Cr • E t.LT __ _ \'.f. c,- •.... T[' AT'. .. G G Gc _ v.. Er ... ../71; June 27, 1961 John Graham & Company, Architects and Engineers 11126 Fifth Avenue Seattle 1, Washington Attention: Mr. A. P. Burzenski Gentlemen: We submit herewith three copies of our "Report of Soils Investigation, Earthslide, South 162nd Street near 53rd Avenue South, Tukwila, Washington, for Puget Western,. Inc." The scope of the investigation is outlined in our proposal dated April 10, 1961. During the course of the investigation supplementary explorations were authorized by Mr. F. W. Kimball of Puget Western, Inc. The results of our field explorations and our preliminary conclusions and recommendations have been presented as they became available during the course of our work. Yours very truly, DAMES & NOORE By JL -JKT: fm cc: Puget Western, Inc. (2) Washington Building Attention: Mr. F. W. Kimball cc: Puget Western, Inc. (2) Washington Building Attention::Mr. Wells McCurdy CAELC ApCFCSE DAMCMC%C :72414oseph Lamont Jr. P0005575 -2- SCOPE The purpose of this investigation is to define the causes of the slide and to provide recommendations for the correction and rehabilitation of the unstable area. Specifically, the scope of the investigation as originally authorized includes the following: (1) determining the nature of the soils within and above the exist- ing slide area, (2) defining zones of subsurface water bearing strata, (3) determining the approximate zone in which the slippage is occurring, (4) determining the feasibility and limitations of excavating the soils within the unstable area, (5) providing recommendations for other possible means of stabiliz- ing and rehabilitating the area, and (6) providing means by which impending damage to adjoining property can be detected and avoided during the time corrective work is being accomplished within the slide area, During the course of the investigation supplemental explorations were author- ized for the purpose of defining the subsurface soil strata and ground water conditions in the borrow area below the toe of the slide. SUBSURFACE CONDITIONS The exploration of the subsurface conditions within and adjacent to the slide area and an evaluation of the depth of slide movements were accomp- lished by drilling a series of eight borings and installing slope indicator casing in four of these borings. The locations of the borings are shown on the aerial photograph on Plate 1. Slope indicator casing was installed in Borings 1 through 1. Details regarding the field explorations and slide cr.nics e F+:ctol:t. -3- :•.mvement observations, including the log of each boring and a record of slide r.rvements, are presented in Appendix A. The slide presently encompasses an area roughly triangular in shape, shown on Plate 1. The width of the slide across the toe is on the order of c.r0 feet. The length of the slide, as measured in a northeast to southwest direction, is also approximately 650 feet. Lateral movements in the central portion of the slide area along South 162nd Street have been on the order of 50 ;.o 60 feet,,,. based on measurements from aerial photographs taken before the slide in July, 1956, and during our investigation. These comparative photographs are presented on Plate 2. The ground surface across the slide area slopes downward to the north- east on an average slope of approximately 52 (horizontal) to 1 (vertical), as indicated by survey measurements obtained in our investigation. The sectia north of South 162nd Street in the toe area of the slide has been developed as a borrow source, as previously noted. Upslope from this street the ground surface is generally quite heavily wooded, although many of the trees have fallen or have been felled after developing a severe leaning as the result of the slope movements. There are several small streams which traverse the slide zone. These streams originate from a series of springs in the hillside above the slide. The soils within and adjacent to the earthslide consist of three general types -- permeable and frequently water bearing granular sand and sandy loam soils, relatively impermeable fine - grained soils ranging from silty loam to silty clay, and firm, dense sandy clay loam or silty clay loam with gravel. Strata of the granular soils were encountered at the ground surface in the bor- ings located out of the borrow area. These strata varied from four to 30 feet in depth, and except in Boring 2, were not water bearing. In Boring 2 ground i,r- w:rs C Psi OOK:: BLSSOOOd water was encountered in the sandy soils at a depth of 19 feet, approximately the same level as a spring which flows out of the hillside immediately below the boring location. Strata of impermeable silty and clayey soils were encoun- tered beneath these upper granular soils. These soils vary from moderately firm to soft, and frequently contain thin sand layers which are generally water - bearing. These impermeable soils are underlain by layers of water bearing sand or sandy loam and firm dense sandy and silty clay loam with gravel. The slope indicator measurements indicate that the slide movement is taking place in the lower portion of the upper layer of the silty and clayey soils. The borings indicate that the dip of these soil strata generally follows the slope of the hill. A subsurface section of the general slide area extending through Borings 1, 2, 4, and 5 is presented on Plate 3. The profile is based on interpolation between borings, and should be considered approximate. In the borrow area the upper granular soils have been excavated, and in some areas the excavation is sufficiently deep so that the water bearing sand stratum which underlies the fine - grained soils is now at relatively shallow depth. This is particularly evident in the vicinity of Boring 8 where seepage from the underlying sand has kept the ground surface in a continually moist condition. The observations of ground water flow encountered as the sand strata were penetrated indicate that there is considerable uplift pressure acting on the bottom of the slide mass. A pressure head in excess of 40 feet is acting on the bottom of the silty clay layer in Boring 1. Water was observed to enter several of the borings at a rate of one to one and one -half gallons per minute from the sand layers. P0005579 1. r. '•RC'r 2• PA0C.. •• CONCLUSIONS AND RECOPMENDATIONS The results of our explorations and observations in the slide area reveal that the zone in which movement is occurring is at a depth of 25 to LtO feet below the ground surface through the central portion of the slide. It is also evident that the large quantity of water in the underlying sand strata has adversely affected the stability of the slope. The zone in which the movement is occurring is in clayey material which is underlain by a deposit of water - bearing sand, or which contains thin layers of water bearing sand. The clay,,' rjkr has become saturated and softened by long -term exposure to this ground water. The Aide movement has resulted from the combined effect of excavation and the substantial hydrostatic pressures existing in the subsurface strata. The effect of the hydrostatic uplift on the soil mass is a large factor in the stability of the soil, and the elimination or reduction of the access of greener liter into the unstable area would do much toward stabilization. We believe that effective drainage of the slide area must be the initial step•in the rehabilitation of the property. Once the water pressure is relieved we expect that the area will become generally stable. However, a period of several months' time may elapse before the soft, saturated clay soils will drain sufficiently to gain enough strength to resist further movement. In , our studies we have considered two principal methods of stabilizing the area by drainage. The first approach involves a reduction in hydrostatic pressure on the base of the moving soil mass by providing relief through drainage dawn slope from the slide. The second approach contemplates the interception of water in a series of deep wells before it gains entrance to the area in which movement has occurred. The suggested initial method of attempting to reduce the hydrostatic pressure in the hillside would be to excavate dawn to the water- bearing sand QAY t2S 8 r%oORE -6- stratum which underlies the ,lower portion of the borrow area. This work could be accomplished in conjunction with the proposed regrading of 57th Avenue South. The excavation should be accomplished so that an adequate buttress of soil is maintained along the existing alignment of South 162nd Street east of 54th Avenue South. The interception of the water bearing stratum appears to be most practical in the vicinity of Boring 8. We recommend that any such excavation begin in this area, and that adequate provisions for drainage of intercepted ground water be provided. Recommended limits for excavation in the area along 57th Avenue South are presented on Plate 4. No excavation should be accomplished in the toe area of the slide until there is substantial evidence that the move- ment has been stabilized. It is our opinion that this excavation and subsequent drainage of the sand stratum will act to relieve some of the pressure which is. acting on the slide mass. Whether it will be possible to sufficiently restrain the slope movement by this procedure alone can only be determined after a period of observation. In order to revaluate the effect that this drainage has on ground water conditions upslope, water level observations should be made in our borings at frequent intervals. Also, we would suggest that a series of survey hubs extending across the slide area be established so that changes in the rate of lateral movement can be detected. A more thorough, but more expensive, method of dewatering the slide is to intercept the groundwater before it gains access to the unstable area. In this instance we believe that such drainage could be best accomplished by installing a aeries of wells above the upper slide scarp and pumping from the water bearing strata. We estimate that these wells would have to be an average of 80 feet deep and spaced on approximately 30 to 50 -foot centers to effectively . intercept the water - bearing strata. The majority of these wells would be G6.MiCS a mooset' h1 YI. LC (SC. T8SS000d -7- located on the Robinson property. The actual spacing and number of wells would have to be determined in the field during installation. It appears probable that. from 15 to 25 wells would be required. In order to ascertain the proper spacing of wells, the initial wells should be drilled at 50 to 60 -foot inter- vals and the drawdown effect determined by observation of the water level in the borings within the slide zone. The removal of water from the deep wells would have to be accomplished by pumping, probably on a continuous basis. Submersible pumps with a capacity of approximately 30 gallons per minute would probably be adequate. A well screen would have to be installed to prevent the infiltration of sand into the pump. Utilizing information obtained from one of our drilling contractors, we estimate that the cost per yell, inoluding well screen and pump, would likely be in the range of $1200 to $1400. This estimate does not include the cost of clearing access roads or maintenance and operating costs of the system. The results of drainage of the subsurface water on the stability of the slide must be carefully considered, and definite assurance that the slide has stabilized must be evident before any earthwork within the slide area can be. started. Once such assurance is obtained the slide area could be cleared and regraded and the remaining proposed excavation in the borrow area accomp- lished. We recommend that regrading of the slide area consist of a series of terraces. Maximum cut slopes within the slide area should not excees three to one. In conjunction with a regrading program, all surface runoff should be collected and channeled around the slide area. This will be required so that the central portion of the slide area can dry sufficiently to permit the move- ment of equipment. P0005582 DAWES G T••:[: • -8- In order to lay out a regrading plan for the slide area, it will be necessary to determine the existing ground surface topography. A suggested plan of regrading in the slide area along a profile through Borings 1, 2, la, and 5 is presented on the subsurface section on Plate 3. During the stabilization and rehabilitation of the slide area refer- ence survey lines should be established, both across and above the slide area. The ends of these lines should be established well outside of the slide area, and the lateral movement of the hubs checked frequently to determine the magni- tude and rate of movement in the slide mass. A line established above the slide will provide a means for detecting any uphill extension of the movement which might develop. As a ftrther check on impending failure of the hillside, slope indicator observations in Boring 2 should be continued. These data should be plotted on a current basis and evaluated frequently so that appropri- ate action can be taken in the event that expansion of the slide area is indicated. June 27, 1961 Respectfully submitted, DAMES & MOORE By Joseph Lamont, Jr . DAMES a µoorir ♦.r�ua r..�'r rc REPORT OF SOILS INVESTIGATION EARTHSLIDE SOUTH 162ND STREET NEAR 53RD AVENUE SOUTH TUKWILA, WASHINGTON for PUGET WESTERN, INC. INTRODUCTION We present in this report the results of our investigation of the earthslide located at South 162nd Street near 53rd Avenue South in Tukwila, Washington. The location of the slide, with respect to adjacent features, is shown on the aerial photograph on Plate 1. The slide conditions shown are those which existed on April 20, 1961. The earthslide developed during the late summer of 1960, after the excavation of borrow material for filling on the Andover Industrial Park. Excavation in the area near where the slide was first noted ranged up to approximately 30 feet in depth. Following the detection of these earth move- ments, six borings were drilled under our supervision. The borings were located, for the most part, on or adjacent to South 162nd Street near the southwest corner of the borrow area. The depths of the borings ranged from 32 to 48 feet. Our conclusions, based on these initial studies, were submitted in our report dated December 1, 1960. Subsequent to the completion of our earlier explorations, the continu- ' ing movement of the slide mass has encompassed an increasingly larger area. The zone of unstable soil now extends over an area many times larger than observed after the initial movements, and has encroached on several pieces of adjacent property owned by others. Recent observations have indicated that some enlargement of the slide is still occurring, particularly at the head of the slide on property owned by Mr. S. A. Robinson. V203557 ... = z , ----:,.- • --,... " " - - ; • :-. • . ' ..... 4 -,4;i.).:-4: - .'.z....::. - - . _,.?...1.:..,:, , • - , , - ',LA- ' ,...5.,..._.-7).. :, .- - . • .", - .---- ).-:,.•:,. j - • s, ' •.:.:.e: ..•:,'--. • x,,•,. 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". .., ..4,.:.;.......„ 7.... -0,' ..7...__,...."......7.: ......=.......... . ,_--- f., _,..-• .1,.., .........._____.• • 7.m ‘,.=•-.,.:. . • - • ... ... !=-5. ,.-.• • .. • • .. 720,-,...,....„-aaar....V:Lici.....;.zz-r.',........ fat: 80....cs sic* I IKICATha • FOnikGS w.1-0.1 SLOPE IKIC•I‘i C•1.9 4. AERIAL PHOTOGRAPH OF SLIDE AREA IC? :C: • 005' DAPIII•IN • Moo. a . . • tqw,it; :,...•1151141 - " • . 4 . • 7.:1) I. ;"•••• . • ' , • . 1i • 7,1. • SURFACE CONDITIONS 7-7-56 COMPARATIVE SCALE 100 FEET SURFACE. CONDITIONS PHOTOGRAPHS MA INTAIN EATING GRADES ut57 Of T1■15 LINE w.TIL SL IDE IS STABIL 'ZED ***, 57 TH AVENUE SOUTH (PROPOSED) PARTIAL GRADING RECOMMENDATIONS BORROW AREA FEET 100 0 100 200 :31:12 3:11=611■4 aCFCRENCE: DRAwlkG 00. C.2 DOMED "BOIRROd •11, SOUln CEATCR, ANC JPACOVER." 8Y JOHN GRAHAM AW COMPANY, DA1C.0 10-14-60. poo05587 DUSS MOOR• •flik ■C• tufal A -1 APPENDIX A FIELD EXPLORATIONS AND SLIDE MOVEMENT OBS'.VATIONS FIELD EXPLORATIONS The soils within, above, and below the unstable area were investigated by drilling eight borings ranging in depth from 20 to 108 feet at the locations shown on Plate 1. The drilling was accomplished utilizing power- operated, cable-tool drilling equipment, and was supervised by one of our field engineers. The soils in each of the borings were classified by visual and textural examina- tion, and a continuous log of the conditions encountered in each boring was maintained by our field engineer. The detailed logs of the borings are pre- sented on Plates A -1 through A -6. The nomenclature used in classifying the soils is described in the Soil Classification Chart on page A -3. Undisturbed samples of the soils were obtained with a sampler of the type illustrated on page A -4 at the depths indicated on the log of each boring., The blows required to drive the sampler a distance of 12 inches are shown immediately above the sample notations on each log. In Borings 1 to 4 a weight of 375 pounds and a stroke of 30 inches was used to drive the sampler. In Borings 5 to 8 a weight of 275 pounds and a stroke of 24 inches was used. Observations of the ground water level were made during and follow- ing the completion of drilling in each boring. In Borings 1, 3, 6, 7, and 8 an artesian flow condition was encountered. In Boring 4 the ground water level rose to approximately one foot beneath the ground surface; however, no artesian flow was observed. The static level of ground water plus other information regarding the flow of ground water into the borings are noted on the boring logs on Plates A -1 through A -6. P0005589 .AP •t S C w,ocrc A -2 SLIDE MOVEMENT OBSERVATIONS In order to determine the zone of movement in the slide mass, heavy plastic casing machined with four longitudinal slots to accomodate a slope indicator device was installed in Borings 1, 3, and 4. Similar casing was also installed in Boring 2, which was located uphill above the existing slide area. This casing was installed for the full length of each boring in order to have assurance that the casing was seated in unyielding soils below the unstable zone. The slope indicator is a precision instrument used to measure the inclination or tilt of,the plastic casing in two directions. The initial series of readings, which are taken at frequent intervals for the full depth of the casing, are used as a reference line. Subsequent readings are then taken and the difference in tilt of the casing relative to the original series of readings is calculated and plotted to show the deflection of the soil mass and define the zone in which sliding is occurring. The casing installed in the four borings in this investigation was oriented so that the north -south axis would approximate the general direction of movement as determined on the basis of field observations. The results of the slope indicator readings are presented to the left of the logs of Borings 1 through la on Plates A -1 through A -la. The orientation of the north -south axis with respect to true north is also shown on these plates. OAF EEO MOOR t. A -3 SOIL CLASSIFICATION CHART PER CENT SAND (PERCENTAGES GIVEN ARE BY DRY WEIGHT) SANDY SOIL FRACTION PARTICLE SIZE LOWER LIMIT UPPER LIMIT MM. INCHES MM. INCHES .0002 CLAY .005 SILT .005 .0002 .05 .002 SAND FINE MEDIUM COARSE ROCK FRACTION THESE FRACTIONS ARE NOT IN SOIL CONSIDERED CLASSIFICATION. DETERMINING GRAVEL COBBLES BOULDERS A -11 SOIL SAMPLER TYPE U FOR SOILS 01FFICULT TO /RETAIN IN SAMPLER U f Mr' NT N0. 2.516,062 DRIVING OR PUSHING MECHANISM COUPLING WATER OUTLETS NOTCHES FOR ENGAGING J FISHING TOOL !♦ NEOPRENE GASKET HEAD Rot[: 'H[Ap [6T[N110R' WM K INTRODUCED REARM' "NERD" AND 'SPLIT SPLIT BARREL TO FACILITATE REMOVAL OF CORE S•N•LE 1 BIT CHECK VALVES VALVE GAGE SPACE TO RECEIVE DISTURBED SOIL CORE-RETAINER RINGS tE 1It 0.0 SY LONG :I CORE- RETAINING DEVICE RETAINER RING DETAINER PLATES ,I•r. (INT[RCNANAEAALE WITH OTNCI TAPES I. .9000 92. OAEh[[5 C iNSOORE 770 SLIDE MOVEMENT DATA 0(lLIC1101 1M MKS 55 1*0 • 270 10- 71 • ►0(1115 Of SOWN 11LT7 LOOM 715_ 15- ]5 ■ BORING 2 (LLTATIUM 215.1 NWT SNOT LON NO 0510MIC M *T7(1 MO OCCGRIOYI GRATIS SOOT TIM( TO Ia1DIUM Sale *ITN OCCASIONAL 5107(1 OCCASIONAL 500TS 10 . TUT 0(01. fAW 514015 711(0 710 77S MOT(. 5105( 1101CATM CASINO INSTAL. LCD 05 5.1.51, SUOSCOU(MT 5CA01K5 OM 5-5, 5.10, 5.15, 650 5.22.1 MATE MOT 5(1(61.10 AMT SIIW ICANT (GRIM MO7(5(M1 IM (175(5 A51*. 065 10— 5• • 15- 15 • 30- 105 • 760 AO —• 10 755 a5If• 150 50— 55 • 745 SS- 10 • AO 60— 15 1• • 65— 55 • 750 TO- 13 • ,115 vs- 135 • 720 iO- 705 • 715 15— 710 10— • 105 1S — ISM■ ( 200_ 100 Mg 1' 115 105— 55■ 110 10 ■ SAW 1ICOM(S •*1GR 5CASI44 AT I1 7((1 5LU15. CAA, SILT7 LOAM 5L1111* GNAT SILTT CLAY LOAM GAT SILTY LOAM GNAT SILTY CLAY LOAM Oa1110MAL L(r(5 OT GNAT TIN( To M(OIUM SAW *ITN GA7(1. 5147 S11T7 LOAM LAMAS OF WILT 51177 CLOT LOAM Nit" GRMMEL MW NATO, 1(60116 !1►( TO •10115 SAW AT S4W7 LOAN IITM GAVEL 345701 1(45",6) AT !INC SAW IrAt(5 0(GRIAS) 01115( CAS(O 10 0(575 Or 106 FM. .......... .1 16 FOOT 55th •575 M.,. r4 50 .44, •M 1•.1.5 n 31 ,75,, 1.16.61 61 f46T •SO7M *I1..5.,. •7 11 Fur .11 (••1.0 75 57 F751, 4.77.1 *4755 1S4L 47 70 LOOT MIN .IT. •..,.. 1O 106 volt 455 (LM15a TI (06 •nr, -1141 LOG OF BORINGS p0005594 w.. . Ne=■ .00. M m5 Ow, PLATF A— 2 ELEVATION IN FEET 110 105 100 95 90 •5 80— 70 65 60 55 50 45 BORING 5 ELEVATION 110 • 58 • 121 • 61 • 62 1 90 • 68 • 90 • 220 • 55 • 115 1 MOTTLED BROWN Al D GRAY SILTY CLAY LoAm WATER LEVEL, 5-18-61 GRAY SILTY LOAM OCCASIONAL POCKETS OF GRAY FINE TO MEDIUM SAND CCCASIONLL GRAVEL GRAY FINE TO MEDIUm SAND WITH °CLASH:MAL GRAVEL (WAT(R BEARING) &&&&& FLOWS 1410 101114 AT SPFOOS1 ...... 041 ASO 041 S&L, LLLLLLL Ppm misUTS., LLLLLLL STATIC 25 FOOT ..... 5.17-61, NO GRAVEL BORING CASED 70 ELEVATION 411 ELEVATION IN FEET 40 35 PO 25 20 15 BORING 6 216 • BO • GRAY FINE 70 MEDIUM SAND (WATER BEARING) ELEVATION 40 GRAY SANDY CLAY LOAM wITM GRAVEL AND OCCASIONAL COBBLES AND BOULDERS r-OLOWS REQURIED TO DRIVE SANDLER ONE FOOT 1.5 wron.T OS4 tits_ BYRE 30 INCmES ttttttt 0 ftele OF OWTO 1101614 ALT 20 01614. OAT( OF /LOW IS AFFO011 041 FPO ORO 441.4 LLLLLLL 011 464W71 AsO FLOW Ottawa, LLLLL IdS BORING CASED TO ELEVATION 21 1) 0 0 0 5 5 9 : ELEVATION IN FEET 70 65 60 55 50 45 40 35 3C 25 BORING 7 12 • 75 • ELEVATION 7a BROWh SILTY LOAM WITH ROOTS AND OCCALICNAL GRAVEL NO RIOTS GRAY SANDY CLAY LOAM WIT* GRAVEL GRAY 11N( TO MEDIU7 SAND WITH GRAVEL AND LAVERS Or GRAY SANDY C►AY LOAM (SLIGHTLY WAICR BEARING) O 440 •41a0(8 AAAAR/ , 144 AROURT INS 00411••: ILOW •RAOUILLT • [00W(• a•Tt•IAA. GRAY SANDY CLAY LOAM WITH GRAVEL (PARTIALLY CEMENTED BORING.CASED TO ELEVATION 32 130 75 W LL 70 1— • 65 60 ' 55 BORING 8 LOG OF BORINGS ELEVATION 00_ WAY SILTY CLAY LOAM •NTH OCCASIONAL GRAVEL GRAY FINE TO MEDIUM SAID Wt7M OCCASIONAL GRAVEL (WATER BEARING) IAA ►LOW OCVtlol• GRAY SANDY CLAY LOAM WITH GRAYEt (PARTIALLY CEMENTED) BORING CASED TO ELEVATION 60 B -1 APPENDIX B LABORATORY TESTS A series of laboratory tests were performed on selected samples of the soils in the zone of movement as defined on the basis of the slope indi- cator readings. These tests were directed primarily towards determining the strength characteristics of the soils, and included direct shear, unconfined compression, and triaxial compression tests. The method used in performing the direct shear tests is described on page B -2. The samples were tested at a constant rate of shearing deflection. The unconfined compression and triaxial compression tests were performed an six -inch long, two and one -half inch dia- meter samples. All tests were performed at field moisture content which, in most oases, was the saturation moisture content for the soils. The tests were performed at a constant rate of shearing deflection. The results of the tests are presented in tabular form on Plate B -1. Moisture and density determinations were performed in conjunction with each of the above tests and on selected additional samples to provide data for correlating the various strata encountered. The results of these tests are also summarized on the table on Plate B -1. B -2 METHOD OF FEF FORM I *; '; I)I -. CT SHEA AN:' FRICTION TESTS Direct shear tests are performed to determine the shearing strengths of soils. Friction tests are performed to determine the frictional resistances between soils and various other materials such as wood, steel, or concrete. The tests are performed in the laboratory to simulate antici- pated field conditions. Each sample is tested within three brass rings, two and one -half inches in diameter and one inch in length. Undisturbed samples of in -place soils are tested in rings taken from the sampling tool in which the samples were ob- ti 2. DIRECT SHEAR TESTING MACHINE tained. Loose samples of soils to be used in constructing earth fills are compacted in rings to predetermined conditions and tested. Direct Shear Tests A three -inch length of the sample is tested in direct double shear. A constant pressure, appropriate to the conditions of the problem for which the test is being per- formed, is applied normal to the ends of the sample through porous stones. A shearing failure of the sample is caused by moving the center ring in a direction perpendicular to the axis of the sample. Transverse movement of the outer rings is prevented. The shearing failure may be accomplished by applying to the center ring either a constant rate of load, a constant rate of deflection, or increments of load or deflec- tion. In each case, the shearing load and the deflections in both the axial and trans- verse directions are recorded and plotted. The shearing strength of the soil is de- termined from the resulting load - deflection curves. Friction Tests In order to determine the frictional resistance between soil and the surfaces of various materials, the center ring of soil in the direct shear test is replaced by a disk of the material to be tested. The test is then performed in the same manner as the direct shear test by forcing the disk of material from the soil surfaces. .' � • 4000 3500 3000 1 2500 N 47 at 2000 1, u W u 1500 A 1000 500 0 1 T 1 i -- -- — -- -.- -- KEY: • CLAYEY • SAND _.__ . .- ..__ --. -- I SOILS • SILTY SOILS 1 1 SOtt INLANALYSIS CV', 1•0111111b6. - IJ 5111. SOtlS i ! ✓ '' .T�— 1 I � � I�•� 1 0 500 1000 1500 2000 2500 3000 3500 NORMAL PRESSURE OR PRINCIPAL STRESS 1N LBS. / 50. FT. MOTE: ALL UNCONFINED AID TIIIAKIAL COMPOESSION TESTS WERE PERFORMED ON CLAYEY 50115. SUMMARY OF TEST DATA 4000 4500 ?0005601 5000 — -. •, • located along South 162nd Street. This line was disconnected and a temporary surface line installed. Subsequent progressive movement resulted in an enlarged unstable area which now extends on to private properties to the south and west of the property owned by Puget Western. Recent measurements made by Morrison - Knudsen Company personnel show that the slide area extends approximately 210 feet on to private property to the south of South 162nd Street, and for a distance of approximately 1x50 feet along South 162nd Street. The upper limits of the slide are near the intersection of this street and 53rd Avenue South, and are readily defined by a scarp which is now eight feet or more in height. In addition, the west side of the slide zone extends for an unmeasured distance north of r.� South 162nd Street. On the basis of field observations, we estimate that significant movements have occurred for a distance of approximately 150 to 200 feet in this direction. A scarp four feet or more in height has developed on the east side of the slide on private property south of South 162nd Street. As movement has occurred during the past 30 to lay clays, several trees have fallen in the upper central portion of the slide area, and several others are leaning badly. Telephone and power lines were moved in late October from along South 162nd Street to prevent damage by falling trees. Field observa- tions have not revealed any movements farther upslope from the major slide scarp, although some widening of the slide area has occurred along South 162nd Street on the southeast side of the slide. At our request, a series of hubs ;:ere established across the slide area on South 162nd Street by Morrison- Knudsen Company personnel. Their observations reveal a lateral shift of the rubs of as much as five feejn_the period between October 21st and 31st. Although additional survey data have not been received concerning the continued harnitude of slide movements, an inspection by one of our engineers on November f nnncnnnd 15 revealed evidence that the earth movement is continuing. SUBSURFACE CONDITIONS aInE AREA Exploration of the subsurface conditions in the slide zone was initiated a short time after earth movements became evident. At that time the major movements which had occurred were generally limited to the area along South 162nd Street adjacent to the borrow area. As a result, the majority of the borings were located in this zone. Borings 1 to 6 were drilled at the locations shown on Plate 1 to explore the soils in the slide mass. These borings revealed an upper stratum of sand and sandy loam rang- ing in depth from one and one -half to seven and one -half feet. In the borings drilled on the roadway, this material generally appears to be fill which was placed in grading the street. South 162nd Street was once a corduroy road, and several logs were encountered in Boring 3 and also just below the surface of the roadway in the vicinity of Boring 6. The soils underlying the upper stratum of granular material consist predominantly of silty clay which frequent- ly contains pockets and lenses of sand. The upper 10 to 15 feet of this clay stratum is generally softer than the underlying clay. All of the clay is saturated with water having gained entrance through both surface and subsurface flow. A stratum of water - bearing sandy Loam was encountered at a depth of 40 feet in Boring IL. From observations of seepage which has occurred farther downslope in the borrow area, it appears that this stratum may be carrying appreciable water. TUBE BORROW SOURCE - SOUTH 162ND STREET Boring 7 was drilled at the location shown on Plate to explore the future borrow source on South 162nd Street east of 54th Avenue South. This L09S000d • • -5- boring revealed strata of sand, sandy loam, and fine- grained clayey soils : :j! tar to but somewhat firmer than those encountered in the other borings ririlled to the west. Below a depth of approximately 27 feet the silty clay and sandy loam strata are saturated. Plater was noted entering the boring at a rirlIth of approximately 52 feet. Although the boring walls caved upon removal casing and thus prevented accurate measurements, the water table is believed i.' at a depth of approximately 45 feet. Additional information regarding the field explorations, including ! !.'• detailed log of each boring, is presented in Appendix A. Water level i-: ' :'.rvations were obtained in most borings and are indicated on the boring logs. CONCLUSIONS AND RECOMMENDATIONS .' AREA SLIDE On the basis of our investigation and analysis of the slide, we : udf: that the basic cause of the earth movement was the removal of toe ' .:r!ance by the excavation which had been accomplished in the borrow area. r.; as a 30 to 40-foot depth of soil has been excavated at this location. '• : da'-a showing the original grades and the grades in mid- October in this •' AN. r;i•c::n on Plate 1. Another factor which we believe has contributed z..r..:r!phntly to the instability of the hillside is the presence of subsurface ro all the infiltration of surface runoff. 'urine the course of the investigation our recommendations for '` .•" :1,asures to aid in the stabilization of the slide were presented in • 'rr p «• ,., ,eptember 29, October 14, October 19, and November 1, 1960. Ter {-;,. •P%-.pions were reviewed and up -dated during this period as the ' a ' n1 :cor :plished and also to reflect the changing conditions of the I I ti '::len the slide area was first defined in its more limited `:onsidered desirable to develop both surface and subsurface DM W:C C ri CSUr L•' -6- drainage. As the slide movement progressed over a larger area, the need for all increasingly extensive subsurface drainage system became evident. We present on Plate 2 a typical cross - section through the slide !wee. The orientation of this cross - section is shown on Plate 1. The exist - ink rround surface line shown upslope from the Puget Western property line is based cn visual field observations. Accurate contour data are not available !'-r this area. Field evidence indicates that the slide is basically a rota - t:ora= -type failure.. The failure arcs shown on Plate 2 indicate the probable Irc',-ression of movements up the slope. We estimate that the major failure arc :':•r the overall slide is approximately as shown, although variations in sub- -::r !'a•:e conditions would likely distort this location. Some of the recommendations presented in our earlier letters have 1e..•:: parried out. These include the installation of a corrugated metal ditch alonr South 162nd Street d a pipe along the west side of the borrow a: �• ; c, carry the major portion of the surface runoff away from the slide area. i•'rt :on of the surface runoff flowing across the upper portion of the slide 4•04 tc::ard South 162nd Street from adjacent private properties has been in lateral pipes and carried to the lined ditch. The slope adjacent i.•roposed Slade Way right -of -way has been regraded to a flatter slope to 1oQalized zones of fractured soil. The aforementioned line of survey ::''- also established to observe the rate and magnitude of slide movements. 0.1,11tion, Morrison- Knudsen Company excavated the soft, saturated soils along Street west of the easterly limits of the slide area, and placed a 1l in this zone in an effort to buttress the hillside. A 8:ation and backfilling procedure has been accomplished in an area ' t' i Street near 53rd Avenue South which had subsided below the level h ' 4 `' "" 1 rround surface to either side. While the placing of a rock n rr: c -s f: Ile: C, r.: -7- buttress would be expected to have a beneficial effect in restraining the origin- ally observed area of movement, it will have only a very limited effect in restraining the larger slide mass now in motion. The stabilization of a slide mass as large as this involves consider- able work and expense. The major effect of the drainage facilities installed thus far has been to reduce the amount of surface water gaining access to the slide area. The continued seepage from the toe of the slide indicates that there is also considerable subsurface water in the unstable area. One means of increasing the stability of the slide mass is to intercept and remove water .:hich has access to the unstable zone. The removal of this water, both surface and subsurface, will reduce the forces tending to cause movement of the soil. In order to collect further surface runoff effectively, pipes could be installed from the lined ditch along South 162nd Street across the upper portion of the area to the slide scarp. An alternate means of intercepting and collecting this runoff would be to construct a ditch around the periphery of the area. We recommend the former means as being more easily accomplished at less cost. A more important feature in stabilizing the area, however, is the drainage of subsurface stater. We recommend that this be accomplished by pump- ing from a series of deep drainage wells. Subsurface drainage should be installed both in the toe area of the slide and around the. periphery of the upper slide scarp. We recommend that the initial installation of subsurface drainage facilities start in the toe area of the slide in the vicinity of Boring u where wells should be drilled into the water - bearing granular stratum encountered in this boring. In order to effectively plan the installation of deep drainage wells above the slide scarp, we recommend that at least one exploratory boring be drilled to define the location of the water - bearing strata. It is likely that such a boring could be subsequently used as a drain- age well. The spacing of such wells cannot be evaluated with the information sv:r;3- : t; .7."! r. -8- now on hand, but it is probable that at least six to eight would be needed. The effect that this drainage has on stabilizing the slide can be best measured by resuming readings on the line of hubs which were placed along South 162nd Street, or if necessary, establishing a new line for subsequent readings. Because of the large amount of material which has been removed from the toe area of the slide, it is problematical whether a sufficient reduction in the driving forces which are acting on the soil mass can be accomplished by drainage alone so that a permanent stable condition will be attained. In addition to the drainage, it may be necessary to re- establish toe resistance by placing fill in the southwest corner of the borrow area. We recommend that this possible action be deferred until the effectiveness of the drainage program can be evaluated. Regrading and backfilling of cracks which have opened on adjacent private properties will be required. We suggest that.this backfilling be accomplished as soon as possible after the slide mass has been stabilized. Because of the shear faces which have been exposed, we recommend that re -slop- ing, backfilling, and compacting be accomplished to fill the tension cracks along the slide scarp and within the slide zone. Large trees should be left in place insofar as possible; however, trees which are leaning badly should be felled. Areas which are denuded in the regrading process should be reseeded promptly. The placement of fill on the upper portion of the slide zone should be avoided. No further excavation should be made at the toe of the slope. The stabilization of the slopes along the proposed Slade Way right - of -way are contingent upon stabilization of the overall slide. Once earth movements have been arrested in the slide area, the roadway may be safely developed. We understand that Slade Way will probably be relocated a short distance farther to the east than is shown on Plate 1. We recommend that this .71 re. V.. = n: c. or. -9- be done so that the roadway will be outside of the slide area insofar as possible. When Slade Way is constructed, permanent drainage ditches should be installed adjacent to the roadway. We recommend that the ditch on the west side of the roadway-be lined so that runoff from the adjacent hillside will not infiltrate beneath the paving. r 0P0SED EXCAVATION EAST OF 511TH AVENUE SOUTH In a recent action by the Tukwila Town Council the portion of South 1•:2nd Street between 511th and 57th Avenues South was vacated. It is planned tic excavate as much as possible of the ridge on which this street is located. : :e have analyzed the stability of the cut slope which would exist if all soil in this zone were removed to the present slope of 12 (horizontal) to 1 (vertical) wh1 rh exists in the adjacent borrow area to the south. We conclude that the of the private property located to the west of 511th Avenue South ti:r::1d be endangered if this amount of soil was removed, particularly in view of :`.c• movement which has occurred farther upslope. The ridge of soil under this ;: tion of South 162nd Street serves as a buttress to the hill above. In lieu of complete removal of the soil in this area, we recommend a restraining mass of soil be left in place to provide protection to the ':i•lride above. For the excavation to be made in the next construction season, • re:coyT:end that the existing surface slope on the soil ridge be maintained 1"r a distance of 100 feet east from the top of the adjacent cut slope on the ": side of South 162nd Street, and that the face of the new cut be established • 11 slope of 2 to 1. The excavation to the east of the toe of this slope may '0 to the full proposed depth. No excavation whatever should be made in u:'r•a during the current winter season, or until movement of the slide to rn 162nd Street has been restrained. P0005612 11.0 nti T. t.. P-. 00 r. r': : . -10- Our computations indicate that some possible further excavation might be made in this area if the hillside to the west is adequately stabilized. However, we cannot recommend that any excavation be made beyond the limits outlined until we have complete assurance that such stabilization is effective. Respectfully submitted, DADS & MOORE By / December 1, 1960 Joseph Lamont, Jr. P0005613 i.74 t :. !; MOO r: 57 IN AVENUE sour./ (PROPOSED) �- - -- STING) f \ SOUTH I [XI \\- AREA 80RR() 7; I \ i / '67 1 l 77 ISO 160 NO 404; , to // i / , y J i / ,.- 54 TN AVENUE SOUTH (EXISTING) • ...JAII 15460 / �BO 1T0- , A- PROP£RT. LINE / / eo 20 APPRQX/MAT( l 60 covrOUR DATA NOT AVAILABLE TOR MIS AREA 1 JJRO. AVENUE SOU1IE 1511 G KEY• _ OR/C•ML CONTOURS IN BORROW AIWA ((/STING COAT0URS NOT(• .A(u ( or ILIA Sd!S(Lt I.4 •/ON 00. CS. cf.-ENENCE• 0007,0 • Or DRAWING SA.1,JO8114E. Of .1 NA• CNANAM AND CO, DAr£D S.8•EC •PPRO>z,MATE LIMITS Or SLIDE AREA ALIGNMENT OT 1 SURVEY NUBS ..� ESTADILISNfO Or MoRR/SON• NNUDS(N PLOT PLAN FEET 100 0 100 200 DlN(CTKAV or GENERAL £ANTN MO✓fM£NT P0005614 DAMES 8 mows ••1a UICM••IC• •.• • B -? METHOD OF FERFOSM.I W; DIRECT SHEAR AND FRICTION TESTS Direct shear tests are performed to determine the shearing strengths of soils. Friction tests are performed to determine the frictional resistances between soils and various other materials such as wood, steel, or concrete. The tests are performed in the laboratory to simulate antici- pated field conditions. Each sample is tested within three brass rings, two and one -half inches in diameter and one inch in length. Undisturbed samples of in -place soils are tested in rings taken from the sampling tool in which the samples were ob- DIRECT SHEAR TESTING MACHINE tained. Loose samples of soils to be used in constructing earth fills are compacted in rings to predetermined conditions and tested. Direct Shear Tests A three -inch length of the sample is tested in direct double shear. A constant pressure, appropriate to the conditions of the problem for which the test is being per- formed, is applied normal to the ends of the sample through porous stones. A shearing failure of the sample is caused by moving the center ring in a direction perpendicular to the axis of the sample. Transverse movement of the outer rings is prevented. The shearing failure may be accomplished by applying to the center ring either a constant rate of load, a constant rate of deflection, or increments of load or deflec- tion. In each case, the shearing load and the deflections in both the axial and trans- verse directions are recorded and plotted. The shearing strength of the soil is de- termined from the resulting load - deflection curves. PO Friction Tests 0 In order to determine the frictional resistance between soil and the surfaces of rn 0 various materials, the center ring of soil in the direct shear test is replaced by a ° disk of the material to be tested. The test is then performed in the same manner as the direct shear test by forcing the disk of material from the soil surfaces. DFLML + t: r:0c::1 c' ELEVATION 320 -• 300 260 260 —' 240 — EAST 220 — 200 — — 160 -- — 160 --- 140 — — BORING 5 120 80 60 BORING I /S. 162 ND STREET / KEY: �� BORING I 5. I62 ND STREET SUGGESTED PROFILE FOR REGRADING SLIDE AFTER STABILIZATION BORING 4 SLIDE SCARP SLIDE SCARPS • APPROXIMATE LOWER LIMIT OF SLIDE MASS SUBSURFACE SECTION A -A FEET 10 5 0 10 20 30 40 50 BORING 4 SLIDE SCARP WE ST BORING 2 (OFFSET 85' SOUTH) 320 � to ':�, eJ ? `•i • t. • • iy5►.r "Y;: -it I 300 2e0 260 240 220 P0005586 2410 180 160 140 120 100 eo 60 ELEVATION 114 FEET oAMNS i MOOR■ •Ota NeCM•MIC• Snell PLATE WE ST SDR;NG 2 (OFFSET 85' SOUTH) . ;ARP • t • r: 320 300 280 260 240 220 P0005586 20.0 180 160 140 120 100 eo 60 ELEVATION IN FEET OAMM• • MOOR el Sou M. $N31 PLATE (i) TEST PITS (05/15/k) + HAND AUGERS (05/23/90) FROM TOPOGRAPHIC SURVEY BY SURVEY PROFESSIONALS & FROM SITE CASCADE GEOTECHNICAL, INC. 12919 N E. 126TH PLACE (206) 821.5080 KIRKLAND, WASHINGTON 98034 FAX (206) 823.2203 Job No. 905-13G LOCATION Dal• 05/29/90 ATTACHMENT D • SLIDE MORTM._EOUTM AXIS MOVEMENT DATA DEFLECTION IN ACMES ♦ •1 0 _2 _A I , 1 J 1 MORTM SOUTH I� I 1 i -r_ ---► 1 i f f I• 1 I . 4-27_61 / f AT i �4•-2f-f1 L --4-25-EL •:00 /�I A... i I I 1 5:00 A... S:00 A.■. ` I T a 4- +- . — — - I I I SMEAR ZONE—, . -f i .....„.*. . i • ` t I • ..,. 1 --- r 17 • T MOTE SLOPE 1ADICATOR WAS 4-24-51. 16171AL •EFEREMCE TAU* AT 3:C0 M.N. INSTALLED 0M READIACS 'I , ' I - AFTER 4.27-61 DEILECTIOM OF CASING -��— IA SMEAR 20AE PREVENTED TMCTAIIMG OF ARV RU•TNER READIAGS. THE TRUE SEARING OF THE MORTM- •OUTN 4115 IS M l0• E. 1 _ I 1 1 I �� l j I NOTE: ELEVATIONS ARE SASEO OW U.S.C. AND O.E. DATUM OF 122I. M.E.L. - ELEVAIICI 0.0. -2 WEST_.EAST 411E • •1 LEST EAST 4-27_- 61 .AT I • :25 4 i AT 1:00 A... 175 -- •2 0 170 — SLOWS REQUIRED TO DRIVE SAMPLER ONE FOOT WEIWI • )S0 Les..STROBE - 24 ACHES I I11C4TES MISSED $PLE 24 maul=IJ K1774 /T IMCN r•Rv1TTRK1 UWL( Ins [I71t,C71E1 LOG OF BORINGS 145 — DE►TN IN FEET 5-- 13 ■ 10 — ibo — 24 ■ I5- 155— 24• 20 — 150— 24• 25 — 145_ 24■ 1♦9— 13• 35 — 135 -- 20 • 40 -- 130 —.- 1S ■ 125 uafg 120 77 SD Z SE — I I S --- 2944 110 �• 105 i — ( BORING I ELEVATION 171.0 SAWN SANDY low WITH GRAVEL MOTTLED •RNA AID 1A*T SILTY CLAY LOA. WITH OCCASIOML GRAVEL GRADES TO ILUJS$ GRAT 10. COLOR M0 GRAVEL GRAY S1L7r CLAY OCC ASIOAAL LAYERS OF GRAY FINE 70 11ED I UM Sup GRAY fINE TO MEDIUM SAND WITH OCCASIOA GRAVEL (WATER SEARING) MAY SANDY CLAY Low WITH GRAVE*. AND LAYERS OF WA7ER_3(W LAG GRAY FINE TO "-MEIIUM SAND Al) WAVES 1•T(•1A■ new ••T10 A, T1• mad/ WA• 315. •.1.ris.y oft* CA41.4 AT 45 I(1- DECREASI*C SAND LAYERS ARAT FINE TO MEDIUM SAND WIT. GRAVEL (WATER SEARING UNDER ARTESIAN MESSDtE GRA1 :Awl' CLAY LOAM WITH (11411TIALL7 CEMER7E01 110100 1111T lb 00.1011 T• A 11.T. •, 5. 111T /.A 1 .1T011AT1.• ., •4.•14 1.01C1• 1♦• 1100 •1 WATT. 1••T.W/0. 0 2 Et- O fA v w 4 4 31' s O • W M J i 113i NI 11113O J. li NI NOI1VA313 W 0 J N • • YJi =J 4 Q� N N / • 3: 8g 1 3 43 .. ~ .. ; 4 .,4 • - 8. 3 S 4. oh A Si MW ':"41 .' l „'.I.IJ4"1" fif/Isof rP0:4f �f 1: Lrlr g. /iP4aaa" IeftrO Ao. •.-,JJ∎ 1./ir/org•100111/ ✓. ": igJJJ01MyorioP. ., 0 :•.:41^ • 61. 1, g4 .`''51 ., ,i.r,. ✓ w5'".r 'le�• ',(4r NO V&TER ENTER INC SOIINC LAYERS OF GRAY SILTY LOAM ► 09..$ 41410 ■r. • • Yt; ■ ..■ .:L 041. ► • O 3 0 2 0 0.o • r = Y • Y • O • ► • ▪ O • • O. f • r • • d •: ■. Y • • • •era • r J Y ▪ ■ . r�■ - i ? A ▪ ► `.if I µ' r --i r n 1 2 1 1 I Ig r r 0 0 w a 4) r • • _•� • • u• 4a n,, H.. n M 0 al N I 1 Illwommmigalmmammft 0- • oft • f Sr • i • — I e IZ J w • t a 0 A • VI 11 • • 41 � ) .111 Intl • • • • O O U O O N 411 1.1 al 1 (9N10311 1131r11) ONrS 3 ,4I1 SOAn CM'S NI VW* O■ 1101 AV13 A11IS M101 A1lIt i w a rat NOILVA31 r • • u ELEVATION IN FEET DEPTH IN FEET • • 0 X) U) r 0 : (1) 1) C;) rti fz) -I III (i) U) 0 la) ill ri NI rtt CI "-i 0 -c Nr011 f-- 0 —.1 •c r. m Z _ (r) 0 rtl r- 11 43 .1 ---, 0 ” Z 1. r- r-• ill Li) > 0_ 1 ro r/1 r11 h' -6 1 Is Is r-■ o 11 Is 43 II. 111 cl CI -1 IV' IZ) 0 U) 0 r Is 7-1 0 .0 ELEVATION IN FEET 0 0 rn NICO)). 173 rri O 0 0 ELEVATION IN FEET y O O O N 0 0 ELEVATION 114 FEET N 0 LG3 -oac03 5,1(00 sLADt vvy LEROY C.. Lo WEE City of Tukwila John W. Rants, Mayor Department of Community Development Rick Beeler, Director City of Tukwila PUBLIC HEARING NOTICE Notice is hereby given that the City of Tukwila Planning Commission will be holding a public hearing on October 14, 1993 at 8:00 p.m. in the Council Chambers located at 6200 Southcenter Blvd. to discuss the following: PLANNING COMMISSION PUBLIC HEARING CASE NUMBER: L93 -01 -APP, (90- 13 -BLA, 91- 3- APRD): Hillcrest APPLICANT: LeRoy Lowe REQUEST: Appealing a decision by the Department of Community Development for slope stability requirements in the Sensitive Areas Ordinance (Section 18.45.080 (E)). LOCATION: Slade Way & S. 160 Street - The site, which has 50 feet of frontage on S. 160th Street and 300 feet of frontage on Slade Way, is an irregularly shaped tract having overall dimensions of 300 feet east /west by 500 feet north /south. CASE NUMBER: L92 -0064, L92 -0065, L92 -0014 APPLICANT: Dujardin Development Company REQUEST: The Planning Commission will formally adopt their findings and conclusions based upon their decision for the Fosterview Estates project. Formal adoption of their findings could either result in reconsideration of or modification to their recommendation to the City Council. LOCATION: Immediately south of Southgate Park between 42nd Ave. S. and 43rd /44th Ave. S. Persons wishing to comment on the above case may do so by written statement or by appearing at the public hearing. Information on the above case may be obtained at the Tukwila Planning Division. The City encourages you to notify your neighbors and other persons you believe would be affected by the above item. Published: Seattle Times October 3 and 10, 1993 Distribution: Mayor, City Clerk, Property Owners /Applicants, Adjacent Property Owners, File. 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431 -3670 • Fax (206) 431-3665 PLANNING COMMISSION MINUTES NOVEMBER 18, 1993 ? - Inaudible word or people Vern Meryhew called the meeting to order at 8:00 p.m. Members present were Messrs. Meryhew, Malina, Flesher, Knudson, Haggerton, and Clark. Mrs. Craft was excused. Representing the staff were Jack Pace, Diana Painter and S lvia Schnu'. I'd like to suggest one change. Back on page 14, about the fourth paragraph from the bottom, where it says "in addition ". Fourth line up from there, second word in, where it says "inaudible ". I'd like to replace "inaudible" with "waiving any claims ". I'd like to also change "deep seeded" to "deep seated ". MR. HAGGERTON MOVED TO APPROVE THE MINUTES OF 10 -14 -93 AS AMENDED, MR. KNUDSON SECONDED THE MOTION AND THE MOTION RTAS UNANIMOUSLY APPROVED. L92 -0084: Blue Star Motel: Ms. Painter- This project appeared before you in June of this year. One of the major revisions was that the applicant can turn the project over to the registered architect or engineer, and make some specific design changes and engineer changes to the project. Some of the specific reconunendations that you made in addition to Living to draw up plans, is that the on -site storm water detention system be revised and the underground system installed. That has been done with a number of revisions to the site plan. One of the major changes that improves the appearance of a project to a great degree, in my opinion, is the change in the roof form. The roof was changed to be regular and it's no longer a massive detailing; it has a much more gradual and graceful appearance. Stepping down toward the west elevation on Highway 99 and stepping down toward the back of the site. Another suggestion we had is That additional landscaping be provided around the east end of the site. This will help screen this area from the multi family neighbors to the backside. The original handicap elevators have the housing that was apparent from the street. There were some kind of awkward roof form that we made suggestions on changing. As you can see, the public view is greatly improved with additions of windows and roof form. We were very pleased with the amount of original modulation that the architect and applicant were able to provide in the submittal. Colors were a bit more subtle, there is texture, and a contrast of materials. That, combined with City of Tukwila John W. Rants, Mayor Department of Community Development Rick Beeler, Director PLANNING COMMISSION MINUTES OCTOBER 14, 1993 Mr. Meryhew called the meeting to order at 8:00 p.m. Members present were Messrs. Meryhew, Knudson, Malina, Haggerton, and Mrs. Craft. Mr. Flesher was excused and Mr. Clark was absent. Representing the staff were Jack Pace, Gary Schulz, Dan Clayton (consultant) and Sylvia Osby. In approving the minutes of September 23, 1993, the Commission addressed the second item on the agenda regarding Dujardin Development Co. L92 -0064, L92 -0065 and L92 -0014. Mr. Haggerton noted that on page 2, the last sentence of paragraph 6, he wanted the sentence changed to read, "Mr. Haggerton said that it doesn't seem staff has as many concerns with the project as he did ". In addition, Mr. Malina stated his rationale for his motion as written in the September 23, 1993 minutes. He said the reason for reducing the number of lots from 41 to 39, falls under the same direction of the minutes of the Vision Tukwila group. In their minutes they were also considering reducing the required lot size from 7200 sq. ft. to 6500 sq. ft. He added that he felt that a lot size of 3954 sq. ft. is too small. Following the minutes from the visioning process, he felt it was very important to increase the lot sizes and try to stay close to the 6500 sq. ft. size. Mr. Malina clarified the motion as follows: FOLLOWING THE DIRECTION OF '111E VISION TUKWILA MINUTES, OF DECREASING THE 7200 SQ. FT. REQUIREMENT TO 6500 SQ. FT., HE FELT IT WAS J PORTANT TO TRY TO STAY AS CLOSE TO THE VISIONING PROCESS AS POSSIBLE. He added that to him, it was inappropriate to have any lot in the City on a PRD that is less than 3900 sq. ft. MR. MALINA MOVED TO APPROVE THE MINUTES OF SEPTEMBER 23, 1993 AS AMENDED AND CLARTFI ED. MR. KNUDSON SECONDED THE MOTION AND THE MOTION WAS UNANIMOUSLY APPROVED. 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665 Planning Commission Minutes Page 2 October 14, 1993 L93- 0001 -APP, 90- 13 -BLA, 91 -3 -APRD: Hillcrest Appeal Jack Pace presented the staff report. He stated that the Sensitive Areas Ordinance requires all appeals regarding the SAO to go before the Planning Commission. There are three factors that need to be addressed when appealing a staff decision. The Commission needs to look at the intent of the sensitive areas ordinance, the technical information provided and when making their decision, they need to give substantial weight to the Director's decision. He continued by saying that this project is before the Commission because the proposed site is on Class 3 and Class 4 slopes which require a geotechnical study. The issue which needs to be resolved between the applicant and staff is the scope of the requirements in the SAO. He then entered into the record a copy of a letter submitted by the applicant after the packets were mailed out. LeRoy Lowe: Mr. Lowe introduced George Lamb, Neil Twelker, Dick Stuth, A.J. Breadberg, and Dick Osborne. On May 21, 1990, the City Council and Mayor unanimously voted to lift the moratorium on this project because it was the kind of project the City wanted. George Lamb, 12016 115 Ave. NE, Kirkland: He stated that his company, Cascade Geotechnical, has prepared a number of reports on this property. Their initial study of the site determined that it could be safely developed, and they performed additional studies regarding the permitting situation. They also performed a slope stability study. He said they concluded that the property is stable now and the proposed improvement will improve both the local and regional stability. The problem which occurred on Slade Way in 1961 was stabilized by a DOT drainage program. Over the past 30 years, the drainage has become clogged and the failure of that drainage caused some reactivation on the slide, which is moving to the northeast. Left unattended, with no improvement, one would expect that this movement would impact Slade Way. It is speculation to think how this movement would impinge beyond Slade Way. He said they have repeatedly analyzed the stability of the slope itself and repeatedly been advised that what they should be addressing is the stability of the property beyond Slade Way which doesn't belong to Mr. Lowe. The site is now stable, the proposed development will improve that stability both on site and down hill. To impact the Hillcrest development, Slade Way would have to fail completely. Neil Twelker, 5645 42 Ave. W., Seattle: The critical question that has arisen is, do we need more investigative effort. A great deal can be learned from the City's consultant, Shannon & Wilson. Their report indicates that the slope below Slade Way has been the subject of intensive geotechnical investigations since 1960. It also indicates that remedial measures to stabilize the hillside were accomplished by WSDOT. These measures consisted of regrading, surface drainage, and installing a system Planning Commission Minutes Page 3 October 14, 1993 of vertical and horizontal drains. These measures were effective in lowering the artisan pressures and stabilizing the slide at that time. There is no dearth of information on this slide, there is no dearth of knowledge or agreement as to what could be done to correct it. Mr. Twelker said that they have put in borings within 100 -200 feet of the Hillcrest property. Dick Stuth, 13232 138 Ave SE, Renton: Mr. Stuth said he is not a geotechnical engineer and that his involvement deals with the utilities and the drainage of the site. He added that this is nothing unusual, a system can be designed to accommodate any storm event that the City would require. Everything that can be done in the way of storm drainage will only enhance the site and the stability of Slade Way. When the City hires a consultant, they hire them with the hidden agenda of how to say "no" in a technical way. Anthony Breadberg, 5800 Soundview Dr., Gig Harbor: He stated that he did the wetland study for the property. He said that the appellant is minimizing the impacts on the natural values and functions of these areas. There is an existing 5 lots and they are trying to re -align the lots to minimize the impacts. They can prevent erosion, the loss of slope and soil stability. They are planning on de- watering the slope to protect the slope for the Hillcrest subdivision. They need to get Hillcrest out of the way and get on to the real issue of protecting Slade Way which has nothing to do with Mr. Lowe's project. He added that several deep- seeded analyses have been completed in the area. He said that there were two professional engineers who are willing to put their reputations on the line to say that these houses will stay. He said that he found it difficult to believe that a recommendation could be made to not accept this information by very esteemed engineers. Dealing with the wetland issue can help the slope stability. The Planning Commission or the City of Tukwila has the option of dealing with the wetland issue for public safety, these wetlands can be exempted from the Sensitive Area Ordinance. Mr. Haggerton asked how much time Mr. Breadberg has spent on this property. Mr. Breadberg said six to eight hours on the property. Mr.. Knudson asked how long the consultant's guarantee or insurance is good for. Mr. Breadberg said that Cascade Geotechnical will be around for a long time and they will have liability insurance. Mr. Twelker said that as a practical matter, as long as one is alive, they are liable for any defects designed into a project or anything they have sealed. There is a new law that says that if one designs a building and they find a defect, but they do not file a litigation against the architect within the first six years, then they do not have a right of remedy. But if they do not know about the defect, they have the right of remedy forever. Planning Commission Minutes Page 4 October 14, 1993 Mr. Lowe said that as a practical matter, one is on the hook forever for something that they put their seal of approval on. Dick Osborne, 21805 .NE 1st, Redmond: He stated that he is a licensed real estate agent. He added that the stability on the slope has an enormous impact on the adjacent property owners and all of the property owners in the area who use Slade Way. Washington is a full- disclosure State and a seller of a home must disclose any and all defects that have to do with that property that might influence the decision of a potential buyer. If Mr. Lowe is not allowed to participate in the stabilization of Slade Way, the instability of Slade Way will be a serious factor in the marketing of anyone's property who uses Slade Way as access. Also, anything affecting the use of the property has to be disclosed as well. Mr. Meryhew asked if any existing properties in the neighborhood have covenants that state that there is a potential for sliding. Mr. Osborne said that he is not specifically aware of what the covenants and the easements, etc. are in that particular area. He added that full- disclosure is retroactive. Mr. Lowe stated that to render a professional opinion on the prognosis of stability of a piece of property is one thing. To deliberately ignore a direct threat to a community by withholding a known remedy is another thing. One is a simple error of judgement, the other a willful derelict of duty. Jack Pace said the Planning Commission and Council had lengthy deliberations on the Sensitive Area Ordinance. This provides a framework for how staff is supposed to process applications, and the criteria for which decisions are made. There are two key parts; the alterations and what is needed regarding a geotechnical report. These are the key elements staff is using to make their decisions. The issue before the Commission is whether they are interpreting the ordinance correctly. Dan Clayton, Shannon & Wilson, 400 N. 34 St., Seattle: Mr. Clayton said that he had been retained by the City to provide an objective peer review of work that had been done as of September 1992 by Mr. Lowe's consultants. That consisted of two geotechnical reports, subsurface investigations of the property itself, and some recommendations for design and ultimate development of the property. In that review, he looked not only at the work that was provided by Mr. Lowe, but also at a number of other studies. The most significant of those involved the hillside below Slade Way and below the Hillcrest development, specifically, a number of studies that were done by various firms, including Shannon & Wilson, beginning in 1960 on the overall slide which occurred in 1960 and 1961. As a result of those various studies, they came to a number of conclusions. Specifically, according to the SAO, the property itself falls into a Class IV area Planning Commission Minutes Page 5 October 14, 1993 of potential geologic instability, meaning landslides. The parameters of the SAO are slopes of 25 degrees or more, seepage on the property, evidence of previous landslides, evidence of landsliding in the near vicinity, all of which have been identified on the Hillcrest development. Specifically, the back -slope is over 25 degrees. A number of consultants have identified that slope as a landslide scarp. In reviewing the subsurface investigations that were performed on behalf of Mr. Lowe, they are inconclusive because there was no attempt to determine if there was a landslide or not. There is some evidence that is suggestive that the interpretation by these various consultants in the past, and Shannon & Wilson, is correct. The interpretation of a scarp being behind, and a slope passing underneath, qualifies the site as being potential landslide materials. There is no indication as to whether they are or are not landslide materials in the Cascade reports and the work that had been done for Mr. Lowe at that time. Down the hillside, there is plenty of evidence of landslide in the vicinity. The requirements of the SAO say that "...prior to permitting alteration of an area of potential geologic instability, the applicant must demonstrate one of the following: 1) there is no evidence of past instability or earth movement in the vicinity of the proposed developments; and quantitative analysis of slope stability analysis indicates no significant risks to the proposed development or surrounding properties. 2) the area of potential instability can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased and the increase in surface water discharge or sedimentation shall not decrease slope stability." As far as modifying it to decrease the potential impact, Mr. Clayton said that he would contend that one would have to understand what that potential impact is before you figure out how to modify it. With regard to decreasing the slope stability, he agrees that this development is not going to diminish the stability of the hillside. And with respect to shallow landsliding, the dewatering of the property should increase the stability. There are two issues: 1) there is the potential for a shallow landslide and; 2) there is the potential for a deep landslide, comparable to what has occurred down the slope. Mr. Twelker has outlined where Shannon & Wilson's postulated landslide is, and no evidence has been offered to argue that landslide is not there. With respect to the retreat of the bluff itself, there are some peats on the property which would argue that is probably not the case, it probably is a landslide scarp. It's up to a developer to determine whether it is or not, and that has not been done. The development will increase the stability with respect to the shallow landslides. What is done in the upper ten feet has very little to do with how the deep stability will behave. The evidence from down slope is that that stability is going to diminish as the dewatering system denigrates and if that's the case, the stability will decrease. Planning Commission Minutes October 14, 1993 In conclusion Mr. Clayton said that he was hired to address the Sensitive Area Ordinance and whether or not the project has met the requirements of that ordinance and with regard to a Class IV landslide area, they have not been met. What is required is to look at.the deep- seeded slope stability of the property itself. Mr. Meryhew asked where in the SAO does it get into the deep seeded slope stability. Mr. Clayton said that he has provided an interpretation that there is a potential for deep - seeded sliding. Mr. Haggerton asked if there was any question in Mr. Clayton's mind that this is a Class IV wetland area. Page 6 Mr. Clayton said that there was no question about that.' Mr. Knudson asked if dewatering the site would decrease the potential for a large slide. Mr. Clayton said that the interpretation of the deep- seeded slide is that a large driving force behind that was hydrostatic up -lift on the soils. The reason why the well system was effective was because it lowered the water by 50 -60 feet, removing the up -lift pressure on the slip plain. For the up -lift pressure to exist, there has to be a confining layer between the surface and where the slip plain is, and its 30 -50 feet in the deep slide plain. Effectively, the water that would be removed, the improvements that would occur on the Hillcrest development, are going to happen in the upper sand, which is not a confined layer and it would be surface water off the hillside. That takes a little load off the hillside, but not a lot. He continued by saying that he doesn't see any evidence to indicate that there is any hydrologic continuity between that water and the high pressure water below. Removing the water from the upper part of the slide will help with the surface slides that are occurring on the property, but that's not the issue that he is concerned with. Mr. Malina asked Mr. Clayton to hi -light for the rest of the Commission what Professor Tubbs had to say with the intent of the SAO. Mr. Clayton explained that the question came up in a meeting with Mr. Lowe and his consultants as to what the intent of the second item in the SAO was about. Specifically, the disagreement was whether they were dealing with the potential impact to the project itself or whether they are talking about adjoining properties. Mr. Tubbs indicated that they did not write the words, but their intention was that the SAO require that there not be an impact to the property. Mr. Malina asked Mr. Clayton what his definition of quantitative analysis is. Mr. Clayton said that a quantitative slope stability analysis is one in which the various Planning Commission Minutes Page 7 October 14, 1993 parameters of the slope and the ground water in the slope and the soil properties are all taken into consideration in a calculation in which a number is generated which is the risk value for slope failure. Mr. Malina asked how deep a deep soil study is. Mr. Clayton said in order to assess hydrostatic up -lift below a confining plain, it would be necessary to know what the water levels are. Based on the information they have, it's probably in the order of 50 -70 feet down. Mr. Malina asked if that wasn't already done. Mr. Clayton said that the slide mass that WSDOT was dealing with ended at Slade Way. The borings that were done for this development went 30 feet. Mr. Malina asked how he felt about Section E of 18.47 and part of 18.48. Is it appropriate to have that section within the SAO. Mr. Clayton said that the cities who ignore slope stability problems find themselves in a lot of trouble. It is very reasonable and provides a means for safeguarding public safety. Mr. Malina asked Mr. Clayton if he was a developer, would he be compelled to comply with the SAO. Mr. Clayton said his recommendation from the out -set would have been to do a deep slope stability analysis. Mr. Malina asked if he was satisfied with Mr. Tubb's interpretation of the direction of the SAO. Mr. Clayton said it was consistent with his interpretation. Mr. Meryhew asked what Mr. Clayton thought the risk would be if they were to develop the site and what might the consequences be. Mr. Clayton said there are a couple of concerns: 1) how will the soil behave under normal static conditions and; 2) how will it behave under dynamic conditions, such as in the event of an earthquake. Mr. Meryhew asked if the deep- seeded analysis will prove conclusively that an earthquake will not create a major problem on the slope. Mr. Clayton responded by saying there is a subjective element to it. Planning Commission Minutes October 14, 1993 Page 8 Mr. Knudson asked where Mr. Clayton would like to see the deep- seeded sample taken. Mr. Clayton said it needs to be down below the interface on the property. He added that the drawings that he has seen would not address a deep- seeded failure. Mr. Malina asked if the study concluded that there is a problem, could that effect the removal of trees and vegetation. Mr. Clayton responded by saying that it would be at the surface level. At this point they are far from understanding the deep- seeded stability and that it's idle speculation to think what removing a couple of trees would do. He said that he didn't think that would have any impact on the deep- seeded stability. Mr. Malina asked if the additional packing of the ground due to the structures that are being built would change the table at all. Would they need to go below 50 -60 feet? . Mr. Clayton said that he didn't think it would affect that. Those things are surficial materials and problems. There is going to be some leakage, but it isn't significant compared to the magnitude of water that exists below that relatively impermeable layer. Mr. Malina asked where does WSDOT come in on all of it and where does the liability lie. Mr. Clayton said that at the time of the Valley View development, it was pointed out that a significant de- watering system needed to be installed in order for the development to occur, or WSDOT had to renew their system and get it working properly. Their response was no, that system is designed for construction of that road, it wasn't designed for residential development or other development on the hillside. Mr. Malina asked if there is a liability issue for a developer who may have to go through a deep- seeded study to approach WSDOT's wells. Does one have to get special permission. Mr. Clayton said that they have to get permission from WSDOT to go into their wells. • Mr. Haggerton asked if there was any documentation regarding how the earthquake affected this property. Mr. Clayton said that he didn't think there was any indication that there was any sliding at that time. Mr. Meryhew said that there was indication that WSDOT may just need to do maintenance on the wells. He asked if that was a normal function that they are supposed to be performing periodically? Planning Commission Minutes October 14, 1993 Page 9 Mr. Clayton said that he thought they do conduct some minimal maintenance. Mr. Meryhew asked if there was any way to put the pressure on WSDOT to take a look at them. • Mr. Clayton said that he could not answer that. Mr. Meryhew asked if the freeway construction after 1960 and 1961 had any affect on this property. Mr. Clayton said that he did not know. There is drainage under Slade Way: There are some horizontal drains that were installed in 1961. Mrs. Craft (inaudible). Jack Pace said that it was because of the history with this site that they decided to have a peer review. Mr. Meryhew asked what WSDOT's responsibilities and obligations are. Mr. Pace said that the issues being dealt with tonight are the applicant's specific request. The City is not asking him to take care of a City problem or WSDOT problem. They are focusing on the stability of those houses on that site. Mr. Twelker said that with regard to the proposed treatment to accommodate for possible loss of grade of the foundations, they would probably stretch that to three feet. Beyond that, things would get tough. He said that he agrees that the water table in the vicinity of the Hillcrest development is not really connected to the water table that causes the problem. The amount of water that escapes from the Hillcrest property is a combination of surface and sub - surface water. Some of the sub - surface water is derived from local infiltration from meteoric water, some comes from off the property. Mr. Twelker read from page 10 of the Shannon & Wilson report, "With regard to the potential for deep- seeded sliding, it is our opinion that the most productive approach the city can take to improve hillside stability, would be to encourage WSDOT to rehabilitate their hillside drainage system. If measures are not taken to improve the system it may also be appropriate for the City to develop an on -going ground water monitoring program to assess the effectiveness of the drainage system ". Mr. Twelker said that he agrees with Mr. Clayton completely there. Mr. Lamb said that he agrees with Mr. Clayton that WSDOT should fix the mess they left or have the city fix it themselves. The site is now stable and the proposed improvement will make it more stable, the downhill problem was stabilized by these drains and they have failed over the years. Left unattended, it will impact Slade Way long before it gets anywhere near these other properties and there's no chance in the world that the highway department Planning Commission Minutes Page 10 October 14, 1993 and the City of Tukwila are going to watch a major access street close. David Morgan, 5190 S. 166: Mr. Morgan said that on his property, there is not any sand. Four feet down there is blue clay which slides like water during an earthquake. On the Hillcrest property there is a drain field which has water running about 90% of the time, while his property is dry. Mr. Morgan said that he has put in a drain field around his property which goes dow five feet and covers almost 400 feet. He said that when he walks off his property, there is water. Where is the water going to go when those houses are built; there is already a twenty -year drain field there that can't handle it. WSDOT is not going to protect the hillside. When will the traffic, water run -off, water retention, sewer, water availability, and sidewalk issues be addressed? Mr. Meryhew asked where Mr. Morgan lived in relation to the proposed site. Mr. Morgan stated that he lived about a 1/4 of a mile south from the property. Darlene West, 5212 S. 164: Ms. West said that the citizens worked so long and hard on the SAO, and there has to be a strong and valid reason for not following it. She said that she felt Mr. Lowe had not shown any valid reasons for not doing the deep- seeded testing to find out how stable his property is. It is an unstable hill and there are a lot of springs and water on the hill. She continued by saying that there has been a lot made of the WSDOT drains and she is not sure if maintaining those is a requirement to maintain Mr. Lowe's property. WSDOT does not give a darn, they have put in writing that their drains were to only maintain the road. They have no intention of upgrading it to do anything else. Mr. Haggerton asked which direction her residence was located from the property. Ms. West said she was located south of the property. Richard Stevens, 800 Bellevue Wy, Suite 400: Mr. Stevens said that he is an attorney representing adjacent property owners, Mr. & Mrs. Iverson and Chris Thakler. Their properties back up to Mr. Lowe's property. If Mr. Lowe's proposal is denied or if he is required to complete a deep seeded analysis, it has an affect on his client's property and everybody who abuts that property. Everyone agrees that the danger at that shallow level is no problem because Mr. Lowe's proposal will get the water off of the property. The problem is somewhere below and would affect his clients' property. If it's determined that this is a danger, it will devalue the neighboring properties. If Mr. Lowe's geotechnical engineers are discounted, it should be on some pretty clear and substantial evidence. All they have is a peer review by Mr. Clayton, who is not licensed in this state as a geotechnical engineer. Therefore, great weight should not be placed on something that is purely speculative. The language in the SAO talks about two different types of situations in which they would allow alteration of an area of potential geologic Planning Commission Minutes Page 11 October 14, 1993 instability. The second situation says that the area of potential geologic instability can be modified, or the project can be designed so that any potential impact to the project and surrounding properties is eliminated. The question is, what is it that needs to be eliminated. Mr. Clayton has advocated that they have to show that the geologic instability is eliminated and Mr. Stevens said that he didn't feel that could ever be shown under any circumstances. Mr. Stevens continued by saying that it is WSDOT's legal responsibility to take care of the wells. The Planning Commission and perhaps City Council, should urge WSDOT to take care of that problem. But that problem does not affect Mr. Lowe's houses any more than it affects his client's houses. If there is a deep- seeded problem, it is going to affect everyone. The short term problem, everyone agrees, will get fixed by the project itself. It seems that the position of the Planning Department is that unless the problem is completely solved, Mr. Lowe cannot do something that is going to help. Everyone agrees that getting the water off the property is going to help and his clients would like to see that stabilized. Mr. Malina clarified that if the applicant falls under the criteria of either item #1 or #2, then that is what triggers this process. Mr. Stevens said that was not his understanding from the staff report. If he has to show that the instability will be eliminated, Mr. Stevens said he didn't think anyone could show that. Everyone agrees that Mr. Lowe has eliminated whatever impacts his project will have on instability. This project will do nothing but make this area safer. Mr. Malina asked if Mr. Clayton was not licensed in the State of Washington. Mr. Stevens said that was his understanding. Mr. Haggerton asked if Mr. Steven's clients have suffered any instability on their property. Mr. Stevens said not that he knows of. Mr. Meryhew asked how long his clients have owned their property. Mr. Stevens said that one client has owned it for several decades, and Mr. Thaekler has owned his less time. There are four properties that abut this project and he represents the owners of two of them. Bob Levinson, Earth Consultants, 1805 136 P1, Bellevue 98005: Mr. Levinson said that he didn't get involved in this situation with an agenda. He stated that he has been involved in the last few months. He stated that this hillside is probably the most studied hillside in the Puget Sound area. He added that he doubted that the deep - seeded potential slide plain exists anywhere near Mr. Twelker's first line. In reading the initial Dames and Moore report, they talked about a clay silth that failed. The information that Planning Commission Minutes Page 12 October 14, 1993 he has seen on Mr. Lowe's property shows about 65 feet of sand. There are two different types of geologic conditions there. It is unlikely that a old sheer plain would go through two different types of geologic conditions. Mr. Levinson said that he has reason to believe that the deep seeded plain does not exist. He said that he is in agreement with everyone with regard to the short term solution when a property is developed, and it is done with a geotechnical engineer, it will always stabilize the slope because the water is being controlled. On the basis of his study, Mr. Levinson said that he thought Mr. Lowe should be able to develop his property. It would give stability to his client's property on the upper portion. He said that he talked to the citizen's committee regarding the SAO and he is against the 15 %, 25 %, 40% slope type of designation that this city has because they do not fully address the potential problems. His recommendation to the citizen's committee was to try to define the sensitive areas and study those instead of going by slope percentages. Mr. Lowe's design on those footings is over -kill and not really needed. It will tend to control the water and stabilize the slope beneath his client's property. Mr. Malina asked if Mr. Levinson has read the City's Sensitive Areas Ordinance since it has been completed. Mr. Levinson said that he had not. He added that he read the portion that was mentioned in the soils report. Mr. Malina asked with regard to section 18.47, the two basic criteria, does the applicant fall into one of those two categories? Mr. Levinson said that there is evidence of past instability. At the same time, that is off the proposed property. With regard to the second criteria, the word, "eliminate" is very strong. You cannot eliminate anything, but you can minimize it. He said he would not have a problem making the statement that impacts based on this development have been minimized where it will not negatively effect any of the adjacent properties. Mr. Malina asked if he felt a deep- seeded study should be required. Mr. Levinson said that on this project, he feels that if the WSDOT situation is taken care of, no further work is needed on Mr. Lowe's property. Although there has been past instability, it is not due to this man's project. It's due to some outside influence. When he talked with the citizen's committee, he was referring to projects that would have impacts on adjacent properties. Mr. Erickson, 16040 51 Ave. S.: He said that his property probably has more frontage on the edge of the hill than anyone Planning Commission Minutes Page 13 October 14, 1993 else's. There are springs all over in the hill. Before wetlands and sensitive areas were identified, he specialized in problem properties and putting structures on wet lots and on hillsides. There is a lot of water coming out of the hillside that needs to be addressed. He said that he was sure that controlling the water on the top would help stabilize the hill. He urged that the Council or city engineers get guarantees that the project is installed correctly. He said that he was not against developing the property as long is it is done correctly. Mr. Haggerton asked where the test hole is in relation to his property. Mr. Erickson said that it was on the lot adjacent to his. Mr. Haggerton asked what type of soil there was. Mr. Erickson said that it was sand and his lot is pure sand. Mr. Knudson asked what his specialty is. Mr. Erickson said it is in excavation. Mr. Knudson asked if he has seen success in developing problem properties. Mr. Erickson said that if it is installed as engineered, then it will be o.k. Generally, he has not seen any failures if it has been installed correctly and maintained. WSDOT is going to have to take care of their system and maintain it. Ray Neilsen, 5108 S. 163 P1.: Mr. Neilsen said that the rear of his property abuts this proposed project. He added that there has been some slippage up there. He said that in the 27 years that he has lived in his home the ground is slipping east, down the hill. Below the proposed property is a swamp all the time. Darlene West asked staff if they were asking the applicant to do the deep seeded testing on their property or on the property down hill from them? Mr. Clayton said that he didn't know what they would have to do to get the required information. There are no wells located on the property that are deep enough. There are wells down hill. Ms. West said it's not fair to expect WSDOT to maintain the drains they put in for a road. She said that no -one should expect WSDOT to support anything that changes what that hill was when they first put in their drain. Mr. Knudson said that because of the cut for the road, they caused some damage and Planning Commission Minutes Page 14 October 14, 1993 impact, and they are responsible. It's a matter of maintaining what they have already destroyed. Mr. Meryhew said that the Commission's decision could not be based on what WSDOT is responsible for. Mr. Meryhew asked for clarification from Mr. Clayton regarding the comment that he is not licensed in the State of Washington. Mr. Clayton said that he is an engineering - geologist and they don't have licenses in the State of Washington. He is a registered geologist in California and Oregon and a certified engineering geologist in Oregon. Mr. Meryhew closed the public hearing. Mr. Meryhew called for a five minute break. Mr. Meryhew called the meeting back to order. He continued by saying that they were looking at a case of potential liability, safety and individual property rights. He said that he was concerned with the safety, but also looking for a reasonable win/win situation for everyone. The development proposed by the applicant should be certified as safe by a geotechnical engineer or geologist and that's in the SAO. Also, the applicant could provide a letter to the City. which is covered in the SAO. Essentially, this letter with plans and specifications, stating that he /she understands and accepts the risk in developing an area with potential unstable soils and he /she will advise, in writing, prospective purchasers of the site or any prospective purchasers of structures or portions of structures on the site of the slide potential. In addition, the owner should execute a covenant within the bounds of the SAO, which that provides a legal description and a statement that the site is an area of potential instability and some of the risks that are associated with it; and any conditions or prohibitions on the development; any features or modifications of the design to address any anticipated soil changes; (inaudible) waive any claims of the owner or his successors for signs that they may have against the City of Tukwila. All of this is allowed within the SAO. In addition to that, the foundation designs that are proposed by Mr. Twelker in his letter to Mr. Lowe dated July 19th should be incorporated as additional safety. Mr. Malina said in his opinion, the Director did what he had to do because of the way the SAO is written. Mr. Meryhew said he agreed, and there is a difference of opinion. Mr. Malina said the appellant is in a catch 22 situation. As far as the peer review group goes, Shannon and Wilson' requirement of an off-site deep seeded study, as well as an on- site deep seeded study, is inappropriate. The appellant is only responsible for his portion. Planning Commission Minutes Page 15 October 14, 1993 If the City or State wants to do an off -site study, that is up to them, it should not have to come out of the applicant's pocket. He added that he personally felt that the applicant has not complied to the conditions of the SAO as the way the SAO is written today. That is not to say that this may be amended at some future date. It may come back to the Planning Commission to be amended. But at this point, the SAO is written, the applicant has not complied and the Director of DCD did the correct thing. Mr. Haggerton said that when they developed the SAO, they paid close attention to the Class III, Class IV and Coal Mine Hazard areas. There was a reason for that. This site has clearly been identified as a Class IV area. The applicant is not responsible for anything off of his property. Because of the way it is classified and the testimony, there should be more on -site tests to prove the stability of the land. Mr. Knudson said that he has problems with the technicalities of what they are trying to achieve. Mr. Meryhew said that there has to be some risk involved, not everything can be guaranteed. Mrs. Craft said that the whole idea of a SAO is so that houses can be built with some responsibility. Part of the wording of the SAO makes it difficult to do that. The whole idea is to develop responsibly, and they are being asked to decide if the appellant's evidence lead to a clear conviction that a mistake was made by the director. She said that she didn't see how they could say that the director made a mistake, he was following the direction of the SAO. She added that she would like to see how.they can help the appellant and what is the next step to make the development happen. Mr. Malina said that they can make the development happen. They are just showing that the director of DCD interpreted the SAO correctly. What the appellant is doing is appealing the decision of DCD and what has been demonstrated here is that the director of DCD was correct. Mr. Meryhew said that this is subject to interpretation. Mr. Malina said that it was apparent as far as 18.47(E), all he has to do is demonstrate and either /or situation. There's no question that there was a landslide and it is a hazard. The applicant, staff, peer review, and citizens know that there is serious concern there. The applicant does have a responsibility to create a covenant when the project is developed. He has a responsibility to the community and there's no doubt he can make the project work. What the Commission is concerned with is what is before them and the Commission can make a recommendation after they make a motion. Mr. Meryhew said that the SAO says that a detailed stability analysis needs to be completed. And what that consists of is what is being interpreted differently. Planning Commission Minutes Page 16 October 14, 1993 Mr. Malina said that the studies have to be within the boundaries of the project. Mr. Meryhew said that it could be interpreted as improving the site without removing eliminating every problem. There has to be a risk factor involved. Mr. Haggerton said that section 18.48 is the most important to him; "the detailed slope stability analysis must be done if it is a Class IV area ". A detailed slope stability analysis consists of a determination of what types of soils exist where the houses are going to be placed, how much water is in the hillside and how can it best be diverted so that it is not a detriment to that hillside. He added that common sense needs to be used along with the ordinance that they adopted. Every word cannot be analyzed. Mr. Meryhew said that is why he is suggesting that they ask for the certification from a geotechnical engineer or licensed geologist that the property is safe. They could also have a covenant that goes along with the land. Mr. Knudson said that it involves more than just someone buying the property, it's what impact will it have on adjacent properties. Mr. Meryhew said that he didn't believe that doing a detailed study on the property alone will provide anymore information than they already have. Mr. Haggerton said that it would provide more information, but would not guarantee it more. Mr. Malina asked for clarification from staff if sensitive area properties have that indicated on titles to the property. Mr. Pace said that existing development has none of that. The only exception is for property owners who want deferment of their taxes. Mr. Haggerton asked what happens if the appeal is approved. Mr. Pace said that if the appeal is approved, staff cannot require further geotechnical analysis and they would proceed ahead. Mr. Haggerton said that apparently, the DCD director is bound by the SAO and the way that it is written to require additional geotechnical studies. Mr. Malina said that's what the peer review is recommending, but they are stepping out of Planning Commission Minutes October 14, 1993 the bounds of the project, and the applicant should not have to. There can be enough of a study done on the site if the deep seeded test were done in the right place. It's important to get the project completed. It's one thing if the slide never occurred, but that's why it has received this hazardous rating to begin with. Mr. Meryhew said that he still felt it was a question of interpretation and some risk still needs to be taken. The covenant also provides liability insurance for the City as stated in the SAO, section 18.48(d) & (e). During the straw vote, Messrs. Haggerton and Malina said they would deny the appeal and Mrs. Craft said she would probably deny the appeal; Messrs. Knudson and Meryhew said they would- approve the appeal. • Page 17 • MR. MALINA MOVED ON 93 -01 -APP: HILLCREST TO UPHOLD STAFF'S FINDINGS, CONCLUSIONS AND RECOMMENDATIONS TO DENY THE APPEAL. MR. HAGGERTON SECONDED THE MOTION. THE MOTION DID NOT PASS BY A VOTE OF 3 -2. MESSRS. HAGGERTON AND MALINA WERE IN FAVOR OF THE MOTION AND MESSRS. KNUDSON, MERYHEW AND MRS. .CRAFT VOTED AGAINST THE MOTION. In order to clarify the motion and provide a positive motion: MR MERYHEW MOVED TO ACCEPT L93 -01 -APP: HILLCREST, APPEALING A DECISION THAT THE APPEAL BE APPROVED AND CARRIED BY A VOTE OF 3 -2 OF THE PLANNING COMMISSION. THE REASON FOR THE MOTION IS THAT THE PLANNING COMMISSION BY A 3 -2 VOTE, FELT THAT THE INTENT AND PURPOSE OF THE SENSITIVE AREAS ORDINANCE HAD BEEN MET SUBJECT TO 'THEIR INTERPRETATION OF THE SAO AND UNDERSTANDING THAT '1'1LE INTERPRETATION IS A DIFFERENCE OF OPINION. WITH A 3 -2 VOTE, THEY FELT THAT THE TECHNICAL INFORMATION AND GEOLOGICAL STUDIES HAVE BEEN ADEQUATE TO MEET THE INTENT OF THE SAO AND THAT THE DCD DIRECTOR'S CONCLUSION AND ORIGINAL DENIAL WAS THE RESULT OF A DIFFERENT INTERPRETATION THAN THE PLANNING COMMISSION'S. Taking a formal vote: JIM HAGGERTON VOTES NO GEORGE MALINA VOTES NO VERN MERYHEW VOTES YES JERRY KNUDSON VOTES YES JOYCE CRAFT VOTES YES. 1'HE APPEAL IS APPROVED. Planning Commission . Minutes October 14, 1993 Vern Meryhew adjourned the meeting. Page 18 Prepared By, Sylvia Schnug '•I l� to u: A :Y:: ri•Gl ?A•C: FJ'a l!L'• _•.`f. ;M.'+ City of Tukwila John W. Rants, Mayor Department of Community Development STAFF REPORT TO THE PLANNING COMMISSION Prepared October 5, 1993 HEARING DATE: October 14, 1993 FILE NUMBER: LOCATION: APPELLANT: REQUEST: STAFF: ATTACHMENTS: 93 -01 -APP Hillcrest 90- 13- BLA/91 -3 -APRD L93 -0063 Slade Way & S. 160th Street Rick Beeler, Director LeRoy Lowe A.I.A. Architect Appeal a decision by the Department of Community Development for slope stability requirements in the Sensitive Areas Ordinance, Section 18.45.080 (E). Jack Pace, Senior Planner Daniel Clayton, Shannon/Wilson, Inc. A. B. C. D. E. F. G. H. I. J. K. Appellant's letter - July 20, 1993 Planning Division letter - July 12, 1993 Neil H. Twelker letter - June 22, 1993 LeRoy Lowe letter - May 25, 1993 Planning Division letter - May 14, 1993 LeRoy Lowe letter - May 4, 1993 LeRoy Lowe letter - March 30, 1993 Cascade Geotechnical, Inc. - March 22, 1993 LeRoy Lowe letter - March 25, 1993 Planning Division letter - March 15, 1993 Shannon/Wilson, Inc. - November, 1992 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 430665 Staff Report to the Planning Commission Page 2 L93- 0063:Hillcrest FINDINGS BACKGROUND On July 2, 1990, a public hearing was held for a waiver request by the applicant from the City's Sensitive Areas Moratorium. The petitioner proposed to re- plat /short plat 5 single -family lots for development. The moratorium Ordinance No. 1544 precluded filing of new development - related applications in sensitive areas, including subdivisions or short sub - divisions, without a waiver. The applicant's property ha slopes over 15 percent. The waiver was granted with conditions by the City Council. Those conditions allowed Mr. Lowe to submit a Boundary Line Adjustment application. On October 16, 1990, the City received applications for a short plat and a boundary line adjustment from LeRoy Lowe. After reviewing both applications, staff concluded that only a boundary line adjustment is required. On June 28, 1991, the SAO became effective. Until this time, staff could not finalize the boundary line adjustment submitted by Mr. Lowe. As a requirement of the SAO an APRD (Administrative Planned Residential Development) was required concurrently with the existing application. The APRD application was submitted on September 9, 1991. On March 19, 1992, the applicant appealed the director's decision regarding the Sensitive Areas Ordinance requirements (TMC 18.45.125). The Planning Commission's conclusions are listed below and denied the appeal. * Based upon TMC 18.45.060 the applicant is required to do a wetland study as part of the boundary line adjustment review. The applicant does not meet the waiver criteria contained in TMC 18.45.020 (F). The proposed development plan could have detrimental impacts on the wetland. * As noted in the report, the applicant has provided adequate geotechnical soil reports for the initial review. Further work may need to be done once the wetland study has been completed. In addition, further stormwater studies need to be done to determine the impacts on down -slope properties. * At this time, the applicant has not provided the needed information to address this issue. Once the wetland study has been completed, there may be justification to use this section of the ordinance. Staff Report to the Planning Commission DECISION CRITERIA L93- 0063:Hillcrest Page 3 Administrative decisions by the Department of Community Development may be appealed as follows: 18.45.125: Appeals (a) Any aggrieved party who objects to or disagrees with Department of Community Development (DCD) decisions or conditions for development in a sensitive area shall appeal to the Planning Commission. Any such appeal shall be made in writing within ten days of the interpretation, conditions or decision being appealed, and shall set forth the basis for the appeal. (b) in considering appeals of decisions or conditions, the following shall be considered: (1) The intent and purposes of the sensitive areas ordinance from which this chapter derives; (2) Technical information and reports considered by the Department of Community Development; and (3) Findings of the DCD Director which shall be given substantial weight. The SAO contains the following requirements for interpretation: TMC 18.45.030: Interpretation: The provision of this chapter shall be held to be minimum requirements in their interpretation and application and shall be liberally construed to serve the purposes of this chapter. The applicant is appealing staff's interpretation of Section 18.45.080(e)(4)(c). (C) Geotechnical reports for Class 3, Class 4 and Coal Mine Hazard Areas shall included a site evaluation review of available information about the site, a surface reconnaissance of the site and adjacent areas, and a subsurface exploration of soils and hydrology conditions. Detailed slope stability analysis shall be done if the geotechnical consultant recommends it in Class 3 or Coal Mine Hazard Areas, and must be done in Class 4 areas. ..In.. -Attachment ..A,_.. the . applicant has explained why he is appealing staff's interpretation of the ordinance. He has stated:, "We appeal your consultants interpretation of the SAO and disagree with his opinion regarding the slope stability analysis prepared by Cascade Geotech.. Mr. Lamb of Staff Report to the L93- 0063:Hillcrest Planning Commission Page 4 Cascade has stated that this site is stable and the potential for landsliding on this property has been addressed." As reflected by the numerous attachments, their has been on -going discussion with the applicant and staff regarding this issue. Attachment A through K contain the applicant's and staff's technical material and position regarding slope stability analysis. In his letter, Mr. Lowe disagrees with the evaluation of the adequacy of the slope stability analysis performed by Cascade Geotechnical Consultants and offers the following opinions as additional basis for his appeal: * Three professional engineers have advised him that, "there will be no adverse impact to the project or surrounding properties"; * "Construction would have a stabilizing effect in controlling on -site water and thus, add a dimension of slope stability"; * "Mr. Lamb of Cascade has stated that the site is stable and the potential for landsliding on this property has been addressed"; and * Various foundation designs are available that "will eliminate any impact to the project as a result of geologic instability." In reviewing Mr. Lowe's letter and attachments, There is no basis for revising the conclusions presented in the peer review and summarized in the consultant's letter of June 28, 1993 to the City of Tukwila. With the exception of the last point listed above, the opinions that Mr. Lowe has offered in his appeal were previously discussed with Mr. Lowe and were considered in our letter of June 28, 1993. To reiterate staff's position, both shallow and deep- seated stability analyses are necessary to evaluate the geologic stability of the proposed project. As of this time, no deep- seated analysis has been performed for the project, although the applicant has been advised repeatedly of the importance staff has placed on this analysis in evaluating the geologic stability of the property. In staff's opinion, a deep- seated analysis is critical to this evaluation because: * The proposed development is situated on an old landslide deposit, with the scarp of the slide crossing the property immediately west of the proposed residences. Staff Report to the Planning Commission Page 5 L93- 0063:Hillcrest * Surficial soils in several areas downslope .from. the site appear to be unstable, including an area which is affecting Slade Way to the south of the proposed development. * Slope indicator data from the vicinity of Slade Way immediately downslope of the property provide questionable evidence of minor, relatively deep- seated slope movement since the 1960 landslide. * A large deep- seated slope failure occurred in 1960 on an adjoining property a short distance downslope. Recurrent movement on this landslide, if it were to occur, might threaten the stability of the site. * Three static an dynamic slope stability analyses conducted during the 1980's indicate that the deep- seated slope stability of the area immediately downslope from the proposed Hillcrest Development is marginal, at best, and likely to diminish over time if the hillside dewatering system operated by WSDOT continued to deteriorate. The slope stability analysis report prepared by Cascade Geotechnical does not address these issues nor does it provide rationale why these considerations were not addressed. Considering the evidence that deep- seated sliding cold potentially affect the project, staff does not believe that the requirements or intent of the SAO have been met. Lacking documentable evidence to the contrary, it would appear prudent to assume that the site is at risk from a deep- seated slope failure and that such a failure could pose a considerable risk to the proposed development and its occupants. With respect to Mr. Lowe's opinion that the foundation design options presented in his appeal could be used to eliminate any impact to the project resulting from geologic instability, we believe that he has not provided an adequate basis for this conclusion. It is staff's opinion that a meaningful evaluation of foundation feasibility and design for this site would require a deep- seated slope stability analysis along with an understanding of the depth of potential movement that could result from sliding. This information has not been provided by the applicant, nor has it been developed to our knowledge. Consequently, there does not appear to be any basis to conclude that foundations could be designed to eliminate the risk of irreparable damage to the structure or possible risk to human health or safety. Along with Mr. Lowe's appeal, his consultants have provided information about a proposed relocation of the wetlands currently on site to a location on the lower part of the hillside. Although staff has concerns about the effect of such a wetland on the Staff Report to the Planning Commission L93- 0063:Hillcrest Page 6 overall slope stability, this aspect of the project has not been considered in our review because it is our understanding that the placement and design of wetlands is not relevant to the requirements for a BLA. CONCLUSIONS As noted in criteria discussion, the Planning Commission needs to consider three items in making their decisions; 1) Intent/Purposes of the Sensitive Areas Ordinance. The applicant has not addressed the following purposes statements contained in the Sensitive Areas Ordinance: (1) Minimize developmental impact on the natural functions and values of these areas. (4) Prevent erosion and the loss of slope and soil stability caused by the removal of trees, shrubs, and root systems of vegetative cover. (5) Protect the public against avoidable losses, public emergency rescue and relief operations cost, and subsidy cost of public mitigation from landslide, subsidence, erosion and flooding. 2) Technical Information As reflected by the numerous reports contained in the attachments, both staff and the applicant have hired technical experts to review this site. Staff believes that both shallow and deep- seated stablity analyses are necessary to evaluate the geologic stability of the proposed project. As of this time, no deep- seated analysis has been performed for the project. 3) DCD Director shall be given substantial weight The City has retained Shannon & Wilson, Inc. to provide peer review of technical material. Based upon their expertise and review with other governmental agencies, the DCD Director has concluded the applicant must demonstrate one of the following: ' 1) There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no significant risk to the proposed development or surrounding properties; or 1,11.81403.1....0.44 r Staff Report to the Planning Commission L93-0063:Hillcrest Page 7 • ■ ' • ..- The .area of potential geologic instatility,,can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or, sedimentation shall not 1,J decrease slope stability. RECOMMENDATION .,r Based upon the evidence and evidence to overturn the Director's interpretation, staff recommends the appeal be denied. SHANNON & * ILSON, INC. impyip Geotechnical and Environmental Consultants w —bib / —01 Attachment to Letter Report Page 1 of 2 Dated: November 23, 1992 To: City of Tukwila Attn: Mr. Phil Fraser Important Information About Your Geotechnical Engineering/ Subsurface. Waste Management (Remediation) Report AN ENGINEERING REPORT IS BASED ON PROJECT - SPECIFIC FACTORS. A geotechnical engineering /subsurface waste management (remediation) report is based on a subsurface exploration plan designed to incorporate a unique set of project - specific factors. These typically include: the general nature of the structure and property involved, its size and configuration; historical use and practice; the location of the structure on the site and its orientation; physical concomitants such as access roads, parking lots, and underground utilities;and the level of additional risk which the client assumed by virtue of limitations imposed upon the exploratory program. To help avoid costly problems, have the consulting engineer/ scientists determine how any factors (which change subsequent to the date of the report) may affect the recommendations. Unless your consulting geotechnical /civil engineer and /or scientist indicates otherwise, your report should not be used: O when the nature of the proposed project is changed; for example, if an office building will be erected instead of a parking garage, or if a refrigerated warehouse will be built instead of an unrefrigerated one; or chemicals are discovered on or near the site; O when the size or configuration of the proposedproject is altered; O when the location or orientation of the proposed project is modified; O when there is a change of ownership; or ❑ for application to an adjacent site. Geotechnical /civil engineers and /or scientists cannot accept responsibility for problems which may develop if they are not consulted after factors considered in their reports have changed. MOST GEOTECHNICAL AND SUBSURFACE WASTE MANAGEMENT "FINDINGS" ARE PROFESSIONAL ESTIMATES. Site exploration identifies subsurface conditions only at those points where samples are taken and when they are taken, but the physical means of obtaining subsurface data precludes the determination of precise conditions. Consequently, the information obtained is intended to be sufficiently accurate for design, but is subject to interpretation. Additionally, data derived through sampling and subsequent laboratory testing are extrapolated by the geotechnical /civil engineer and /or scientist who then renders an opinion about overall subsurface conditions, their likely reaction to proposed construction activity, and /or appropriate design. Even under optimal circumstances actual conditions may differ from those opined to exist, because no geotechnical /civil engineer and /or scientist, no matter how qualified, and no subsurface exploration program, no matter how comprehensive, can reveal what is hidden by earth, rock and time. For example, the actual interface between materials and /or chemicals may be far more gradual or abrupt than the report indicates, and actual conditions in areas not sampled may differ from predictions. Nothing can be done to prevent the unanticipated, but steps can be taken to help minimize their impact. For this reason, most experienced owners retain their geotechnical /waste management consultant through the construction stage to identify variances, conduct additional tests which may be needed, and to recommend solutions, to problems encountered on site. Prudent owners establish contingencies to account for such variations in subsurface conditions as exposed during construction. SUBSURFACE CONDITIONS CAN CHANGE. Subsurface conditions maybe affected as a result of natural changes orhuman influence. Because a geotechnical /waste management en ineering report is based on conditions which existed at the time of subsurface exploration, construction decisions should not be Page 2 of 2 based on an engineering report whose adequacy may have been affected by time. Speak with the geotechnical /waste management consultant to learn if additional tests are advisable before construction starts. For example, groundwater conditions commonly vary seasonally. Construction operations at or adjacent to the site and natural events such as floods, earthquakes or groundwater fluctuations may also affect subsurface conditions and, thus, the continuing adequacy of a geotechnical / waste mana gement report. The geotechnical / civil engineer and /or scientist should be kept apprised of any such events, and should be consulted to determine if additional tests are necessary. - THE GEOTECHNICAL ENGINEERING /SUBSURFACE WASTE MANAGEMENT (REMEDIATION) REPORT IS SUBJECT TO MISINTERPRETATION. Costly problems can occur when other design professionals develop their plans based on misinterpretation of a geotechnical engineering /subsurface management(remediation)report. To help avoid these problems, the geotechnical/civil engineer and /or scientist should be retained to work with other appropriate design professionals to explain relevant geotechnical, geological, hydrogeological and waste management findings and to review the adequacy of their plans and specifications relative to these issues. BORING LOGS SHOULD NOT BE SEPARATED FROM THE ENGINEERING/WASTE MANAGEMENT RE- PORT. Final boring logs are developed by the geotechnical /civil engineer and /or scientist based upon interpretation of field logs (assembled by site personnel) and laboratory evaluation of field samples. Only final boring logs customarily are included in geotechnical engineering /waste management reports. These logs should not, under any circumstances, be redrawn for inclusion in architectural or other design drawings, because drafters may commit errors or omissions in the transfer process. Although photographic reproduction eliminates this problem, it does nothing to minimize the possibility of contractors misinterpreting the logs during bid preparation. When this occurs, delays, disputes and unanticipated costs are the all- too-frequent result. To minimize the likelihood of boring log misinterpretation, contractors should be given ready access to the complete geotechnical engineering /waste management report. Those who do not provide such access may proceed under the mistaken impression that simply disclaiming responsibility for the accuracy of subsurface information always insulates them from attendant liability. Providing the best available information to contractors helps prevent costly construction problems and the adversarial attitudes which aggravate them to a disproportionate scale. READ RESPONSIBILITY CLAUSES CLOSELY. Because geotechnical engineering /subsurface waste mangement (remediation) isbased extensively on judgment and opinion, it is far less exact than other design disciplines. This situation has resulted in wholly unwarranted claims being lodged against geotechnical /waste management consultants. To help prevent this problem, geotechnical /civil engineers and /or scientists have developed model clauses for use in written transmittals. These are not exculpatory clauses designed to foist the engineer's or scientist's liabilities onto someone else. Rather, they are definitive clauses which identify where the engineer's or scientist's responsibilities begin and end. Their use helps all parties involved recognize their individual responsibilities and take appropriate action. Some of these definitive clauses are likely to appear in your report, and you are encouraged to read them closely. Your engineer /scientist will be pleased to give full and frank answers to your questions. OTHER STEPS YOU CAN TAKE TO REDUCE RISK. Your consultingengineer /scientist will be pleased to discuss other techniques which can be employed to mitigate risk and to provide a variety of materials which may be beneficial. Contact your engineer /scientist for further information. The preceding paragraphs are based on information provided by the • . Association of Soil and Foundation Engineers, Silver Spring, Maryland 1/92 SHANNON EIWILSON, INC. TABLE OF CONTENTS Page 1.0 INTRODUCTION 1 2.0 SCOPE OF WORK 1 3.0 SITE CONDITIONS 2 3.1 Surface Conditions 2 3.2 Subsurface Conditions 3 4.0 PREVIOUS GEOTECHNICAL INVESTIGATIONS AND REMEDIATION 4 4.1 1960 Landslide Investigations 4 4.2 Geotechnical Investigations for the Proposed Valley Vue Estates 5 4.3 Geotechnical Investigations for the Hillcrest Development 5 5.0 DISCUSSION OF FINDINGS 6 5.1 Slope Stability 6 5.1.1 Shallow Landslide Potential 6 5.1.2 Deep - Seated Landslide Potential 8 5.2 Geotechnical Considerations for Slade Way Stability 9 5.3 Geotechnical Considerations for the Proposed Hillcrest Development. 11 5.3.1 Slope Stability Considerations 11 5.3.2 Seismic Hazard 12 5.3.3 Site Drainage 12 5.4 Alternative Considerations for Hillcrest Development 13 5.5 Stormflow Drainage 14 6.0 CONCLUSIONS 14 7.0 LIMITATIONS 16 LIST OF FIGURES Figure No. 1 Site Plan 2 Generalized Geologic Profiles i W- 6367 -01 L... .wouMINannr n:nwmw:+a�w� !srroic�xi SHANNON FIWILSON, INC. TABLE OF CONTENTS (Cont.1 LIST OF APPENDICES APPENDIX A - SEQUENTIAL LIST OF DOCUMENTS REVIEWED APPENDIX B - IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL ENGINEERING REPORT ii W- 6367 -01 SHANNON &WILSON, INC. GEOTECBNICAL REVIEW OF PROPOSED LEROY LOWE DEVELOPMENT, TUKWILA, WASHINGTON 1.0 INTRODUCTION This report presents the results of our geotechnical engineering review of the proposed Hillcrest residential development at South 160th Street and Slade Way, Tukwila. The purpose of the review was to provide the City of Tukwila with an independent evaluation of the stability of Slade Way and the proposed development, to evaluate the need to dewater the wetlands on the site in order to develop the site as has been proposed by the developer, and to evaluate the need for a drainage detention system. Our review focused on: a) development plans and recent geotechnical investigations done for the Hillcrest developer /owner, Mr. LeRoy Lowe; b) geotechnical investigations performed in the 1980's for the proposed Valley Vue Development downslope from the Lowe property, and c) geotechnical investigations and remedial actions that were taken in the 1960's to stabilize a large landslide that extended eastward to the vicinity of Slade Way from a borrow excavation at the toe of the slope. 2.0 SCOPE OF WORK The scope of our services is described in detail in Exhibit A to your Standard Consultant Agreement. This work was authorized by your signed Standard Consultant Agreement and notice to proceed dated October 2, 1992. Our scope consisted of a field reconnaissance of the property; a review of documents, reports and analyses prepared by others; and preparation of a report presenting the findings of our study. Our scope of services originally included: 1. Review of the existing geotechnical reports, wetland reports, and site plans and drawings for the proposed Hillcrest Development which were provided by the City. 2. Review of geotechnical reports from the site vicinity, including those prepared in conjunc- tion with the construction of Interstate Highway 5 and the proposed Valley Vue Develop- ment. 3. Estimate' of- site runoff under existing .conditions, and combined runoff and dewatering system discharge for the proposed Hillcrest Development. 1 W- 6367 -01 SHANNON &WILSON, INC. 4 Provision of a professional opinion regarding the present stability of Slade Way in the im- mediate vicinity of the proposed Hillcrest Development, and if appropriate, discussion of alternatives that are available for improving the stability of the slope. 5. Determination whether stability analyses by others for adjacent property may be applied to assess.the stability of Slade Way and provision of a professional opinion as to how these factors of safety might be affected by the proposed Hillcrest Development. 6. Evaluation of the feasibility of various alternatives for site dewatering and runoff manage- ment that might exist with respect to slope stability of the proposed development and the adjacent part of Slade Way. 7. Evaluation of the geotechnical suitability of measures that could be taken to prevent an in- crease in peak flows from the property when it is developed. Once the work was begun, it became clear that there was not sufficient site - specific data available to make a reliable estimate of site runoff, so it was agreed to drop this task from the scope of work. A list of the documents that were reviewed in the study is provided in Appendix A of this report. Appendix B provides important information about your geotechnical report. 3.0 SITE CONDITIONS 3.1. Surface Conditions The proposed Hillcrest Development (or LeRoy Lowe Development) is located'on the west side of Slade Way, just south of South 160th Street, in Tukwila, Washington. It is situated approxi- mately half -way up the steep, east- facing slope of the hill, which is located southwest of the Tukwila freeway interchange. The property consists of approximately three acres of undeveloped land bounded on the north, west, and south by single family residences and on the east by Slade Way. The site is wooded and has about 64 feet of topographic relief across it. A steep slope (2H:1V) about thirty feet high separates a relatively flat upland in the western part of the site from an adjoining lowland to the east. The lowland area is hummocky and poorly drained, with an- overall gentle - eastward slope toward Slade. Way. Much. of this lowland area is boggy, and at the time .of _our _reconnaissance, .locally ._contained .standing. water. Large parts of the eastern part of the site have been classified as a Type 3 Wetland as the result of a wetland study in 1992 by Bredberg and Associates, Inc. 2 W- 6367 -01 SHANNON &WILSON. INC. The steep slope that separates the upland and lowland portions of the site is interpreted to be the southern end of the headscarp of an ancient extensive landslide. This landslide escarpment extends about 3000 feet northwest of the site and typically displays about 30 to 40 feet of topographic relief. At the base of this escarpment, the ground is typically hummocky and poorly drained and commonly is the site of springs and boggy conditions, as it is on the Lowe property. This disrupted drainage probably has contributed to the numerous smaller slides within the ancient slide mass. To the east of the site and Slade Way, the hillside slopes moderately steeply down to the Duwamish Valley. This densely wooded, vacant property below Slade Way was modified in the early 1960s by regrading and other remediation measures after a large, deep - seated landslide propagated upslope from the base of the hill in 1960. The extent of this 1960 landslide and subsequent movements through April 1961 is shown in Figure 1. 3.2 Subsurface Conditions Borings drilled on the site by Cascade Geotechnical, Inc. (1990) and downslope by others indicate that there are three main geologic deposits of concern in the site vicinity. These geologic units are portrayed in Figure 2 in generalized geologic profiles that extend eastward from Slade Way. The uppermost deposit consists of fine to medium sand and silty sand with scattered gravel. This upper unit varies from about 10 to 30 feet thick, blankets the slope, and thins eastward, downhill from Slade Way. Groundwater within this layer is perched on the underlying clayey silt and comprises the surficial aquifer at the site. The upper sand unit overlies a relatively thick layer of clayey silt and silt that ranges from about 30 to 70 feet thick. This intermediate layer of clayey silt typically is laminated, fractured, and slickensided. The clayey silt unit, as a whole, slopes eastward, roughly paralleling the slope of the hillside, as do bedding planes within it. This clayey silt unit has a low permeability and serves as an aquitard betweeri the water table aquifer above and a confined aquifer below it. Underlying the clayey silt unit is a lower sand and gravel deposit that is typically about 10 to 40. feet thick. Although it varies in grain size, this sand and gravel is continuous throughout the -area. Based °n its • prolific capacity. as -an aquifer, it appears to .extend far into the hillside to the .west„where_.it is recharged .and.from.which it has. considerable hydraulic head. The water table in the surficial sand unit is about 25 to 30 feet below the upland part of the proposed development and seeps out of the base of the steep slope on the property. However, on the lower, eastern part of the site, this sand is saturated essentially to the ground surface, 3 W- 6367 -01 SHANNON F&WILSON, INC. as it also appears to be in lower areas to the east of Slade Way. The underlying clayey silt is an aquitard, confining the groundwater in the deeper sand and gravel deposit and restricting hydraulic communication between . this deeper aquifer and the overlying water table aquifer. Hydrostatic uplift on the base of the clayey silt from confined groundwater in the underlying sand and gravel aquifer has been an important contributing factor to landsliding on the hillside. 4.0 PREVIOUS GEOTECHNICAL INVESTIGATIONS AND REMEDIATION 4.1 1960 Landslide Investigations The slope below Slade Way has been the subject of intensive geotechnical investigations since 1960, when gravel borrow pits were developed at the base of the hill. Excavation of the natural buttress at the toe of the hillside resulted in a series of progressive landslides which extended upslope to the approximate location of Slade Way (see Figure 1). The center of this slide mass moved laterally as much as about 70 feet, with the main slide plane located about 30 to 50 feet below ground surface within the clayey silt unit. Geotechnical investigations of the 1960 and subsequent landslides revealed that the main contributing geologic factors to sliding were: 1) the pre- existing landslides on the hillside, 2) the downslope dipping structure of the laminated clayey silt unit, 3) the presence of a natural reservoir of water in deep sand and gravel deposits uphill and behind the unstable slopes, and 4) high artesian water pressure in a confined aquifer underlying the clayey silt unit. Remedial measures to stabilize the hillside were accomplished by the Washington Department of Transportation (WSDOT). These measures consisted of regrading, surface drainage, and installing a system of vertical and horizontal drains. These measures, mostly accomplished by the fall of 1961, were effective in lowering the artesian pressures and stabilizing the sliding at that time. Additional related geotechnical investigations by Shannon & Wilson on behalf of WSDOT continued through 1968 to stabilize other parts of the hillside further to the north and west of the 1960 landslide. The same general geologic conditions that contributed to the 1960 slide also exist in °these areas. —These slopes were also stabilized by various combinations of vertical and horizontal: drains, regrading, soldier pile: walls, and :rock buttresses:- • 4 W- 6367 -01 SHANNON &WILSON, INC. 4.2 Geotechnical Investigations for the Proposed Valley Vue Estates Between 1982 and 1989, a multiple- family condominium complex was proposed on the site of the 1960 landslide;- directly east of the proposed' Hillcrest Development. GeoEngineers, Inc. was retained to conduct - geotechnical investigations of that site. Their work involved a limited subsurface exploration program, installation of slope indicators and piezometers, slope stability analyses, and development of recommendations for measures that would be necessary, in their opinion, to successfully build and maintain the proposed complex. At the time of their investiga- tions, GeoEngineers, Inc. noted small offsets a short distance below Slade Way as well as local surficial slides within the large area below Slade Way. Dames & Moore and WSDOT provided review of GeoEngineers, Inc. investigations and findings for the Valley Vue Estates, particularly with respect to slope stability analyses and requirements for remediation and maintenance of the WSDOT drainage system. The ongoing deterioration of the WSDOT hillside drainage system, installed some 20 years earlier to stabilize the hillside, was a primary concern addressed in their reviews. Dames & Moore, WSDOT, and Geo- Engineers, Inc. all indicated that without a functional drainage system, there would be an unacceptable risk of a deep - seated slope failure on the site. The lack of achieving a feasible and acceptable means of improving and maintaining that drainage system apparently was a major consideration in the disapproval of the proposed Valley Vue Development. 4.3 Geotechnical Investigations for the Hillcrest Development. Geotechnical investigations conducted thus far for the proposed Hillcrest Development consist of: 1) two phases of site explorations and evaluations reported by Cascade Geotechnical in May and August of 1990, 2) subsequent letter reports from Cascade Geotechnical providing review of site drawings and plans, 3) a letter describing site reconnaissance observations from Mr. Stuth of Resco Mechanical and Civil Engineering, and 4) a letter from Mr. Dennis Joule, Civil Engineer, describing his site observations and conclusions from reviewing earlier geotechnical reports. A wetlands study on the property by Bredberg and Associates, Inc., dated April 1992, also offered several observations relevant to geotechnical issues, albeit from a biologist's perspective and expertise. 5 W- 6367 -01 5.0 pISCUSSION OF FINDINGS 5.1 Slope Stability SHANNON &WILSON, INC. Slade Way and the proposed Hillcrest Development are situated on a hillside that has experienced repeated slope movements since pre - historic time. An extensive landslide or sequence of slides in pre- historic time downdropped the eastern part of the proposed development, Slade Way, and the area to the east by at least 30 feet. The failure plane for this slide is believed to have been deep - seated, probably within the clayey silt unit. It is within the slide mass of this ancient slide that numerous subsequent slope movements have occurred. The largest known subsequent slope failure within the boundary of the ancient slide mass occurred in 1960 on the hillside below Slade Way. This landslide was initiated by excavation of the toe of the hillside about 700 feet east and 200 feet below Slade Way. The slide progressed upslope to within less than 100 feet east of the north- trending part of Slade Way and cut across the area where the road turns eastward (see Figure 1). The failure plane for the main part of this slide was also deep - seated, about 30 to 50 feet below ground surface within the clayey silt unit. The failure was caused by the combined effects of undercutting of the toe of the hillside and high uplift pressures from the confined groundwater in the deep sand and gravel aquifer. Numerous relatively small shallow slides also have occurred on the hillside, mostly within the disturbed topography of the ancient slide mass. Evidence of shallow soil creep, including bowed and leaning tree trunks, is widespread in the area, most notably on the eastern part of the proposed Hillcrest Development and immediately to the east below Slade Way. Based on a review of previous investigations and a site reconnaissance of the hillside, it is our opinion that Slade Way, the proposed Hillcrest Development, and the adjoining properties could be damaged by either shallow- seated or deep seated landslides. The potential for these two types of landslides to affect Slade Way and the adjoining properties is discussed below. 5.1.1 Shallow Landslide Potential. A shallow slide in the vicinity of Slade Way would likely occur within or at the base of the upper sand unit. -Conditions,that -could contribute to such a.failure include high groundwater levels..in..this water_, table.. aquifer, ..poor. ..surface..drainage conditions which in turn contribute to a high water table, weak soils, and ground modifications that either undercut the toe or overload the head of a potential slide area. All of these conditions exist to varying degrees in the immediate vicinity of Slade Way. 6 W- 6367 -01 SHANNON &WILSON. INC. A short distance below the north - trending segment of Slade Way where it borders the pro- posed Hillcrest Development, the slope was oversteepened by the 1960 landslide. In September 1961, a series of horizontal drains were installed, and slope grading was accomplished in this area below the present location of Slade Way to stabilize existing slide area and to protect the road alignment and adjoining properties. The horizontal drains extended westward from a bench that was excavated at approximately elevation 240 feet on the slope east of the roadway. Although documentation of their exact orientation or lengths has not been found, their locations and elevation (see Figure 1) indicate that these drains were installed in the upper sand unit to provide drainage to the shallow water table aquifer, primarily in the area immediately east of what is now the proposed Hillcrest Development. The site grading and horizontal drains were effective in 1961 in stopping the westward progression of the slide. As of 1968, the horizontal drains were flowing freely, and there was no surficial evidence of slide movement (Shannon & Wilson, 1968). However, by 1982, GeoEngineers, Inc. (1982) reported that a 200 -foot long, 12- to 20 -inch high scarp had formed just below the road. At that time the drains were flowing, although there is no indication of how well they were working. Dames & Moore concluded in 1989 that all but four of the horizontal drains within the proposed Valley Vue Development had deteriorated significantly and that groundwater levels recorded in the piezometers were significantly higher than they had been when the drainage system was installed. WSDOT also noted in 1989 that a slope indicator in this area (location F on Figure 1) indicated movement down to a depth of 24 feet. During our reconnaissance in October 1992, flow was minimal from the horizontal drains that could be found. At that time there was no surficial evidence of recent soil movement below the north- trending part of Slade Way other than widespread bowing of tree trunks caused by shallow creep. It appears from this review that, in the three decades since the horizontal drains were in- stalled, their effectiveness in draining the soils underlying Slade Way has diminished. If the water table has risen as is expected, it follows that the stability of the road has decreased correspondingly. GeoEngineers, Inc. (1985) calculated that for a 3H:1V slope (the approximate slope below Slade Way), the minimum depth to the water table required for a factor of safety of greater than unity is about 6 feet, given the soil strength they assigned to the upper soils (0 = 25 degrees) at a groundwater depth of 12 feet, the factor of safety for Slade Way would be only 1.4. Without specific information on the depth to groundwater and the depth of the upper sands in the roadway area, it is not possible to calculate a meaningful factor of safety for a shallow slope failure that would involve Slade Way. Nevertheless, the field observations over the past 7 W- 6367 -01 SHANNON &WILSON, INC. several years, as described above, suggest the north - trending section of the road is at risk of movement, particularly during the rainy season when the perched water table below the road and seepage pressure at the slope face are highest. These field observations include the very shallow groundwater levels measured on the adjoining proposed Hillcrest Development, evidence that the horizontal drains below the road have deteriorated, small slope movements that have occurred below the roadway in the past several years, and the wet conditions observed locally on the slope below Slade Way. Further to the south, where Slade Way turns eastward, there is an area of the roadway which is currently slipping, apparently within the shallow soils on the slope below the roadway. Within the last year the outside half of the outside lane of the road in this bend subsided about 10 inches. Rather than excavating or recompacting the fill under this section of the road, the City of Tukwila road maintenance crew placed an overlay to reduce the immediate hazard to traffic. Since that repair, the fill and edge of the pavement edge has continued to subside in response to shallow soil movements on the slope below the road. This sliding at the bend in Slade Way is occurring in an area that failed in the 1960 landslide (see Figure 1). Unlike the deep - seated sliding further downslope, movements here probably occurred as relatively small, shallow slips that may be related to a high groundwater table or to loose slide material or fill. It is not known if the slide material in this area was subsequently excavated and filled, or if fill material was simply placed over the downdropped slide material. Unlike the area along Slade Way immediately to the north, there apparently were no horizontal drains installed below this part of the roadway. It appears that the current instability at the bend in Slade Way can be attributed in part to the earlier movement, the lack of subsurface drainage, and, perhaps, the placement or composi- tion of the materials that were used to repair the sliding. Without remediation, this area can be expected to continue to slide. The rate or extent to which this will happen cannot be predicted without an understanding of the subsurface conditions in this area. It appears likely, though, that drainage improvements further north along the roadway would not improve the stability of this area. 5.1.2 -Deep-Seated - Landslide Potential. The risk of a deep - seated landslide on the hillside below Slade Way, like that which oc- curred in 1960, was evaluated by slope stability analyses conducted for WSDOT by Shannon & Wilson, Inc. in 1964 and for the proposed Valley Vue Development by GeoEngineers, Inc., and WSDOT in 1989. This latter analysis, which was based on 1989 groundwater levels, 8 W- 6367 -01 SHANNON &WILSON, INC. provided computed factors of safety ranging from 1.18 to 1.24 for static conditions and 0.86 to 1.08 for dynamic loading (seismic coefficient of 0.15 g). The more conservative factors of safety correspond closely with Profile A -A' and the less conservative correspond roughly with Profile B -B' on Figure 2. WSDOT concluded that the maximum static factor of safety reason- ably achievable by groundwater drawdown would be 1.29 and, conversely, that a 20 -foot rise in groundwater in the confined aquifer would result in a static factor of safety of only 0.82. Given the deterioration of the WSDOT drainage system and the resulting rise in groundwa- ter levels that has occurred over the last 30 years, it is reasonable to conclude that the factor of safety for a deep - seated landslide will continue to decrease without remediation of the drainage system. It is not known what plans WSDOT has for such remediation. It was apparent in 1989, however, that WSDOT was not prepared to make or allow repairs or modifications to the drainage system for the benefit of a private development. 5.2 Geotechnical Considerations for Slade Way Stability Slade Way and the adjoining areas appear to be at some risk of sliding from two different mechanisms: a shallow slope failure, resulting largely from shallow groundwater in the surficial aquifer, and a deep- seated landslide that would result from buildup of artesian pressure in the underlying confined aquifer. It is likely that the risk of deep- seated sliding has increased since 1961 and will continue to increase over time if remedial measures are not taken. There is also evidence to suggest that the potential for shallow sliding has increased. However, it is not clear that this risk will continue to increase without remediation. It is our opinion that, during periods of high rainfall and a corresponding high groundwater level, the stability factor of safety would be close to unity for a shallow slide surface beneath Slade Way. Typically, factors of safety in excess of 1.25 are desired for the design of permanent slopes on highway projects. This philosophy was expressed by WSDOT in their 1989 review of the Valley Vue Estates Project. Based upon stability analyses performed for an adjoining area by GeoEngineers, Inc. (1989), it is inferred that the water table would need to be at or below a depth of 10 feet in order to provide a static factor of safety in excess of 1.25 for the shallow slope stability. Based on these considerations, it is our opinion that soil samples and groundwater levels should be obtained from the Slade Way area to better assess the stability of the road. This could be accomplished by a series of shallow monitoring wells installed in borings drilled on either side of the roadway. The wells should be installed in time for water level measurements during. the rainy season when groundwater is shallowest and most significant in reducing slope stability. 9 W- 6367 -01 SHANNON &WILSON, INC. If stability analyses based on these water levels and soil conditions indicate an unacceptable risk, several approaches are available for improving the stability of the road. The least costly would likely be an attempt to clean out existing horizontal drains below the roadway and monitor the new piezometers (discussed above) for any change in water levels. If such a cleanout is not successful in lowering the water level, additional horizontal drains could be installed. Flow from any horizontal drains should be conveyed in a lined ditch or tightline to the storm sewer. Another relatively simple measure that would likely improve conditions would be to line the ditch along the west side of Slade Way in order to reduce infiltration of surface runoff. A more elaborate method to improve the shallow stability of the roadway and adjoining proper- ties would be to collect surface water and shallow groundwater from the base of the steep slope on the Lowe property by installing a French drain system that is tightlined to the storm sewer. This would likely be the most effective passive solution for improving the hillside stability, but would require dewatering of the Type 3 wetland on the Lowe property. Alternatively, a somewhat deeper French drain could be placed along the west side of Slade Way with similar results. One possible advantage to a drain in this location is that it would probably not complete- ly eliminate the wetland, as would a drain located further to the west. In order to reduce the potential for continued slippage in the area of the bend in Slade Way, the soils underlying the pavement and shoulder should be removed and replaced with compacted fill. Depending on the subsurface conditions encountered, subdrainage measures may be required to improve this section of the road. If remediation of this section of the roadway is seriously considered, it is recommended that additional subsurface explorations (borings and test pits) be conducted in this area to define subsurface conditions, the mechanism of the soil movement, and the details of the site remediation. Depending on the subsurface conditions that are observed in both the north- trending section and in the bend in Slade Way, it is possible that more elaborate measures may be required to achieve a level of safety desired by the City. Such measures might include rock buttresses or retaining walls which would have to be designed to match site conditions. With regard to .the potential for deep - seated sliding, it is our . opinion that the most productive approach the . City can take to improve .hillside , stability would be to encourage WSDOT to rehabilitate their hillside drainage system. If measures are not taken to improve the system, it may also be appropriate for the City to develop an ongoing groundwater monitoring program to assess the effectiveness of the drainage system. In the absence of any remediation by WSDOT, the City may prefer to defer any remediation until such time that movements affect 10 W- 6367 -01 SHANNON &WILSON, INC. Slade Way. At that time, remedial schemes could be considered ranging from fill placement in the failed sections of the roadway to installation of subsurface drains (horizontal drains and /or French drains) to improve the area. Any remedial action should be based on performance standards and requirements established by the City for this section of the hillside. 5.3 Geotechnical Considerations for the Proposed Hillcrest Development. 5.3.1 Slope Stability Considerations Based on our field reconnaissance and review of previous geotechnical reports for the site and vicinity, the proposed Hillcrest Development is interpreted to be situated on the uppermost part of an ancient landslide. This ancient slide mass has been reactivated in the form of smaller slides within the debris from the large ancient slide. The most significant of these slides occurred in 1960, when the reactivation of the slide mass extended nearly to the eastern boundary of the proposed development. Given this history of sliding, it is our opinion that the subject property should be considered an area of potential geologic instability, as defined in the City's document entitled " Areas of Potential Geologic Instability Development and Report Criteria." Specifically, we would consider it to be a Class 4 area, where landslide potential should be considered high unless a sufficient geotechnical evaluation, including site - specific slope stability analyses, demonstrates otherwise. We recommend that slope stability analyses be performed for both shallow and deep sliding and for both pre - and post- development conditions. Such analyses will require additional geotechnical studies to obtain the necessary subsurface and groundwater data. We anticipate that a slope stability analysis for a deep - seated slope failure will require deeper monitoring well installations than currently exist on the property and rainy season water level measurements from such wells. The stability analyses would also require information on the seasonal fluctuation of the near- surface water table. This may require the installation of additional shallow monitor- ing wells. This work should be accomplished soon if the results are to be available in time for construction during the next dry season. We ,anticipate' that. the slope stability analysis. for a deep - seated landslide may indicate similar- results,.to those-obtained by WSDOT:for..the hillside below Slade Way. We also believe it is likely that this analysis will indicate that construction of the development would have little effect on reducing the potential for deep - seated sliding. If the stability analyses for a deep - seated slope failure indicate a significant risk, either considerable work and cooperation from WSDOT 11 - W- 6367 -01 SHANNON &WILSON, INC. in rehabilitating the existing drainage system or installation of deep on -site dewatering wells would be required to reduce the risk. Either approach may not be economically feasible for this development. With respect to the results of a stability .analysis for shallow sliding, the site development could have significant effects, either increasing or decreasing stability depending on the construc- tion methods and site design. However, it is likely that the potential for shallow sliding on site and adjacent to the property can be minimized by proper drainage, construction, and maintenance of the site. 5.3.2 Seismic Hazard The developer of the property will need to address the potential for seismically induced geological hazards, such as liquefaction potential and the potential for earthquake- induced slides as a result of liquefaction in underlying soils. Earthquake- induced landsliding should be addressed by dynamic stability analyses for both deep and shallow sliding and for both existing and post - development conditions. The developer also should consider potential amplification of ground motions in the areas underlain by peat and the minimal lateral resistance the peaty soils may provide to any pile supported structures. 5.3.3 Site Drainage We understand that the owner /developer of the proposed Hillcrest Development has pro- posed to drain the wetlands and capture both surface and groundwater in a French drain system along the base of the steep slope on the property. We also understand that the Tukwila Sensitive Areas Ordinance (SAO) specifies that filling of a Type 3 wetland requires either on -site or off - site mitigation to include a 50 percent increase in total wetland area. As we are not aware of any plans the developer may have for off -site wetlands mitigation, we have assumed that this ordinance will effectively require that, for each acre of wetland which is filled or otherwise removed, construction of 1 -1/2 acres of on -site wetlands would be required to comply with the SAO. Based on our site reconnaissance, review of the wetlands study, and geotechnical reports of the site and vicinity, it is our opinion that the complete drainage of the wetlands, as proposed by the developer, would be necessary :- forsuccessful .development of the property as it currently is planned. Relocation of wetlands within the property does not appear to be feasible or advisable given the proposed layout of five residences in the lowland area of the property. This 12 W- 6367 -01 SHANNON FJWILSON, INC. is particularly the case because of the SAO requirement to increase by 50 percent any wetlands that are relocated. From the - perspective of constructibility; it is-our-opinion that dewatering of the site would be required for foundation•preparation construction;: access: roads, and utilities associated with the five proposed residences as they are currently planned. Consequently, it does not appear to be feasible to develop the site as planned without elimination of much of the existing wetlands. Also, the drainage systems that would be required for the foundations, and surface structures would likely continue to affect surface water to the detriment of any wetland area that could be retained or re- established. With respect to the slope stability of the site and adjacent roadway, removal of the surface water and lowering of the water table on the property would improve the shallow stability of both the site and the adjacent north - trending section of Slade Way. Conversely, a partial relocation of the wetlands within the property may do little, if anything, to improve the long -term stability of the hillside and Slade Way. Until site - specific slope stability analyses are accom- plished, the importance of this drainage issue remains unknown. However, we believe it could be a significant factor in the feasibility of the project. 5.4 Alternative Considerations for Hillcrest Development In lieu of the proposed development, which would require the elimination of the wetlands, it is our opinion that a reduced -scale development may be accomplished outside of the wetlands which would then save this ecological area. Such a reduced -scale development could include building on the uplands portion of the site, with access provided through easements on existing property to the west or possibly along a roadway off Slade Way at the south end of the property. In our opinion, such alternative development schemes could both maintain the existing wetlands and allow development of the property. Regardless of the final proposed development plans, it will be imperative for the owner's geotechnical consultant to perform slope stability analyses for pre- and post- development conditions for both shallow and deep landslide potential. The geotechnical consultant also will have to provide recommendations for_drainage,.foundations, and cuts and fills and retaining walls that are based on the stability analyses. 13 W- 6367 -01 SHANNON 6WILSON, INC. 5.5 Stormflow Drainage We understand from the City of Tukwila that the existing storm sewer systems available to service the proposed Hillcrest Development are operating at capacity. Therefore, as a part of the development it will be necessary to provide- sufficient drainage retention to prevent an increase in peak flows to the storm drains. As proposed by the developer, the permanent drainage of the site would involve collection of both the surface runoff and the shallow groundwater. Surface runoff from the property flows eastward to Slade Way via an 18 -inch culvert on the northern end of the site and along a few minor stream paths further south. This runoff, along with shallow seepage from the surficial soils, enters an unlined ditch along the west side of Slade Way. This ditch drains northward to the storm sewer at the intersection of South 160th Street and Slade Way. This runoff is fed by precipitation directly on the property and, more significantly, by a series of springs at the base of the steep slope that divides an eastern lowland and western upland. Perched groundwater in the lowland area of the property is very shallow. Over much of the area, the water table intersects the ground surface, resulting in shallow ponds and bogs. In this area the shallow groundwater flows eastward under Slade Way where the groundwater is partially drained by horizontal drains that extend below the road and partially surfaces as springs further down the hillside. A large percentage of this perched water is probably intercepted by the WSDOT surface and subsurface drainage system on the hillside below Slade Way. Runoff and spring flow data are not available from the site to use in developing reliable, quanti- fiable estimates of the amount of runoff under either existing or developed conditions. However, it is clear that development of the site will lead to an increase in surface and subsurface water flows into the existing storm /sanitary sewer system and roadway culverts. Therefore, construc- tion of the proposed project will require construction of a runoff detention system to reduce peak storm flows. The developer will need to perform additional subsurface hydrogeologic and geotechnical studies to determine the volume of flow that a detention system will have to accommodate. 6.0 - CONCLUSIONS ► Slade Way and the proposed Hillcrest Development are situated within and near the head of an ancient landslide, and immediately upslope from the head of a deep - seated landslide that failed in 1960. Based on our reconnaissance of the area and review of existing data, it is our opinion that there is a potential for both deep - seated and 14 W- 6367 -01 SHANNON 6WILSON, INC. surficial landsliding that could involve Slade Way and the proposed Hillcrest Develop- ment. ► .. The . deep- seated slope stability of the hillside below Slade Way is believed to be decreasing because of deterioration of an extensive drainage network that was installed in the 1960's to relieve artesian pressure within the hillside. As of 1989, the factor of safety for a deep - seated landslide downslope from Slade Way apparently was unacceptably low for a proposed residential development in that area. ► Additional site - specific geotechnical evaluations, including slope stability analyses, are necessary to evaluate the potential for deep - seated landsliding that could involve the proposed Hillcrest Development. Both static and dynamic stability analyses for existing and post- development site conditions are needed. These analyses will require additional geotechnical and hydrologic data from on -site monitoring wells installed in the deep confined aquifer. ► If slope stability analyses indicate an unacceptably low factor of safety for the proposed Hillcrest Development, remedial measures might include rehabilitation and maintenance of the WSDOT drainage system, or installation of a new dewatering system to achieve drawdown of the confined artesian aquifer. In either case, such remediation may not be feasible for the proposed Hillcrest Development. ► The stability of the shallow soils on the hillside is controlled largely by the level of perched water in the surficial soils and by the angle of the slope. It is likely that groundwater below Slade Way has risen over the past 30 years as a result of clogging of horizontal drains installed below the roadway. Such a rise in the water level would reduce the stability of this section of Slade Way and adjoining properties. ► Site - specific slope stability analyses are also needed to assess the static and dynamic stability of the site and the adjoining section of Slade Way with respect to shallow landsliding. Stability analyses should be performed for both existing conditions and for the proposed development conditions, including cuts, fills, and drainage measures. Shallow piezometer installations will be needed along Slade Way to provide water level information during the rainy season for use in the shallow slope stability analyses. ► If the slope stability analyses for shallow landsliding indicate a risk of slope failure for Slade Way that is unacceptable to the City, a variety of methods are available to improve.- drainage conditions, and lower the. risk of a slope failure.. These methods include: a) rehabilitating the existing horizontal drains or installing new horizontal drains below the road; b) installing a deep French drain along the west side of Slade Way; c) paving the ditch along the west side of Slade Way; and d) installing a French drain at the toe of the slope as has been . proposed by Mr Lowe for the Hillcrest Development. Assuming one or more of these measures are taken, their impact on 15 W- 6367 -01 SHANNON &WILSON, INC. slope stability should be evaluated by water level monitoring in the piezometers installed along the roadway. Slade Way. has recently. experienced .minor slippage or settlement in the area where the road turns eastward. This movement, which occurred in an area of previous sliding, can be expected to continue if corrective action is not taken. At a minimum, reconstruction of the fill material underlying the roadway would be required to stabilize this area. If remediation is desired, explorations are recommended to better assess the cause of sliding and to develop an appropriate remedial scheme. Sliding in this area does not appear to be related to groundwater or surface water on the LeRoy Lowe property. • In our opinion, the proposed Hillcrest Development, as shown on the site plans submitted for the Boundary Line Adjustment (received by City of Tukwila on Septem- ber 9, 1991), is not feasible from a geotechnical perspective unless the wetlands are drained and a subsurface drainage system is installed to substantially lower the water table on the site. • Alternative approaches to site development, involving relocation and reduction of the number of residences, appear feasible without significant impact to the wetlands. If stabilization, of Slade Way adjacent to the wetlands area is warranted, it is likely that this could be accomplished by off -site drainage enhancement which would minimize impact on the wetlands. • Development of the site will result in an increase in peak discharge from both surface runoff and from the groundwater collection system. A runoff detention system will be required within the drainage system to accommodate the peak flows. The design of this system will require shallow geotechnical and groundwater investigations to determine the detention capacity that will be required to handle the added flow from the subsurface drains and surface runoff from the development. 7.0 LIMITATIONS This report has been prepared for the City of Tukwila in accordance with generally accepted professional engineering principles and practices for use by the City of Tukwila in review of the proposed LeRoy Lowe Development, also known as the Hillcrest Development. No other warranty, either expressed or implied, is made as to the nature of the conclusions and recom 16 W- 6367 -01 .w�vesutu..+.vS »t<... Li[iYr::t(a }}iu; >YtiH.li v "M4[MUS]IYnHVStA'V nw..: bsf M' n'.tNJt+f�f.ro:wnvwavA.M1t�wrxwro SHANNON 6WILSON, INC. mendations. The content of this report is not intended for the use of other parties or for other purposes. It may or may not contain sufficient information for other uses. .- SHANNON & WILSON, INC. Daniel N. Clayton, RG, CEG Senior Associate DNC:WPG /dnc W6367 -01.RPT/W6367- 1kd /eet 17 W- 6367 -01 i.r • \ \ 1(Ri{ \ • tat Drive��. • ►1: • • • • • -�:� o N • HILLCREST -�\ DEVELOPMENT �.y` �`:•: SITES e• Approximate Extent Ancient - • of Slide 4- 20 -61— .•.::5r:c x . Landslide ♦ . '��- � • Scarp s. 1960 -1966 SLOPE IMPROVEMENTS o Original Test Drains Recommended Drain (Grade: 1 -3 %) = Cylinder Pile Wall f'"""4.. Existing or Proposed.Right of Way Additional Vertical Drains (6 -9 -in. dia.) +e Large Diameter (5') Deep Well Final Excavated Slope (horizontal:vertical) A Piezometer Installed During 1966 Investigation F• Slope Indicator Profile Location 0 100 200 400 Scale in Feet NOTE Modified from Shannon & Wilson, Inc., 1966. Geotechnical Review LeRoy Lowe Development Tukwila, Washington 1960 LANDSLIDE LOCATION AND REMEDIATION October 1992 W- 6367 -01 SHANNON & WILSON, INC. Gedadnical and Ernirom»ntal Consultants FIG. 1 •9961 "oul 'uosUM q uouue4S wa) P;J!PoW e z 0 0 �o 0 8 a 5 7 5. < f-t 8 le' Elevation In Feet Elevation In Feet gg it • z S I _ Elevation In Feet SHANNON FiWILSON, INC. APPENDIX A SEQUENTIAL LIST OF DOCUMENTS REVIEWED W- 6367 -41 SHANNON &WILSON, INC. APPENDIX A SEQUENTIAL LIST OF DOCUMENTS REVIEWED 1960 Landslide Investigations Documents Dames & Moore, September 29, 1960, Letter to John Graham & Company, re: Borrow Area Slide, South 162nd Street near 54th Avenue South, Tukwila, Washington, 2 p. Dames & Moore, October 14, 1960, Letter to John Graham & Company, re: Borrow Area Slide, South 162nd Street near 54th Avenue South, Tukwila, Washington, 2 p. Dames & Moore, October 19, 1960, Letter to John Graham & Company, re: Borrow Area Slide, South 162nd Street near 54th Avenue South, Tukwila, Washington, 3 p. and map. Dames & Moore, November 1, 1960, Letter to John Graham & Company, re: Borrow Area Slide, South 162nd Street near 54th Avenue South, Tukwila, Washington , 2 p. Dames & Moore, December 1, 1960, "Report of Stability Investigation, Borrow Area Slide, 54th Avenue South and South 162nd Street, Tukwila, Washington," report to John Graham & Company for Puget Western, Inc, 10 p. Dames & Moore, May 19, 1961, Letter to Puget Western, Inc., re: Earthslide South 162nd Street near 53rd Avenue South, 1 p. Dames & Moore, June 27, 1961, "Report of Soils Investigation, Earth Slide, South 162nd Street near 53rd Avenue South, Tukwila, Washington," report and drawings and 2 appendices. Dames & Moore, August 8, 1961, Letter to Puget Western, Inc., re: Slide Control Measures, Earth Slide, South 162nd Street near 53rd Avenue South, Tukwila, Washington, 2 p. Dames & Moore, September 14, 1961, Letter to John Graham & Company re: Slide Area Grading and Drainage , 2 p. Dames & Moore, September 18, 1961, Letter to Puget Western, Inc., re: technical comments, 1 p. Dames & Moore, September 30, 1961, Letter to Puget Western, Inc., re: technical comments, 2 p. Dames & Moore, October 6, 1961, Letter to Puget Western, Inc., re: technical comments, 1 p. (appears to include 2 poor reproductions of maps). A -1 W- 6367 -01 SHANNON &WILSON, INC. Shannon & Wilson, Inc., June 12, 1964, "Report on Foundation Investigation, Existing Slide Area, Tukwila Interchange, PSH 1 (SR5)," for Washington State Highway Commission, Department of Highways, District No. 1, 14 p. Shannon & Wilson, Inc., July 14, 1964, "Report on Foundation Investigation, North of Existing Slide Area, Tukwila Interchange, PSH 1 (SR5), Supplement No. 2" for Washington State Highway Commission Department of Highways District No. 1, 9 p. and figures. Shannon & Wilson, Inc., March 1, 1965, "Report on Foundation Conditions, Certain Parcels of Land in Tukwila," for Washington State Highway Department District No. 1, 7•p. and figures. Dames & Moore, March 10, 1965, Letter to Puget Western, Inc., re: comments on borrow area stability, 1 p. Shannon & Wilson, Inc., April 30, 1966, "Slope Stability Investigation, Tukwila Interchange, PSH 1 (SR5)," for Washington State Highway Commission, Department of Highways, District No. 1, 116 p. Shannon & Wilson, Inc., June 21, 1968, "Summary Report, Soil Conditions and Earth Move- ments, Vicinity of the Tukwila Interchange," prepared for Washington State Highway Commission, District No. 1, 8 p. Shannon & Wilson, Inc., February 6, 1973, Letter report to Don Koll Company, Inc., re: Slade Way Property, Tukwila, Washington, 3 p. and map. Pr posed Valley Vue Development Documents G Engineers, Inc., May 3, 1982, "Report, Geotechnical Design Consultation, Proposed Condominium Development, Slade Way and 53rd Avenue South, Tukwila, Washington ", report to The Mithun Associates, 9 p. G Engineers, Inc. Undated 1983 ?, Geotechnical and Hydrological Studies, Proposed Valley j View Estates, Slade Way and 53rd Avenue South, Tukwila, Washington," Report for Dr. H. M. Allenbach, 17 p. and 2 appendices (Report included as an Appendix C to Tukwila Department of Community Development, 1984 ?). kwila Department of Community Development, undated 1984 ?, Valley View Estates Working Draft Environmental Impact Statement, Part III.. "Existing Conditions, Impacts and Mitigating Measures," pp. 24 -31, Including Appendix C (see GeoEngineers, Inc. undated 1983 ?), report prepared by R. W. Thorp and Associates, Inc. A -2 W- 6367 -01 SHANNON &WILSON, INC. Dames & Moore, January 2.4, 1985, "Review of Draft Environmental Impact Statement, Proposed Valley View Estates" draft report of engineering consultation for the City of Tukwila, 10 p. Dames & Moore, March 10, 1985, Letter to Puget Western, Inc., re: technical comments,1 p. GeoEngineers, Inc. April 25, 1985, Geotechnical and Hydrological Studies, Proposed Valley View Estates, Slade Way and 53rd Avenue South, Tukwila, Washington," Report to Stepan & Associates, Inc. (and Puget Western), 32 p. and 3 appendices (Report included as an Appendix A to Tukwila Department of Community Development, January 1986). Stepan & Associates, Inc. May 1, 1985, "Valley View Estates Utility Report," in Tukwila Department of Community Development, January 1986, Appendix B, 4 p. Tukwila Department of Community Development, January 1986, Valley View Estates Final Environmental Impact Statement, Part III. "Existing Conditions, Impacts and Mitigating Measures," pp. 50 -63, Including Appendix A (see GeoEngineers, Inc., April 25,1985) and Appendix B (see Stepan & Associates, May 1, 1985). Haggard, Joel E. (Attorney), January 27, 1989, Letter to Washington State Department of Transportation transmitting "Instrumentation Plan" and "Monitoring Action Plans" for the Valley View Estates Development, 2 p. and attachments prepared by GeoEngineers, Inc. Washington State Department of Transportation, April 19, 1989, Letter to City of Tukwila re: Valley View Estates, including attachment of intradepartmental letter dated April 19, 1989, reviewing GeoEngineers, Inc., geotechnical analysis of the proposed Valley View Estates, 5 p. and drawings. Attorney General of Washington, February 7, 1989, Letter to Joel E. Haggard re: Valley View Estates Instrumentation Plan and Monitoring Action Plan, 2 p. Dames & Moore, March 6, 1989, "Review of Plans, Proposed Valley View Estates," report to City of Tukwila, 8 p. Tukwila Department of Community Development, March 10, 1989, Letter to Puget Western, Inc., re: Valley View Estates Development, Request for Resubmittal of Plans per Initial Geotechnical /Hydrological Report from Dames & Moore, 6 p. Washington State Department of Transportation, March 23, 1989, Letter to GeoEngineers, Inc., re:. Valley.. View .Estates,..review. of..stability. analysis, 3 p. and drawings. Tukwila Department of Community Development, April 6, 1989, Letter to Puget Western, Inc., re: Valley View Estates building permit application, 6 p. A -3 W- 6367 -01 SHANNON iWILSON, INC. GeoEngineers, Inc., April 10, 1989, Letter to Washington State Department of Transportation re: Valley View Estates, Tukwila, Washington, slope stability analysis review, 4 p. Dames & Moore, April 20, 1989,-"Valley -View-Estates Geotechnical/ Hydrological Review," letter report to .Ci y_of.Tukwila, 8 p. Proposed Hillcrest Development Documents Cascade Geotechnical Inc., May 30, 1990, "Preliminary Geotechnical Investigation for Hillcrest, Slade Way, South of South 160th Street, Tukwila, Washington," prepared for LeRoy C. Lowe, 10 p. and Appendices A, B, and C. Cascade Geotechnical Inc., August 27, 1990, untitled - additional subsurface site investigations for Hillcrest, Slade Way, South of South 160th Street, Tukwila, Washington," prepared for LeRoy C. Lowe, 10 p. and Appendices A, B, C and D. Cascade Geotechnical Inc., April 24, 1991, Letter to LeRoy Lowe, re: Plan Review, Hillcrest, Slade Way, Tukwila, Washington, 3 p. LeRoy C. Lowe, A.I.A. Architect, September 3, 1991, Application for Planned Residential Development for Hillcrest. Bredberg and Associates, Inc., January 8, 1992, Letter to LeRoy Lowe, A.I.A. Architect, re: Hillcrest Boundary Line Adjustment, 2 p. Joule, Dennis, P.E., January 15, 1992, Letter to LeRoy Lowe, A.I.A., re: Hillcrest Dewatering /Stability of Slade Way, 2 p. Resco Mechanical & Civil Engineering, February 17, 1992, Letter re: Hillcrest Boundary Line Adjustment, Dewatering and Stability, 1 p. LeRoy C. Lowe, A.I.A. Architect, February 18, 1992, Letter to Tukwila Planning Commission, re: Hillcrest, 1 p. LeRoy C. Lowe, A.I.A. Architect, February 18, 1992, Letter to Tukwila Planning Department re: Hillcrest, 2 p. Bredberg and Associates, Inc., April 16;1992, "Wetlands Study, City of Tukwila, Hillcrest," report prepared. for .LeRoy : Lowe,: A. I. A, Architect; 17-p. Bredberg and Associates, Inc., May 15, 1992, Letter to LeRoy Lowe, A.I.A., re: Slade Way and S 160th St., Tukwila, 1 p. and attached map. A -4 W- 6367 -01 SHANNON ,WILSON, INC. Cascade Geotechnical Inc., June 27, 1992, Letter to LeRoy Lowe, A.I.A., re: Supplemental Letter, Hillcrest, Tukwila, Washington, 3 p. LeRoy C. Lowe,,A.I.A.. Architect ,:,June.29,::.1992, letter to. Tukwila Department of Community Development, re: Hillcrest B.L. A.., .2 p. Tukwila Department of Community Development, July 17, 1992, Letter to LeRoy Lowe A. I. A Architect, re: Hillcrest Boundary Line Adjustment/Administrative Planned Residential Development, 5 p. LeRoy C. Lowe, A.I.A. Architect, July 18, 1992, Letter to Tukwila Planning Commission, re: Hillcrest Boundary Line Adjustment, Reasonable Use Exception, 2 p. with attachment. LeRoy C. Lowe, A.I.A. Architect, July 27, 1992, Letter to Tukwila Planning Commission, re: Hillcrest Boundary Line Adjustment, 5 p. Tukwila Department of Community Development, August 19, 1992, Letter to LeRoy Lowe, re: Hillcrest Boundary Line Adjustment/Administrative Planned Residential Development, 1 p. Tukwila Department of Community Development, undated, "Wetland and Watercourse Special Studies Report Criteria," 2 p. Tukwila Department of Community Development, undated, "Areas of Potential Geologic Instability Development and Report Criteria," 3 p. 11- 23- 92/APPENDIX.A/W6367- Ikd/eet A -5 W- 6367 -01 SHANNON &WILSON, INC. APPENDIX B IMPORTANT INFORMATION ABOUT YOUR ,SUBSURFACE WASTE MANAGEMENT (REMEDIATION) REPORT TO: FROM: DEPT. OF COMMUNITY DEVE DEPT. OF PUBLIC WORKS DATE: FEBRUARY 7, 1994 RE: City of Tukwila John W. Rants, Mayor Department of Community Development Rick Beeler, Director CITY COUNCIL MEMORANDUM •Pu- / // Ap REBUTTAL FOR THE CITY COUNCIL HEARING REGARDING HILLCREST FILE # 93 -01 -APP STATE DRAINAGE SYSTEM WSDOT in February of 1989 provided a letter on the functioning of the drainage system on SR5, MP 154.0 at Valley Views Estates. We are not aware of any change in the WSDOT position on the system. We have no regulatory authority which the city could impose on WSDOT to improve this system. (See attached Attorney General of Washington letter). CITY STAFF ENGINEERS The City of Tukwila employs several registered professional civil engineers. They are registered in the state of Washington. We have many years of experience in municipal engineering, primarily providing recommendations on development projects and on the design and construction of public capitol projects. Our recommendations on development projects are based on the information provided by the developers consultants. When the facts are available to support a recommendation, we will make one. On this project, we believe additional information is required before recommendations can be made. The Public Works Director and City Engineer are registered civil engineers and support the recommendations of Shannon and Wilson, that additional information is required prior to making final recommendations on this site. BOUNDARY LINE ADJUSTMENT A Boundary Line Adjustment (BLA) is the realignment of property lines between parcels. The purpose of a BLA is to accommodate a minor transfer of land between adjacent, legally create lots of record. The City's role is to approve legal, safe buildable lots. 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665 • :< . OFF -SITE STUDIES The City .often requires off -site studies to be done by the applicant to identify-off-site impacts. This is typically-done for roads, stormwater, water, and sewer. WHETHER TO ALLOW DEVELOPMENT The issue is "not" whether to allow development. The issue "is" whether additional information is needed to make a reasoned decision under the requirements of the Sensitive Areas Ordinance. The deep seeded slope stability analysis that the staff is asking for will provide information that is needed to make sure that any structure or change to the land will be done safely. RECOMMENDATION The Planning Commission's decision to lift the burden from the applicant is conforming to TMC 18.45.125. Based upon the information presented at the hearing on October 14, 1993, and at the City Council hearing January 10, 1994, staff recommends the decision by the Planning Commission be reversed. MAYOR RANTS CITY COUNCIL LEROY C. LOWE A.I.A. ARCHITECT .O •O. 111•1 ..•..1.. w•.Hi..O.CN • ••11 REBUTTAL ADDENDUM DCD APPEAL FEB. 4, 1994 MAYOR /CITY COUNCIL TUKWILA WA. MY REBUTTAL TO MR. BEELER'S APPEAL WAS FOCUSED ON HIS STATEMENT THAT THE PLANNING COMMISSION SUBSTITUTED THEIR JUDGMENT FOR THE CITY'S PEER REVIEWER AND DID NOT GIVE SUBSTANTIAL WEIGHT TO THE DCD'S DECISION. WE RESPONDED TO THIS MISLEADING STATEMENT. AT THE PUBLIC HEARING MR. BEELERS APPEAL WAS EXPANDED TO REVISIT ALL ASPECTS OF THE BLA. THE MOST DISTURBING PROPOSAL WAS THAT I BE REQUIRED TO STUDY THE ENTIRE HILLSIDE ; QUOTING MR. CLAYTON, " - -- IN AN ONGOING STUDY PROGRAM - -" . I HAVE TRACED FROM MR. CLAYTON'S DRAWING, HISTORICAL CROSS SECTIONS THRU THIS HILLSIDE SHOWING ALL OF THE TEST BORINGS PREPARED FROM 1960 TO PRESENT, SUPPORTING MR. CLAYTON'S OWN STATEMENT THAT - -" THE SLOPE BELOW SLADE WAY HAS BEEN THE SUBJECT OF INTENSIVE GEOTECHNICAL INVESTIGATION SINCE 1960, - -" MAY 1 SUBMIT THAT THIS HILLSIDE HAS BEEN STUDIED TO THE POINT THAT IT WOULD BE DIFFICULT TO FIND A VIRGIN PIECE OF GROUND TO DRILL A HOLE IN. FURTHER INVESTIGATION IS NOT LIKELY TO SHOW ANYTHING NEW. VERYC S LEROY C. l.O,E �• :J�AJ N « o �r rm n < mm X m 0 r • 'A r 4) 0 Pa • 0 0 ELEVATION IN FEET m 0 N 0 ELEVATION IN FEET • a N 2 yr n PI m X pi A :V A. N II, iE1 0 WA!. _ _ -N 1 . . =E1 IIIIII m rn t!1 0 N � N 0 0 0 0 S $: PROPERTY LINE PROPERTY A r r 1 CO„ Cy PROPERTY LINE PERTYY LI ti 1' 4. a m 0 m z r z v U) v VV V•.r. -.• -• - -- 5 -17 -90 SITE SURVEY COMPLETED 5 -21 -90 MORITORIUM WAVER REQUESTED 5 -30-90 Isy GEO REPORT COMPLETED 7-2 -90 MORITORIUM WAVER GRANTED 7-0-90 PRELIM. PLANS SUBMITTED 9- +27 -90 GEO REPORTS COMPLETED 9-0-90 GEOTECH REPORTS TO CITY 10 -20-90 SITE SURVEY WITH LOTS •5 & 06 ADDED 1991 4 -24 -91 LETTER FROM CASCADE AT THE REQUEST OF THE CITY RE : GEOTECH & PLANS REVIEW 9-6 -91 B.L.A. APPLICATION FILED 1992 1-6 -92 A.J. BREDBERG WETLAND STUDY 3 -26 -92 PLANNING COMM. HEARING 4 -20-92 WETLAND STUDY FILED WITH CITY 6 -15 -92 REASONABLE USE EXCEPTION REQUEST FILED WITH CITY 1993 1 -13 -93 LOWE 'DEFENDS POSITION THAT ADDITIONAL STUDIES ON OTHER ' PROPERTY . NOT HIS RESPONSIBILITY 3 -30 -93 SLOPE STABILITY ANALYSIS FILED WITH CITY 3 -30 -93 CONTRACT FOR PIER REVIEW OF SLOPE STABILITY ANALYSIS FILED WITH CITY LEROY C. LOWE • A.IA. ARCHITECT •..w 1••• ••••■• ••••••••• 1994 MAY JUN JUL AUG SEP 06 T NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OC T NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB PRELIM PLANS REJECTED BY DARREN WILSON. LOTS ARE NOT CONTIGUOUS. LOWE, DISAGREES. 7 -0-90 DARREN WILSON FINALLY CONCEEDS & ACCEPTS PRELIM. DOC. FOR B.L.A. 10 -20 -90 • JACK PACE CANCELS MEETING 12 -31 -91 JACK PACE CANCELS MEETING 1 -10 -91 JACK PACE CANCELS MEETING 1 -16 -91 JACK PACE DEMANDS WETLAND STUDY 6Mos AFTER B.L.A. SUBMITAL NEEDS INVENTORY OF WHAT IS TO BE DEWATERED. 3 -26 -92 JACK PACE DEMANDS PIER REVIEW OF ALL GEO REPORTS 22MOS AFTER GEO REPORTS WERE SUBMITTED 9-0 -90 DARREN WILSON -NEVER SEEN THESE PLANS BEFORE) 6 -15 -92 PLANNING COMM HEARING 7 -30 -92 SHANNOM & WIL•tSON GEOTECH PIER REVIEW 11 -24 -93 JACK PACE DEMANDS ADDITIONAL GEOTECH STUDIES BE PREPARED ON PUGET WESTERN'S PROPERTY. 12 -9 -92 SHANNON & WILSON DEMAND SLOPE STABILITY, STUDY WITH A PIER REVIEW OF SAME. 3 -2 -93 THIS DEMAND IS MADE. OVER TWO (2) YEARS AFTER GEOTECH REPORTS ORIGINALLY SUBMITTED. SEP 23Ro MEETING CANCELED OCT 14TH PLANNING COMMISSION PUBLIC HEARING • LEROY LOWE'S BLA APPROVED, DCD FILES APPEAL JAN 10TH MAYOR /CITY COUNCIL PUBLIC HEARING ( 4MOS SHORT OF A 4 YEAR BOUNDARY LINE APPLICATION FILING ) FEB. 7TM MAYOR /CITY COUNCIL PUBLIC HEARING (REBUTTAL) tfILLCZ6T CHRONOLOGY OF EVENTS & DATES NILLCREST BOUNDARY LINE ADJ. BOUNDARY LINE ADJUSTMENT 5-17-90 SITE SURVEY COMPLETED 5-21-90 MORITORIUM WAVER REQUESTED 5-30-90 Irr GEO REPORT COMPLETED 7-2-90 MORITORIUM WAVER GRANTED 7-0-90 PRELIM. PLANS SUBMITTED 9-27-90 GEO REPORTS COMPLETED • 9-0-90 GEOTECH REPORTS TO CITY 10-20-90 SITE SURVEY WITH LOTS 05 & 06 ADDED 1991 4-24-91 LETTER FROM CASCADE AT THE REQUEST OF THE CITY RE : GEOTECH & PLANS REVIEW , • 9-6-91 B .L .A. APPLICATION FILED 1992 1-6-92 A.J. BREDBERG WETLAND STUDY 3-26-92 PLANNING COMM. HEARING 4-20-92 WETLAND STUDY FILED WITH CITY 6-15-92 REASONABLE' USE EXCEPTION REQUEST FILED WITH CITY 1993 1-13-93 LOWS DEFENDS POSITION " THAT ADDITIONAL STUDIES ON OTHER PROPERTY • NOT HIS RESPONSIBILITY 3-30-93 SLOPE STABILITY ANALYSIS FILED WITH CITY 3-30-93. CONTRACT FOR PIER REVIEW OF SLOPE STABILITY ANALYSIS FILED WITH CITY LEROY C. LOWC • FEB MAR APR MAY JUN JUL. AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL City of Tukwila PRELIM PLANS REJECTED BY DARREN WILSON. LOTS ARE NOT CONTIGUOUS. LOWE, DISAGREES. 7-0-90 DARREN WILSON FINALLY CONCEEDS & ACCEPTS PRELIM. DOC • FOR B .L..A B.L.A. 10-20-90 • JACK PACE CANCELS MEETING 12-31-91 JACK PACE CANCELS MEETING1-10-91 .JACK PACE CANCELS MEETING 1-16-91 JACK PACE DEMANDS WETLAND STUDY 6mos AFTER B.L.A. SUBMITAL NEEDS INVENTORY OF WHAT IS TO BE DEWATERED. 3-26-92 JACK PACE DEMANDS PIER REVIEW OF ALL GEO REPORTS 221405 AFTER GEO REPORTS WERE SUBMITTED 9-0-90 DARREN WILSON -(NEVER SEEN THESE PLANS BEFORE) 6-15-92 PLANNING COMM HEARING 7-30-92 SHANNOM & WleSON GEOTECH PIER REVIEW 11-24-93 JACK PACE DEMANDS ADDITIONAL GEOTECH STUDIES BE PREPARED ON PUGET WESTERN'S PROPERTY. 12-9-92 SHANNON 6 WILSON DEMAND SLOPE STABILITY, STUDY WITH A PIER REVIEW OF SAME. 3-2-93 THIS DEMAND IS MADE, OVER TWO (2) YEARS AFTER GEOTECH REPORTS ORIGINALLY SUBMITTED. • DAMES & MOORE BORING LOGS BORING # 1 59.5' 4 -24 -61 BORING # 2 106' 4 -27 -61 BORING # 3 100' 5 -3 -61 BORING # 4 100' 5 -8 -61 BORING # 5 49.5' 5 -17 -61 BORING # 6 29' 5 -0 -61 BORING # 7 3$' 5 -0 -61 BORING # 8 22' 5 -0 -61 GEO ENGINEERS BORING LOGS BORING "C" 73.5' 10 -20 -83 BORING "D" 58.5' 10 -19 -83 BORING "F" 63.1' 10 -21 -83 BORING "G" 78.5' 10 -18 -83 BORING "L" 63' 10 -14 -83 BORING "H" 62.5' 10 -13 -83 BORING "J" 84' 10 -15 -83 BORING "K" 88.75' 10 -17 -83 CASCADE GEOTECH BORING LOGS BORING # 1 31.5' 7 -24 -90 BORING # 2 26.5' 7 -25 -90 BORING # 3 26.5' 7 -25 -90 TEST PIT # 1 12' 5 -23 -90 TEST PIT # 2 13' 5 -23 -90 HAND AUGERS # 1 #1 5' 5 -23 -90 #2 5' 5 -23 -90 #3 5' 5 -23 -90 • #4 5' 5 -23 -90 #5 3' 5 -23 -90 #6 5' 5 -23 -90 #7 5' 5 -23 -90 #8 5' 5 -23 -90 #9 5' 5 -23 -90 #10 5' 5 -23 -90 #11 5' 5 -23 -90 #12 5' 5 -23 -90 #13 3' 5 -23 -90 FIlLGCZ6T LEROY C. LOWE A.1.A. ARCHITECT January 18, 1994 City of Tukwila Department of Public Works Mr. Daniel N. Clayton Senior Associate Shannon & Wilson, Inc. P. O. Box 300303 Seattle, Washington 98103 Dear Mr. Clayton: NOTICE TO PROCEED: John W. Rants, Mayor Ross A. Eamst P. R, Director Soils and Geotechnical Evaluation of Leroy Lowe Development Supplement No. 1 to Contract No. 92 -115.1 for $6,000.00 Project No. 92 -DR10 Budget Line Item No. 000113.532.200.41 Your executed original 2 of 2 of Supplemental Agreement. No. 1 to Contract No. 92 -115 is enclosed. The supplement provides for additional review of soils studies and geotechnical evaluation of the proposed Leroy Lowe short plat and opinions and recommendations on the stability of Slade Way and the hillside. 1. Please list the project name and Tukwila project number (92 -DR10) on . general correspondence; 2. Please list the project name, Tukwila project number (92- DR10), contract number (92- 115.1), Supplement No. 1 and the budget line item number (000/13.532.200.41) on all billings. We look forward to working with you toward the successful completion of this project. Sincerely, Ron Cameron, P. E. City Engineer /kjp Enclosure CF: City Clerk Finance Dept. (W /encumbrance notice) Jack Pace, Senior Planner File: 92- DR10 -2 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • Phone: (206) 433-0179 • Fax (206) 431-3665 THIS AGREEMENT after referred referred to as and conditions SuPPLEMEN ? Ala / CONSULTANT AGREEMENT FOR qa -115 Soil and Geotechnical Evaluation SERVICES, 'ii, al *" Leroy Lowe Shortplat, Tukwila, Washington (Addendum to Shannon & Wilson, Inc. proposal dated '/22/92) is entered into between the City of Tukwila, Washington, herein - to as the City ", and Shannon & Wilson, Inc. , hereinafter "the Consultant ", in consideration of the mutual benefits, terms, hereinafter specified. 1. Project Designation. The Consultant is retained by the City to perform geotechnical services in connection with the project designated Leroy Lowe Shortplat Soils and Geotechnical Evaluation 2. Scope of Services. The Consultant agrees to perform the services, identi- fied on Exhibit "A" attached hereto, including the provision of all labor, materials, equipment and supplies. 3. Time for Performance. Work under this contract shall commence upon the giving of written notice by the City to the Consultant to proceed. The Consultant shall perform all services and provide all work product required pursuant to this Agreement within ninety (90) calendar days '7'-' from the date written notice is given to proceed, unless an extension . of such time is granted in writing by the City. 4. Payment. The Consultant shall be paid by the City for completed work and for services rendered under this Agreement as follows: A. Payment for the work provided by the Consultant shall be made as pro- vided on Exhibit "B" attached hereto, provided that the total amount of payment to the Consultant shall not exceed $6,000 (supplemental funding) without express written modification of the Agreement signed by the City. B. The consultant may submit vouchers to the City once per month during the progress of the work for partial payment for that portion of the project completed to date. Such vouchers will be checked by the City and, upon approval thereof, payment will be made to the Consultant in the amount approved. C. Final payment of any balance due the Consultant of the total contract price earned will be made promptly upon its ascertainment and verifica- tion by the City after the completion of the work under this Agreement and its acceptance by the City. D. Payment as provided in this section shall be full compensation for work performed, services rendered, and for all materials, supplies, equip- ment and incidentals. necessary to complete the work. E. The Consultant's records and accounts pertaining to this Agreement are to be kept available for inspection by representatives of the City and state for a period of three (3) years after final payments.. Copies shall be made available upon request. CONSULTANT AGREEMENT Page 2 5. Ownership and Use of Documents. All documents, drawings, specifications and other materials produced by the Consultant in connection with the services rendered under this Agreement shall be the property of the City whether the project for which they are made is executed or not. The Consultant shall be permitted to retain copies, including reproducible copies, of drawings and specifications for information, reference and use in connection with the Consultant's endeavors. The Consultant shall not be responsible for any use of the said documents, drawings, specifications or other materials by the City on any project other than the project specified in this Agreement. 6. Compliance with laws. The Consultant shall, in performing the services contemplated by this Agreement, faithfully observe and comply with all federal state and local laws ordinances and regulations applicable to Qg% m4 the services to be rendered under this Agreement. sole n -;1% e oA 7. Indemnification. The Consultant shall indemnify, defend the City, it's officers, agents and employees, from and a claims, losses or liability, including attorney's fees, or death to persons or damage to property occasioned by failure of the Consultant, its officers, agents and employees, in performing the work required by this Agreement. With respect to the performance of this Agreement and as to claims against the City, its officers, agents and employees, the consultant expressly waives,ks immunity under Title 51 of the Revised Code of Washington, the Industrial Insurance Act, for injuries ��� to its employees, and agrees that the obligation to indemnify,ef�r- and hold harmless provided for in this paragraph extends to any claim brought by or on behalf of any employee of the Consultant. This waiver is mutually negotiated by the parties. This paragraph shall not apply to any damage resulting from the sole negligence of the City, its agents and employees. To the extent any of the damages referenced by this paragraph were caused by or resulted from the concurrent negligence of the City, its agents or em- ployees, this obligation to indemnify,idcfcnd and hold harmless is valid and Pe-i enforceable only to the extent of the negligence of the Consultant, its officers, agents, and employees. an hold harmless inst any and all sing from injury , omission Tr- 8. Insurance. The Consultant shall secure and maintain in force throughout the duration of this contract comprehensive general liability insurance, with a minimum coverage of $500,000 per occurrence and $1,000,000 aggregate for personal injury; and $500,000 per occurrence /aggregate for property damage, and professional liability insurance /tin the amount of $1,005,,00 I ctedjm- self irm.rgu, aL I ?Or 3 Said general liability policy shall name the City of Tukwila 1:s an addition- al named insured and shall include a provision prohibiting cancellation of said policy except upon thirty (30) days prior written notice to the City. Certificates of coverage as required by this section shall be delivered to the City within fifteen (15) days of execution of this Agreement. 9. Independent Contractor. The Consultant and the City agree that the Con - sultant is an independent contractor with respect to the services provided pursuant to this Agreement. Nothing in this Agreement shall be considered CONSULTANT AGREEMENT Page 3 to create the relationship of employer and employee between the parties hereto. Neither the Consultant nor any employee of the Consultant shall be entitled to any benefits accorded City employees by virtue of the services provided under this Agreement. The City shall not be responsible for with- holding or otherwise deducting federal income tax or social security or for contributing to the state industrial insurance program, otherwise assuming the duties of an employer with respect to the Consultant, or any employee of the Consultant. 10. Covenant Against Contingent Fees. The Consultant warrants that he has not employed or retained any company or person, other than a bonafide employee working solely for the Consultant, to solicit or secure this contract, and that he has not paid or agreed to pay any company or person, other than a bonafide employee working solely for the Consultant, any fee, commission, percentage, brokerage fee, gifts, or any other consideration contingent upon or resulting from the award or making of this contract. For breach or vio- lation of this warranty, the City shall have the right to annul this con- tract without liability, or in its discretion to deduct from the contract price or consideration, or otherwise recover, the full amount of such fee, commission, percentage, brokerage fee, gift, or contingent fee. 11. Discrimination Prohibited. The Consultant, with regard to the work per- formed by it under this Agreement, will not discriminate on the grounds of race, color, national origin, religion, creed, age, sex or the presence of any physical or sensory handicap in the selection and retention of employees or procurement of materials or supplies. 12. Assignment. The Consultant shall not sublet or assign any of the services covered by this Agreement without the express written consent of the City. 13. Non - Waiver. Waiver by the City of any provision of this Agreement or any time limitation provided for in this Agreement shall not constitute a waiver of any other provision. 14. Termination. A. The City reserves the right to terminate this Agreement at any time by giving ten (10) days written notice to the Consultant. B. In the event of the death of a member, partner or officer of the Con- sultant, or any of its supervisory personnel assigned to the project, the surviving members of the Consultant hereby agree to complete the work under the terms of this Agreement, if requested to do so by the City. This section shall not be a bar to renegotiations of this Agree- ment between surviving members of the Consultant and the City, if the City so chooses. 15. Attorneys Fees and Costs. In the event either party shall bring suit against the other to enforce any provision of this Agreement, the prevailing party in such suit shall be entitled to recover its costs, including reason- able attorney's fees, incurred in such suit from the losing party. • . CONSULTANT AGREEMENT Page 4 16. Notices. Notices to the City of Tukwila shall be sent to the following address: • City of Tukwila 6200 Southcenter Boulevard Tukwila, WA 98188 Notices to Consultant shall be sent to the following address: Shannon & Wilson. Inc. 400 North 34th Street, Suite 100 P.O. Box 300303 Seattle, Washington 98103 17. Integrated Agreement. This Agreement, together with attachments or addenda, represents the entire and integrated Agreement between the City and the Consultant and supersedes all prior negotiations, representations, or agree- ments written or oral. This Agreement may be amended only by written instrument signed by both the City and the Consultant. DATED this 8th day of December , 19 93 • CITY OF TUKWILA CONSULTANT John Rants, Mayor Attest /Authenticated: City Clerk 'Jane Cantu, Office of the City Attorney By: Daniel N. Layton, C.E.G., R.G. Senior Associate Title Approved as to Form: (32 /FM.CNSLT.1 &.2) LR 02/19/87 JANUARY 13, 1994 WALLY RANTS, MAYOR CITY OF TUKWILA 6200 SOUTHCENTER BOULEVARD TUKWILA, WA. 98188 Dear Mayor Rants; It is our understanding that the Council meeting of January 10, 1994 concerning the HILLCREST appeal was a "quasi judicial" hearing. When this matter came up on the agenda the City Attorney asked if any council member had a conflict or "appearance of'fairness" problem. Mrs. Craft spoke saying she had served on the Planning Commission when the appealed vote was taken on the Hillcrest matter. Mrs. Craft said she would step down bait wished to stay in the room to hear what was said. The City Attorney said, "that's fine...I have no problem with that ". Unfortunately, I do have a problem, along with at least three others in the audience.,_The City Attorney did not inquire if anyone was in agreement with her. At a previous "quasi- judicial" hearing a Councilman was required to not just step down but to leave the room during any and all discussion on the matter. The then City Attorney said he had to leave the room because his expression and posture might be seen by the rest of the council members and have an effect on their decision. This Councilman had not been involved in the decision that was being appealed. We feel the same rules should apply in the instance of Councilwoman Craft. It's not that we are distrustful of the City Council, but as all our City Attorneys have said, "It's the "appearance" of fairness that is important. The City Attorney's decision of January 19, 1994 does not appear to be fair. We respectively ask that Councilwoman Craft be excused from the Counci]. Chambers whenever this appeal on the Hillcrest matter is discussed. Sincerely, Dharlene West, Pres. Tukwila McMioken•Action Committee 5212 So. 164th Tukwila, Wa. 98188 cc; Rick Beeler RECEIVED JAN I d 1g !if COivitvIuNi ! Y DEVELOPMENT CASCADE GEOTECHNICAL INC. 12016 115TH AVENUE N.E., BLDG. H (206) 821.5080 KIRKLAND, WASHINGTON 98034 FAX: (206) 820 -6953 January 12, 1994 Leroy Lowe, AIA Box 1241 Seattle, WA 98111 Re: Tukwila City Council Hearing, January 10, 1994 Hillside Property Dear Leroy, From our discussions and my attendance at the above hearing, I believe it is appropriate for me to provide you with a response to some of the points raised. 1. As I pointed out before, NO ONE has suggested that your property is unstable, or that it has ever been unstable in historical times. 2. EVERYONE technically involved in the project or in the peer review agrees that the project, as proposed, will improve stability for the entire hillside. 3. Your property is presently zoned and subdivided for 13 single family homes. Your proposed boundary adjustment reduces the number of homes to five, and arranges them more suitably on the hillside. 4. In my opinion, the requirements of the City of Tukwila Sensitive Area Ordinance have been fulfilled by the work performed to date. This is based on my work as a professional engineer in this area over the past 28 years. I have worked throughout that time with zoning and sensitive areas ordinances and their application to sloping sites. I have also been engaged to work with several public bodies in the development of SAOs and similar regulations. 5. I understand that some concerns were expressed as to why you do not wish to perform the extensive exploration recommended by the DCD peer reviewer. I believe it would be wise to recap the process which led us to this point. You first engaged us to perform a preliminary investigation at the site. The results indicated that further work and deeper borings would be desirable. You authorized this work and we performed it, providing you with recommendations for the safe development of the site. These recommendations involved a number of site improvements and other items to be included in the design. My analysis and design were reviewed by a number of my competitors, and found to be appropriate and conservative. The plan was submitted to the City of Tukwila and given a "peer review" by Mr. Clayton, a geologist with Shannon & Wilson. Mr. Clayton requested further analytical work on the site, and that additional borings 4200 MERIDIAN, SUITE 216 (206) 647.8588 BELLINGHAM, WASHINGTON 98226 FAX: (206) 671 -4329 CASCADE GEOTECHNICAL INC. and analytical work be done to deal with problems outside the site and beyond your control. We performed the additional analysis requested on the site and found that it supported our earlier analysis and design. My competitors reviewed it again, and again found it to be appropriate and conservative. We did not perform the additional "deep" borings requested by Mr. Clayton, since ample information is available regarding deep conditions at the site and on nearby properties. After all, the area has been intensively studied for over thirty years. 6. Mr. Clayton's expressed concerns are that my report did not address the stability of the nearby property above Klickitat Way. In my opinion, and that of three of my competitors, the stability of this other property is irrelevant to Mr. Lowe's request for a boundary line adjustment on the Hillside property. Nonetheless, thanks to the voluminous records on the other property, I can properly provide a professional opinion as to the stability of the property below Slade Way. Previous work on this other site has been performed by Shannon and Wilson, Dames and Moore, GeoEngineers and perhaps others. In other words, we have the opinions of much of the geotechnical engineering community in the Puget Sound Area. The first recorded movement on the property below Slade Way occurred over thirty years ago and was probably associated with removal of material from the toe of the slope. This movement interfered with the construction of Klickitat Way and destroyed a city street on the property. WSDOT (Then the Washington Department of Highways) installed an extensive drainage system which stabilized the slope, and the City of Tukwila built Slade Way around the outside of the slide area to serve the area beyond. At no time was the Lowe property involved in any soil movement. The movement on the property below Slade Way ceased for about thirty years. Recently, the movement began again on the property below Slade Way, and began to encroach on Slade Way in an area beyond the Lowe property to the South and East. Some movements were also observed on the other side of Slade Way across from the Lowe Property. Investigations by other firms indicated that the drainage system installed to protect Klickitat Way had begun to fail, and that this was the most likely cause of the renewed movement. Our review of these reports leads us to the same conclusion. Mr. Clayton has requested that Mr. Lowe conduct deep borings and extensive analysis to evaluate the stability of the property below Slade Way. In the same report, regarding the same area, he advised his client, the City of Tukwila, only to observe water levels in existing wells and to approach WSDOT to get them to repair their drainage system. He did not recommend that the City of Tukwila conduct any further borings or analysis, even in the event that Mr. Lowe was forced to abandon his property for lack of permits. I am unable to understand why Mr. Lowe, whose property is not in any danger, should need additional borings and extensive analysis to adjust lot lines, while the City of Tukwila, whose street and utilities are presently threatened, are urged to perform only monitoring and administrative efforts to protect city property. Mr. Clayton's firm, Shannon and Wilson, would not knowingly give the city inadequate recommendations to protect city property. I must conclude that Shannon and Wilson does not CASCADE GEOTECHNICAL INC. believe that Slade Way is in such danger that immediate, forceful action by the city is required. I must also conclude that Shannon and Wilson believes that the Lowe property, which cannot be damaged until after the failure of Slade Way, is secure against sliding. I am mystified by the request for additional borings and analysis by Mr. Lowe under these circumstances. I have reviewed the work done in this area by respected engineering firms. I have also reviewed our own work. I believe I can properly provide a professional opinion about the stability of the property below Slade Way without further borings or analysis. This is based on deep borings and extensive analysis done by others and on my own observation of the property. The drainage system installed years ago appeared to stabilize the property below Slade Way. The recent reactivation of the movement below Slade Way appears to be entirely associated with the deterioration of the drainage system. The present Factor of Safety of the area of movement below Slade Way is slightly below 1.0. That is, movement is now taking place. Any analysis which shows a different value must, by definition, be seriously flawed, since it would not reflect the observed conditions at the site. This movement can be arrested again in the same fashion that it was done earlier. That is, by drainage. As stated before, this movement presents no hazard to the Lowe property, and will not do so unless the other parties, including the City of Tukwila, deliberately allow it to happen. 7. 1 wish to emphasize the objectivity of the review done of my work by Neil H. Twelker, Ph.D., P.E.; Dennis Joule, P.E., and Robert Levinson, P.E. Each of these gentlemen is a direct competitor of mine and would have nothing to gain by favorably; reviewing my work. t sic() emphasize that an endless, inconclusive investigation of the Lowe property would be quite a profitable venture for my firm. However, I cannot justify such an investigation and would consider it a total waste of my client's money. CASCADE GEOTECHNICAL INC. 8. I believe that this letter, plus my other correspondence, has answered not only the peer reviewer's concerns about the Lowe property itself, but has also adequately addressed the purported concerns about the property below Slade Way. I see no professional justification for further analysis of the property below Slade Way as part of the development of the Lowe property. Sincerely, CASCADE GEOTECHNICAL, INC. by • George E. President an I EXPIRES 11 / 21 195 t xtrntjes Pave January 8, 1994 Honorable Mayor and Members of the City Council City of Tukwila Re: DCD's Appeal of Planning Commission Decision Proposed Hllicrest Residential Development Ladies and Gentlemen: In the DCD's letter of 10/25/93 the Director makes the statement that •TMC 18.45.125(b)(3) requires the Planning Commission to give the decision of the DCD Director 'substantial weight' He goes on to state that ' ... deep - seated soil borings .. were found necessary to substantiate the Applicant's undocumented claim that the hillside will be stable after development. Documentation has still not been provided to substantiate that the deep - seated stability concems of staff and Shannon and Wilson are unfounded.' I believe that the transcript of the Planning Commission hearing will show very clearly that the members d give, at the very least, serious concern (In a two -hour long, penetrating and agonized discussion) to the DCD's decision. (If 'substantial weight' were to mean that the DCD cannot be overruled, the 'appeal process• would be devoid of meaning). As the Director states in his letter 01 10/25/93, The DCD's decision was based upon a report by Dan Clayton, an engineering geologist with a license to practice engineering geology in the State of Oregon. Mr, Clayton's testimony (at the appeal hearing) to the effect that additional deep borings were required, was controverted by 5 of the most senior practitioners of Geotechnical Engineering in the Pacific Northwest (both registered Civil Engineers in the Sate of Washington), who pointed out that previous studies of the hillside in question have not only produced an unprecedented wealth of geotechnical information, but have resulted in the installation of a drainage system which now has a thirty -Year history of success. All parties agree that maintenance of the WSDOT- installed drainage system is now required. Mr. Clayton has even advised the City of Tukwila concerning the means of correcting the deficiencies in stability without mentioning any need whatever for additional deep borings. It was in this context that the Planning Commission decided that the burden of insuring the future stability of the hillside below Slade Way was not to be the price Leroy Lowe should pay for a adjustment of the Hillcrest property. cc: Leroy Lowe LEMMA 31131 1 q 94 1 Respectfully submitted, NEIL H. VNELKER AND ASSOC., INC. Neil H. Twelcer, Pres. CASCADE GEOTECHNICAL INC. 12016 115TH AVENUE N.E., BLDG. H (206) 821.5080 KIRKLAND, WASHINGTON 98034 FAX: (206) 820.6953 January 5, 1994 Leroy C. Lowe, A.I.A Architect P.O. Box 1241 Seattle, WA 98111 Dear Leroy, I have reviewed your recent correspondence with the City of Tukwila and Neil Twelker's letter of December 18, 1993. Neil's letter presents a very plausible hypothesis as to the origin of the steep slope on your property. He also presents a very. credible picture of events in the development of this unfortunate case. I would like to add the following: 1. 1 fully endorse Neil Twelker's last paragraph: The requirement of the SAO that a stability analysis be performed has been met. No real threat to the Hillside Property will occur until Slade Way is destroyed. The program recommended by Dan Clayton for the Hillcrest Development will not be carried out because it exceeds the value of the development. The preservation of Slade Way must and will be accomplished through the aegis of the City of Tukwila (either directly by the City or by WSDOT at the City's urging). With the stability of Slade Way assured, the Hillcrest Property is in no danger. 2. It appears to me that the City Council is not obliged to examine the technical details of this case. As I understand it, the City Council's review function only requires them to examine whether the Planning Commission has acted in good faith and within its authority. 3. I will be happy to provide further geotechnical input into any problems associated with your property itself. However, none of the engineers or geologists involved in this case have suggested that your property is unstable nor that it has ever been unstable in historical times. I am very reluctant to take your money for the futile 4200 MERIDIAN, SUITE 216 (206) 647.8588 BELLINGHAM, WASHINGTON 98226 FAX: (206) 671 -4329 CASCADE GEOTECHNICAL INC. effort of determining how you, as an individual landowner, might-somehow solve geotechnical problems associated with other nearby landowners, including the City of Tukwila. I would be especially reluctant to do this in the light of the DCD's intransigence as to this project. 4. I believe that this whole process would be greatly advanced if the City of Tukwila took the advice of their own consultant as given in Mr. Clayton's 1992 report: "With regard to the potential for deep - seated sliding, it is our opinion that the most productive approach the City can take to improve hillside stability would be to encourage WSDOT to rehabilitate their hillside drainage system. If measures are not taken to improve the system, it may also be appropriate for the City to develop an ongoing groundwater monitoring program to assess the effectiveness of the drainage system. In the absence of any remediation by WSDOT, the City may prefer to defer remediation until such time that movements affect Slade Way. At that time, remedial schemes could be considered ranging from fill placement in the failed sections of the roadway to installation of subsurface drains (Horizontal drains and /or French drains) to improve the area...." As indicated above, this would remove any doubts about the stability of your property and represents work that the City will undertake regardless of the disposition of your property. Please let me know if you need further information. Sincerely, CASCADE GEOTI?CHNICA,L, I -r b i Geode E. Lamb, President and PrinciliatEngineer 1 FIILLCZeST I HAVE APPEALED THE DEPT. OF COMMUNITY DEVELOPMENT'S DETERMINATION REGARDING MY PROPOSAL. I HAVE JUST RECEIVED A COPY OF THE STAFF REPORT AND WOULD LIKE TO SUBMIT THIS SHORT WRITTEN RESPONSE PRIOR TO THE HEARING. I OBJECT TO THE CONCLUSION THAT A DEEP- SEATED ANALYSIS IS NECESSARY BECAUSE THE DEPARTMENT IS ESSENTIALLY REQUIRING US TO PROVE A NEGATIVE - -- WHICH IS IMPOSSIBLE FOR ANYONE TO DO. ADDITIONAL TESTS ARE UNLIKELY •TO TELL US ANYTHING MORE ABOUT THE DEPARTMENTS THEORY OF THE UNDERLYING LAND CONFIGURATION. THE-CONCLUSION THAT SUCH ANALYSIS IS NECESSARY IS BASED ON A CONCLUSION THAT SOIL DOWNSLOPE, SUCH AS SLADE WAY , IS UNSTABLE. OF COURSE, IF THE INSTABILITY OF SLADE WAY IS USED AS THE BASIS FOR DETERMINING THAT MY PROPOSAL MUST BE REJECTED, THE CITY OF TUKWILA, REMAINS LIABLE SINCE IT IS THE CITY'S RESPONS!BILITY TO PROVIDE LATERAL SUPPORT TO UPSLOPE PROPERTIES SUCH AS MINE . HOWEVER, WHAT IS NOT CLEAR FROM THE STAFF REPORT IS AN AGREEMENT BY ALL TECHNICAL PEOPLE WHO HAVE LOOKED AT THIS PROPOSAL , INCLUDING MR. CLAYTON ON BEHALF OF THE CITY, THAT MY PROPOSAL ADDS TO THE STABILITY OF THE AREA AND WHATEVER "DANGER" TO MY PROPOSED DEVELOPMENT OR NEIGHBORING HOMES WILL BE LESS THAN WHAT ALREADY EXISTS THE ASSERTED RISK TO HUMAN HEALTH OR SAFETY STATED IN THE STAFF REPORT WAS NOT SUPPORTED BY ANY TECHNICAL REVIEW OF MY PROJECT. THE SAME IS TRUE IN REGARD TO DAMAGE TO STRUCTURES -- THE "DANGER" IS LESS THAN WHAT ALREADY EXISTS ON NEIGHBORING PROPERTY. IN REGARD TO THE "CONCLUSIONS" SECTION OF THE STAFF REPORT ON PAGE #6 AND #7, "THE DCD DIRECTOR HAS CONCLUDED THE APPLICANT MUST DEMONSTRATE ONE OF THE FOLLOWING (2) THE AREA OF POTENTIAL GEOLOGIC INSTABILITY CAN BE MODIFIED OR THE PROJECT CAN BE DESIGNED SO THAT ANY POTENTIAL IMPACT TO THE PROPOSED PROJECT AND SURROUNDING PROPERTIES IS ELIMINATED, SLOPE STABILITY IS NOT DECREASED, AND THE INCREASE IN SURFACE WATER DISCHARGE OR SEDIMENTATION SHALL NOT DECREASE SLOPE STABILITY ." LEROY C. LOWE A.I A. ARCHITECT 2 FIILLCRMT I AND MY ENGINEERS BELIEVE WE HAVE DEMONSTRATED THIS REQUIREMENT. THE DISAGREEMENT WITH STAFF APPEARS TO HINGE ON THE MEANING OF THE PHRASE "ANY POTENTIAL IMPACT TO THE PROPOSED PROJECT AND SURROUNDING PROPERTIES IS ELIMINATED." STAFF SUGGESTS THAT ANY POTENTIAL IMPACT FROM THE UNDERLYING INSTABILITY MUST BE ELIMINATED. WE BELIEVE THAT IS AN UNREASONABLE INTERPRETATION OF THE SAO BECAUSE NO ONE COULD EVER COMPLETELY ELIMINATE ALL INSTABILITY . THE SAO SHOULD NOT BE INTERPRETED TO HAVE ABSURD RESULTS OR IMPOSE REQUIREMENTS WHICH ARE IMPOSSIBLE TO MEET UNDER ANY CIRCUMSTANCES. WE CONTEND THAT THE PHRASE MEANS THAT ANY POTENTIAL IMPACT TO STABILITY CAUSED BY THE PROJECT MUST BE ELIMINATED. OUR INTERPRETATION OF THE ORDINANCE MAKES SENSE BECAUSE THE SAO SHOULD BE CONCERNED WITH WHAT IS PRACTICAL - - -- MAKING SURE THAT THE PROPOSED PROJECT DOES NOT ADD TO THE INSTABILITY OF THE LAND AND SURROUNDING PROPERTIES. EVERYTECHNICAL REVIEW OF OUR PROPOSAL HAS CONCLUDED THAT MY PROJECT WILL DO NOTHING TO ADD TO THE INSTABILITY OF THE AREA, BUT WILL ONLY INCREASE THE STABILITY OF THE AREA BY DEWATERING THE SITE. THERE ARE MANY HOMES IN THE AREA WHICH THE CITY HAS NOT SEEN FIT TO REQUIRE PEOPLE TO ABANDON UNDER ANY CONCERN FOR HEALTH AND SAFETY OF THE OCCUPANTS. ONE HOME HAS BEEN BUILT OVER THE TOP OF THE ORIGINAL LAND -SLIDE WITH THE CITYS PERMISSION. WHILE EVERYONE AGREES THAT MY PROPOSED HOMES AND THE HOMES IN THIS AREA WILL BE SAFER, MORE STABLE, THE STAFF'S INTERPRETATION OF THE SAO IS SUCH THAT IT WILL PROHIBIT ADDING TO THE STABILITY OF THE AREA. THIS MAKES NO SENSE I VERY TRULY YOURS LEROY C. LOWE A.I.A. LEROY C. LOWE A.I.A. ARCHITECT MAYOR RANTS CITY COUNCIL REBUTTAL DCD APPEAL MAYOR /CITY COUNCIL TUKWILA, WA. LEROY C. LOWE A.I.A. ARCHITECT •O .03 1341 ••*TTL•. W••MI1407014 0•111 DEC. 4, 1994 THE DIRECTOR OF COMMUNITY DEVELOPMENT HAS TAKEN THE POSITION THAT THE PLANNING COMMISSION HAS NO POWER TO GRANT AN APPEAL OR APPLICATION. (BLA) 1 UNDERSTAND THAT THE PLANNING COMMISSION MUST GIVE SUBSTANTIAL WEIGHT TO THE DCD'S DECISION BUT THAT DOES NOT MEAN THE PLANNING COMMISSION CAN NEVER OVER -RULE THE DCD; IF SO THERE WOULD BE NO REASON FOR ANY APPEAL TO THE PLANNING COMMISSION. THE DCD STATED - - "THE PLANNING COMMISSION SUBSTITUTED THEIR JUDGMENT FOR THE CITY'S PEER REVIEW GEOTECHNICAL CONSULTANT - -" ( MR. DAN CLAYTON ) . THIS MISLEADING STATEMENT CAN NOT GO UNCHALLENGED. WHAT THE PLANNING COMMISSION DID WAS TO CAREFULLY REVIEW AND ANALYZE REPORTS, STUDIES, AND TAKE OVER 12 HOURS OF TESTIMONY FROM PROFESSIONAL ENGINEERS, CITIZENS, AND ATTORNEYS SPANNING MORE THAN (2i) TWO & ONE HALF YEARS BEFORE REACHING AN INFORMED AND THOUGHTFULL DECISION. THE CITY'S PEER REVIEW CONSULTANT, ( DAN CLAYTON ), IS NOT A PROFESSIONAL ENGINEER AND IS NOT LICENSED TO PROVIDE ANY PROFESSIONAL OPINION IN THIS STATE I I BELIEVE THE QUESTION BEFORE THE MAYOR CITY COUNCIL IS A SIMPLE ONE " DID THE PLANNING COMMISSION EXCEED IT'S AUTHORITY BY RENDERING A DECISION ? " IF THE MAYOR AND CITY COUNCIL DENY THE COMMISSION'S AUTHORITY TO MAKE DECISIONS THEY THEN DISMISS AND DISBAND THIS COMMISSION. RESPECIEUJ,J,Y SUBMITTED A.I._A,.._._..__.__...._... I HAVE INCLUDED A LIST OF THOSE PROFESSIONALS WHO HAVE TESTIFIED BEFORE THE COMMISSION filLLCZc5T DESIGN PROFESSIONALS GEORGE LAMB ENGINEER B.E. M.S. LIC. WASHINGTON, ALASKA, CALIFORNIA, IDAHO, OREGON, DIST. OF COLUMBIA. NEIL TWELKER ENGINEER B.S. M.S. PHD. LIC. WASHINGTON, ALASKA, OREGON. DENNIS JOULE ENGINEER B.S. M.S. LIC . WASHINGTON, CALIFORNIA. ROBERT LEVINSON ENGINEER B.S. A.A.S. LIC. WASHINGTON, OREGON, CALIFORNIA, ALASKA, IDAHO, HAWAII, NEW JERSEY. RICHARD STUTH ENGINEER M.S.ME B.S. CE LIC. WASHINGTON ANTHONY JAY BREDBERG.. SOIL. SCIENTIST, BIOLOGIST /BOTANIST NATIONAL REGISTRY , WASHINGTON . JOHN W. JENNINGS SOIL SCIENTIST B.S. NATIONAL REGISTRY FORESTER RICHARD OSBORN LICENSED PROFESSIONAL REALESTATE AGENT WASHINGTON LEROY C. LOWE A.I.A. ARCHITECT 1 CONSULTANTS - TESTIFYING BEFORE THE PLANNING COMMISSION 1991 - 1993 GEORGE LAMB P.E. PROFESSIONAL ENGINEER LICENSED IN : WASHINGTON, ALASKA, CALIFORNIA, IDAHO, DIST. OF COLUMBIA, AND OREGON . EDUCATION : B.E. CIVIL ENGINEERING YALE UNIVERSITY M.S. CIVIL ENGINEERING U. OF WASHINGTON GRADUATE COURSES : ENVIRONMENTAL PROTECTION LAW & PROJECT MANAGEMENT . PROFESSIONAL SOCIETIES : UNIVERSITY OF WASHINGTON ENGINEERING ALUMNI BOARD . FIRST CHAIRMEN OF NOMINATING COMMITTEE . FORMER CHAIRMEN NATIONAL ENGINEERING MANAGEMENT COMMITTEE ON PROFESSIONAL ACTIVITIES . PROJECT MANAGEMENT INSTITUTE INTERNATIONAL SOCIETY FOR SOIL MECHANICS & FOUNDATION ENGINEERING . SECURITY CLEARANCE : U.S. NAVY STRATEGIC INTELLIGENCE DIVISION, SECRET. PUBLICATIONS : CO- AUTHOR OF SIX PROFESSIONAL PAPERS AND PUBLICATIONS ON TECHNICAL AND PROGRAMMATIC ISSUES. WORK EXPERIENCE : 34 YEARS. NEIL H. TWELKER P.E. PHD PROFESSIONAL ENGINEER LICENSED IN WASHINGTON, ALASKA, OREGON. EDUCATION : B.S. CIVIL ENGINEERING UNIVERSITY OF WASHINGTON A.M. SOIL MECHANICS HARVARD UNIVERSITY PHD. SOIL MECHANICS HARVARD UNIVERSITY W /GEOLOGY MINOR PROFESSIONAL SOCIETIES : AMERICAN SOCIETY OF CIVIL ENGINEERS GEOLOGICAL SOCIETY OF AMERICA STRUCTURAL ENGINEERS ASSOC. OF WASHINGTON. UNITED STATES COMMITTEE ON LARGE DAMS TAU BETA P1 SIGMA XI LECTURER /PROFESSOR : UNIVERSITY OF WASHINGTON COLLEGE OF ARCHITECTURE & URBAN PLANNING. WORK EXPERIENCE ; 49 YEARS 2 DENNIS JOULE P.E. PROFESSIONAL ENGINEER LICENSED IN : WASHINGTON, AND CALIFORNIA. EDUCATION : M.S. CIVIL gNGINEERING /GEOTECHNICAL ENGINEERING SAN JOSE STATE UNIVERSITY . B.S. AGRICULTURAL ENGINEERING CALIFORNIA STATE POLYTECHNIC AL U. CURRENT TECHNICAL POSITIONS : KING COUNTY BOARD OF HEALTH ADVISORY COMMITTEE . REPRESENTATIVE FOR THE WASHINGTON STATE PROFESSIONAL ENGINEERS . CHAIRMEN - TECHNICAL SUBCOMMITTEE. WORK EXPERIENCE : 20 YEARS. RICHARD STUTH P.E. PROFESSIONAL ENGINEER LICENSED IN : WASHINGTON : CIVIL ENGINEER MECHANICAL ENGINEER EDUCATION : B.S. MECHANICAL ENGINEERING SEATTLE UNIVERSITY B.S. CIVIL ENGINEERING SEATTLE UNIVERSITY . PROFESSIONAL SOCIETIES : AMERICAN SOCIETY OF PROFESSIONAL ENGINEERS . NATIONAL SOCIETY OF PROFESSIONAL ENGINEERS CURRENT TECHNICAL POSITIONS : CHAIRMEN WASHINGTON STATE DEPT. OF HEALTH TECHNICAL REVIEW COMMITTEE. WORK EXPERIENCE :. 28 YEARS. ROBERT S. LEVINSON P.E. PROFESSIONAL ENGINEER LICENSED IN .: WASHINGTON, OREGON, IDAHO, ALASKA, CALIFORNIA, HAWAII, NEW JERSEY. GEOTECHNICAL ENGINEER : CALIFORNIA. EDUCATION : B.S. CIVIL ENGINEERING CALIFORNIA STATE UNIVERSITY A.A.S. STRUCTURAL TECHNOLOGY N.Y. STATE SOIL MECHANICS UNIVERSITY OF CALIF. U.S.C. BERKELY PROFESSIONAL SOCIETIES : AMERICAN SOCIETY OF CIVIL ENGINEERS BOARD OF DIRECTORS CONSULTANT : CITIZENS COMMITTEE OF TUKWILA SENSATIVE AREA ORDINANCE. WORK EXPERIENCE : 36 YEARS ANTHONY JAY BREDBERG- REGISTERED PROFESSIONAL SOIL SCIENTIST WASHINGTON CERTIFIED SOIL SCIENTIST AMERICAN REGISTRY OF CERTIFIED PROFESSIONALS IN AGRONOMY WETLANDS BIOLOGIST SOIL SCIENTIST, BOTANIST. JOHN WESLEY JENNINGS 3 EDUCATION : M.S. NORTH CAROLINA STATE UNIVERSITY RALEIGH N.C. SOIL AND PLANT SCIENCE B.S. WESTERN ILLINOIS UNIVERSITY PLANT SCIENCE AND BOTANY GRADUATE STUDIES : SOIL SCIENCE UNIVERSITY OF ILLINOIS FORESTRY UNIVERSITY OF NEW HAMPSHIRE WETLANDS UNIVERSITY OF MASSACHUSETTS CURRENT TECHNICAL POSITIONS : NATIONAL SOCIETY OF CONSULTING SOIL SCIENTISTS (NSCSS) BOARD MEMBER SINCE JAN 1990 SOIL AND WATER CONSERVATION SOCIETY OF AMERICA PROFESSIONAL SOCIETIES : INTERNATIONAL SOCIETY OF SCIENTISTS SOCIETY OF WETLAND SCIENTIST WORK EXPERIENCE : 17 YEARS PROFESSIONAL LICENSES CERTIFIED SOIL SCIENTIST AMERICAN REGISTERY OF CERTIFIED PROFESSIONALS IN AGRONOMY. SOIL SCIENTIST - 1974 FOREST SOIL SCIENTIST - OLYMPIC NATIONAL FOREST • EDUCATION : B.S. UNIVERSITY OF WASHINGTON FOREST SCIENCE GRADUATE STUDIES : SOIL SCIENCE OREGON STATE UNIVERSITY FORESTER : SMITH RIVER RANGER DISTRICT REEDSPORT OREGON WORK EXPERIENCE : 20 YEARS LEROY C. LOWE A.I.A. ARCHITECT P.O. BOX 1241 SEATTLE. WASHINGTON D 8111 JAN -Li 0 994 COiviiviuN; ; j DEVELQPMEf .,1 LYNDA COHEN ATTORNEY CITY OF TUKWILA 6200 SOUTHCENTER BLVD. TUKWILA, WA. 98188 DEAR Ms COHEN ; DEC. 29, 1993 ON DEC. 14m 1993 JACK PACE CITY PLANNER AND 1 DISCUSSED THE HEARING WHICH IS TO BE HELD IN YOUR CHAMBERS ON JAN. 10 , 1994. IN THE COURSE OF THIS DISCUSSION HE STATED THAT HE WOULD BE SUBMITTING A WRITTEN ARGUMENT TO THE TRIERS, AND THAT I WOULD BE PERMITTED TO RESPOND WITH A WRITTEN REBUTTAL. HE STATED THAT BOTH SUBMITTALS SHOULD BE MADE, - By DECEMBER 27m 1993. 1 STATED THAT MY REBUTTAL WOULD BE SUBMITTED PROMPTLY, WHEN HIS ARGUMENTS WERE MADE"KNOWN TO ME; I THEN COMMENCED A DRAFT OF MY REBUTTAL, BASED ON WHAT I ANTICIPATED HIS ARGUMENTS WOULD BE, SO THAT I WOULD BE IN A POSITION TO AMEND IT AS NECESSARY, AND COMPLETE MY SUBMITTAL WITHOUT DELAY. MR. PACE HAS NOT PROVIDED ME WITH A COPY OF HIS ARGUMENT, AND NOW CLAIMS THAT HE NEVER INTENDED TO MAKE HIS ARGUMENTS KNOWN TO ME. MR. PACE HAS INFORMED ME BY TELEPHONE CONVERSATION, ON DEC. 27TH THAT BECAUSE MY REBUTTAL WAS NOT RECEIVED BY DEC , 27TH , I AM NOT TO BE ALLOWED ''TO SUBMIT A WRITTEN REBUTTAL PRIOR TO THE HEARING. I MUST PROTEST IN THE STRONGEST MANNER POSSIBLE THIS LATEST PIECE OF DOUBLE- DEALING ON THE PART OF MR . PACE LET ME SIMPLY POINT OUT THAT WHAT MR. PACE SAID HE WANTED FROM ME WAS A REBUTTAL. THIS MEANS AN ANSWER TO AN ARGUMENT. HE IS OBLIGATED ( BY ALL THE RULES OF PUBLIC DEBATE AND COMMON SENSE) TO PRESENT HIS ARGUMENT BEFORE HE IS ENTITLED TO HEAR THE REBUTTAL. EVEN THOUGH I THINK I KNOW APPROXIMATELY WHAT IT IS, I MUST CONFESS THAT MY MIND -READING SKILLS MAY NOT BE UP TO A FULL REVELATION OF WHAT MR . PACE HAS TO SAY; I AM SURE YOU CAN APPRECIATE MY CONSTERNATION AT THIS LATEST MANEUVER ON HIS PART. MR. PACE HAS STATED THAT HE WILL NOT GIVE ME A COPY OF HIS SUBMITTAL FOR THE HEARING ON JAN IO, UNTIL JAN 6TH WHICH WILL LEAVE ME NO TIME TO RESPOND. SINCE HE ALREADY HAS A COPY ON HIS DESK GIVING ME NO TIME TO RESPOND IS APPARE TLY HIS PURPOSE. VERY TRULY YOURS 1t; /Q1�IfE-Th ;1/4 rA. k LERO ('C . L LEROY C. LOWE A.I.A. ARCHITECT P.O. BOX 1241 SEATTLE. WASHINGTON 98111 COii: ivi:jt's Y DE`JELOPMEN JACK PACE PLANNER CITY OF TUKWILA 6300 SOUTHCENTER BLVD TUKWILA, WA. 98188 DEAR MR . PACE : DEC. 21, 1993 THANK YOU FOR THE INFORMATION YOU SHARED WITH ME DURING YOUR PHONE CALL OF DEC. 14, 1993. YOU STATED THAT YOUR SUBMITTAL TO THE MAYOR AND CITY COUNCIL WILL BE COMPLETED ON DEC. 27, 1993 AND THAT YOUR DOCUMENT WILL BE LIMITED TO YOUR INTERPRETATION OF THE S.A.O. (SENSATIVE AREA ORDINANCE), AS IT APPLIES TO MY PROJECT. YOU OFFERED TO INCLUDE MY RESPONSE TO YOUR APPEAL AT THAT TIME. I WOULD BE DELIGHTED TO PROVIDE A REBUTTAL TO YOUR APPEAL, BUT FIRST 1 MUST READ, REVIEW, AND ANALYSE YOUR PAPER TO PROPERLY PREPARE A RESPONSE TO SAME, MY REBUTTAL, WHEN COMPLETED, WIL BE DELIVERE TO THE CITY CLERK FOR DISTRIBUTION. VERY TRULY YOURS : LEROY C . LOV�E�M Telephone: (206) 284 2410 Ne i I K Twelker and Associates, Inc. Consulting Soils Engineers 5645 42nd Ave. West Mail to : P.0 Box 99086 Seattle, Washington 98199 December 18, 1993 Honorable Mayor and Members of the City Council City of Tukwila Re: Review of Planning Commission Decision Proposed Hillcrest Residential Development Let us first review the basic facts of this dispute. The Hillcrest property lies uphill of Slade Way. The ground which lies downhill of Slade Way was involved in a very large landslide which occurred more than 30 years ago. The landslide at that time destroyed a City Street (S. 160th St.). The slide was brought under control by an extensive drainage system installed by WSDOT. The drainage system has been successful in stabilizing the hillside for the past 30 years but is now showing some signs of deterioration (presumably because of clogging of the drainage interfaces); maintenance and /or replacement of some or perhaps all of its components is indicated. Dan Clayton, in reviewing the circumstances of this project, has concluded that the steep sandy slope on the Hillcrest property is a scarp formed by a prehistoric landslide (similar in character and configuration to the slide of 30+ years ago below Slade Way. We (on the other hand) have expressed the opinion that this feature is probably caused by something other than a deep - seated slide (i.e., sapping of the slope by emerging groundwater). Even if a prehistoric landslide had been the cause of the steep slope, the present position of the feature in question could not be in any way indicative of the location of the original slip plane. This is because the steep slope on the Hillcrest property exists at the angle of repose, and the slope is composed of easily erodable material; it is very obviously in a state of retreat under the continuous process of natural weathering agencies (and other disturbances caused by plants, animals, and human activity). The original location of the headscarp of that hypothetical slip surface would have been many feet downslope of the present feature, in all probability below the present location of Slade Way. The Sensitive Areas Ordinance (SAO) requires that a stability calculation must be performed for this property. Cascade Geotechnical did perform such a calculation; however their study was restricted to the actual Hillcrest property. Dan Clayton (in a Peer Review) has criticized this effort as not sufficient, on the grounds that the entire slope below Slade Way should have been studied. The deliberations of the Planning Commission were directed entirely to the manner in which thewording of the SAO should be interpreted: Does a stability calculation restricted to the actual Mayor and City Council (Tukwila) Dec. 18_1993 Page_2 property satisfy the requirements of the act or should the weaknesses of the surrounding area have been taken into account? The Commission decided that the work which had been done conformed with the intent of the Act. "Stability calculation" is a term of art, the procedures and meaning of which are not readily understood outside of the engineering profession. The following paragraphs are intended to shed more light on this important subject. A stability calculation starts with a scenario in which earth movement is envisioned in a manner prescribed by the investigator. For example, the analysis performed by Cascade Geotechnical postulated a number of earth masses bounded by circular arcs, in the vicinity of the steep slope on the Hillcrest property, and then proceeded to arrive at an evaluation of the margin of safety for each presumed possibility. Each scenario was then modified to include the effects of a strong motion earthquake of a given intensity. What finally emerges from this effort is a series of calculations for a multitude of similar (but not identical) scenarios. Cascade Geotechnical 's scenarios were limited to the actual site, whereas Mr. Clayton believes that off-site instability should be investigated. Let us now examine the scenario in which the Hillcrest property is endangered by off-site instabil- ity. This would, of course, be caused by a reactivation of the old slide which occurred in 1960 when a large amount of soil was excavated from the hillside above Klickitat Drive, resulting in the destruction of S. 160th St., and its subsequent replacement with the street now known as Slade Way. The hillside was stabilized with a large -scale drainage system installed by WSDOT. The drainage system functioned perfectly for nearly 30 years; however, it is presently understood to require maintenance and /or replacement of some of its elements. If the failure of the drainage system were to become complete, and the slide were to resume, Slade Way would ultimately be threatened with destruction. If Slade Way were to be succumb to the slide, the Hillcrest property would, without a doubt, be threatened with loss of stability. Mr. Clayton is (in effect) saying that before the Lot Line Adjustment is approved by the City, the developers must first demonstrate either that (1) the drainage system which everyone knows to be in a state of disrepair is actually functioning well, or that (2) the Hillcrest property would not be endangered if Slade Way were to be destroyed. They even outline some of the information which they think should be obtained in this effort! One is tempted to wonder if this advice is being offered seriously or facetiously. If Slade Way were to be destroyed, who would not be concerned for the safety of the Hillcrest property? Moreover, who would not be concerned for the safety of the dozens of other nearby properties whose total value must surely be measured in the millions of dollars? Just picture the consternation which would prevail throughout the entire neighborhood (not to mention City Hall)! Slade Way is a major thoroughfare for hundreds of citizens, for emergency vehicles, and for buried utilities (sewer, water, communications, etc.); its loss would be a calamity of unthinkable proportions. We cannot imagine a geotechnical engineer attempting to "prove" that a calamity of this magnitude would present no threat to uphill property; Mr. Lowe has been told this by us and by other members of the Geotechnical community. Mr. Clayton states (report of November 1992, p. 15) that the consequences of an unacceptably low factor of safety may require remediation which "... may not be feasible for the proposed Hill- crest Development." His message to Mr. Lowe is quite clear: "Just go away!" Let us now suppose that Mr. Lowe accepts this rather pointed suggestion and does go away. Does the concern for the safety of Slade Way simply evaporate? Was the deep - seated sliding scenario Mayor and City Council (Tukwila) Dec. 1$, 1993 Page 3 nothing more than a passing thought, on the order of, say, an impact with an asteroid from outer space? If no further concern is to be felt for the existence of Slade Way, would that not suggest that no serious concern was felt for it in the first place and that the real purpose of Mr. Clayton's report was simply to accommodate Jack Pace by getting rid of Leroy Lowe? If, on the other hand, the concern for Slade Way is genuine, who is going to undertake the investigative program and the ensuing remediation program referred to in Mr. Clayton's 1992 report? He actually suggests an answer to this question in that same report (p. 10) where he states, "With regard to the potential for deep- seated sliding, it is our opinion that the most pro- ductive approach the City can take to improve hillside stability would be to encourage WSDOT to rehabilitate their hillside drainage system. If measures are not taken to improve the system, it may also be appropriate for the City to develop an ongoing groundwater monitoring program to assess the effectiveness of the drainage system. In the absence of any remedia- tion by WSDOT, the City may prefer to defer remediation until such time that movements affect Slade Way. At that time, remedial schemes could be considered ranging from fill placement in the failed sections of the roadway to installation of subsurface drains (hor- izontal drains and /or French drains) to improve the area..... " (emphasis added) Interestingly enough, Mr. Clayton does not suggest that the City of Tukwila embark on the same program of investigation, testing, and monitoring that he thinks Mr. Lowe should perform, nor does he suggest that the City could simply abandon this important artery. Surely some valid reason must underlie this disparity of viewpoint; perhaps he would be kind enough to tell us what it might be. In any event, Mr. Clayton does appear to take for granted that the City of Tukwila will somehow maintain the integrity of Slade Way. We heartily endorse his selection for the "most productive approach" to the problem; the lessons to be learned from 30 years of successful stabilization should not be ignored or cast aside! Summarizing the foregoing discussion, we reiterate that (1) the requirement of the SAO that a stability analysis be performed has been met; that (2) no real threat to the Hillcrest property will occur until Slade Way is destroyed; that (3) the program recommended by Dan Clayton for the Hillcrest Development will not be carried out because it exceeds the value of the development; and that consequently (4) the preservation of Slade Way must and will be accomplished through the aegis of the City of Tukwila (either directly by the City or by WSDOT at the City's urging). With the stability of Slade Way assured, the Hillcrest property is in no danger. cc: Leroy Lowe Pr' of %VAS;, • .�t a .1Ce ;. c; [EXPIRES 3n3I 14 4+ Respectfully submitted, NEIL H. TWELKER AND ASSOC., INC. Neil H. Twelker, Pres. R.. City of Tukwila Department of Community Development October 25, 1993 TO: City Council FROM: Rick Beeler, Director -DC RE: Hillcrest Development - 93 -01 -APP. John W. Rants, Mayor Rick Beeler, Director LI OCT 25 1";3 o !TY OF T U:,, , „i,_ . On behalf of the City of Tukwila I hereby appeal the October 14, 1993, decision of the Planning Commission upholding the appeal by Leroy Lowe of my decision to require deep- seated soil borings pursuant to the Sensitive Areas Ordinance (Chapter 18.45, Tukwila Municipal Code - "TMC "). The basis for this appeal is that substantial weight was not given my decision and that the Planning Commission substituted their judgment for the City's peer review geotechnical consultant. BACKGROUND Hillcrest is a reconfiguration (boundary line adjustment) of 7 existing single family zoned lots into 5 lots in order to accommodate the wetlands and a Class 3 hillside on the property at the southwest corner of Slade Way and S. 160th St. This property is part of the Puget Western property on the east side of Slade Way, on which the Valley View Estates multifamily development failed to achieve a satisfactory stability coefficient to support development. The applicant, Leroy Lowe, submitted geotechnical reports which were subjected to peer review by the City's consultant, Shannon & Wilson, Inc. That review culminated in the July 28, 1993, finding of Shannon & Wilson that: "...the applicant has not adequately addressed the potential for landsliding on this property, nor has he demonstrated that the property meets the requirements set forth in the City of Tukwila Sensitive Areas Ordinance... ". On July 12, 1993, the applicant was notified that staff, on my behalf, agreed with Shannon & Wilson, thereby affirming Shannon & Wilson's May 5, 1993, conclusion that: "The slope stability analysis report for the proposed Hillcrest Development addresses the stability of the north - trending steep slope that runs through the center of the property rather than the stability of the property as a whole. In our opinion, the primary concern for stability of the property is related to the potential 6300 Southcenter Boulevard, Suite. #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665 Hillcrest Appeal, 10 -25 -93 Page 2 for deep seated failure that could affect much if not all of the property, rather than the smaller, more localized type of failures with are addressed in this report." On July 20, 1993, Mr. Lowe filed a timely appeal of staffs decision pursuant to TMC 18.45.125 to the Planning Commission. The Planning Commission heard the appeal on October 14, 1993, and upheld the appeal (3:2 vote). SUBSTANTIAL WEIGHT TMC 18.45.125(b)(3) requires the Planning Commission to give the decision of the DCD Director "substantial weight ". Attached is the staff report to the Commission, which contains the above referenced documents. Staffs request for the deep- seated soil borings was based upon the peer review recommendations of Shannon & Wilson. Those borings were found necessary to substantiate the applicant's undocumented claim that the hillside, and upslope and downslope, will be stable after construction of the development. Documentation has still not been provided to substantiate that the deep - seated stability concerns of staff and Shannon & Wilson are unfounded. The Planning Commission erred in not giving "substantial weight" to the staff decision. The applicant provided no documentation that overcame the "substantial weight" burden or proof. This is a geotechnical issue required to be resolved before construction can proceed on the steep hillside or wetland areas (relocated) (TMC 18.45.080(e)(4)(c). This is not a matter of a "flexible interpretation" of TMC 18.45. GEOTECHNICAL JUDGMENT The applicant's property continues to be owned by Puget Western and is the westerly extension, under Slade Way, of the "Valley View Estates" portion of the large Puget Western holding. A significant landslide occurred in the early 1960's that required installation of a substantial deep - seated drainage system to stabilize the undeveloped Puget Western property. That system is deteriorating and the Valley View Estates development proposal failed because the property could not be safely stabilized. In addition Slade Way displays signs of sloughing. Based on this information, the question became how much of the westerly portion of Puget Western's property (applicant's ownership) possessed similar deep- seated slippage potential or difficulty in achieving a safe stability coefficient. Therefore, staff requested of the applicant geotechnical studies to demonstrate the existing and future stability of the hillside and upslope and downslope properties. Once that information was provided, peer review was conducted, a normal practice in Class 3 hillside conditions. That peer review pointed out deficiencies in the prior geotechnical studies as stated above. :�liul= ^w.�3� ;,:Am..tSt:- "i;4;'i i.; iiS' Q::: MUaz!: v... w.•...., vr., ..W.u+xr.�::,u.o:cex:rn.v:.,,,. Hillcrest Appeal, 10 -25 -93 Page 3 • n,.aar.!.•.z 0.4 ...W... n..+ s< w. ex�-. v:< cn.,,...... ,.,.,.,.....,....w..r.!......�. Staff and Planning Commissioners lack geotechnical engineering qualifications to judge geotechnical reports. That is why peer reviews are conducted to give staff a second opinion or added insurance that public life and safety are protected. The Planning Commission erred in offering their opinion that this specific Class 3 hillside did not need deep - seated testing to document hillside stability. No conclusive documentation, only opinion, was provided to demonstrate what would be found in that testing. Without the testing in this instance, no firm conclusions can be drawn regarding the stability of the hillside. Not to require the testing incurs an undue risk to the future residents of Hillcrest, surrounding property owners, and the City in general. CONCLUSION This appeal is unusual in that staff rarely contests a decision of the Planning Commission who works hard to make reasoned decisions. Normally those decisions do not have public life and safety ramifications for residents, but in the specific situation.ofHlllcrest, peer review by an expert clearly indicated that danger exists unless documented otherwise. It is the applicant's responsibility to demonstrate that life and safety are not endangered. The Planning Commission's decision to lift that burden from the applicant, in this specific instance, is contrary to TMC 18.45 and their expertise. Therefore, the decision must be reversed. LEROY C. LOWE A.J.A. ARCHITECT P.O. BOX 1241 SEATTLE. WASHINGTON 90111 • VERN A. MERYHEW 4431 SO , I4TH SEATTLE , WA. 98188 DEAR MR . MERYHEW ; OCT. 13, 1993 I HAVE APPEALED THE DEPT, OF COMMUNITY DEVELOPMENT'S DETERMINATION REGARDING MY PROPOSAL. I HAVE JUST RECEIVED A COPY OF THE STAFF REPORT AND WOULD LIKE TO SUBMIT THIS SHORT WRITTEN RESPONSE PRIOR TO THE HEARING. I OBJECT TO THE CONCLUSION THAT A DEEP-SEATED ANALYSIS IS NECESSARY BECAUSE THE DEPARTMENT IS ESSENTIALLY REQUIRING US TO PROVE A NEGATIVE--- WHICH IS IMPOSSIBLE =-.. FOR ANYONE TO DO. ADDITIONAL TESTS ARE UNLIKELY TO TELL( US ANYTHING MORE ABOUT THE DEPARTMENTS THEORY OF THE UNDERLYING LAND CONFIGURATION. THE CONCLUSION THAT SUCH ANALYSIS IS NECESSARY IS BASED ON A CONCLUSION THAT SOIL DOWNSLOPE, SUCH AS SLADE WAY, IS UNSTABLE. OF COURSE, IF THE INSTABILITY OF SLADE WAY IS USED AS THE BASIS FOR DETERMINING THAT MY PROPOSAL MUST BE REJECTED, THE CITY OF TUKWILA, REMAINS LIABLE SINCE IT 1S THE CITY'S RESPONSIBILITY TO PROVIDE LATERAL SUPPORT TO UPSLOPE PROPERTIES SUCH AS MINE. rTIcLcZsT HOWEVER, WHAT IS NOT CLEAR FROM THE STAFF REPORT IS AN AGREEMENT BY ALL TECHNICAL PEOPLE WHO HAVE LOOKED AT THIS PROPOSAL; INCLUDING MR. CLAYTON ON BEHALF OF THE CITY , THAT MY PROPOSAL ADDS TO THE STABILITY OF THE AREA AND WHATEVER "DANGER" TO MY PROPOSED DEVELOPMENT OR NEIGHBORING HOMES WILL BE LESS THAN WHAT ALREADY EXISTS. THE ASSERTED RISK TO HUMAN HEALTH OR SAFETY STATED IN THE STAFF REPORT WAS NOT SUPPORTED BY ANY TECHNICAL REVIEW OF MY PROJECT. THE SAME IS TRUE IN REGARD TO DAMAGE TO STRUCTURES -- THE "DANGER" IS LESS THAN WHAT ALREADY EXISTS ON NEIGHBORING PROPERTY. IN REGARD TO THE "CONCLUSIONS" SECTION OF THE STAFF REPORT ON PAGE #t6 AND #7, "THE DCD DIRECTOR HAS CONCLUDED THE APPLICANT MUST DEMONSTRATE ONE OF THE FOLLOWING • (2) THE AREA OF POTENTIAL GEOLOGIC INSTABILITY CAN BE MODIFIED OR THE PROJECT CAN BE DESIGNED SO THAT ANY POTENTIAL IMPACT TO THE PROPOSED PROJECT AND SURROUNDING PROPERTIES IS ELIMINATED, SLOPE STABILITY IS NOT DECREASED, AND THE INCREASE IN SURFACE WATER DISCHARGE OR SEDIMENTATION SHALL NOT DECREASE SLOPE STABILITY." I AND MY ENGINEERS BELIEVE WE HAVE DEMONSTRATED THIS REQUIREMENT. THE DISAGREEMENT WITH STAFF APPEARS TO HINGE ON THE MEANING OF THE PHRASE "ANY POTENTIAL IMPACT TO THE PROPOSED PROJECT AND SURROUNDING PROPERTIES IS ELIMINATED." STAFF SUGGESTS THAT ANY POTENTIAL IMPACT FROM THE UNDERLYING INSTABILITY MUST BE ELIMINATED. WE. BELIEVE THAT IS AN UNREASONABLE INTERPRETATION OF THE SAO BECAUSE NO ONE COULD EVER COMPLETELY ELIMINATE ALL INSTABILITY. THE SAO SHOULD NOT BE INTERPRETED TO HAVE ABSURD RESULTS OR IMPOSE REQUIREMENTS WHICH ARE IMPOSSIBLE TO MEET UNDER ANY CIRCUMSTANCES. 2 ii[GGCRCe5T WE CONTEND THAT THE PHRASE MEANS THAT ANY POTENTIAL IMPACT TO STABILITY CAUSED BY THE PROJECT MUST BE ELIMINATED. OUR INTERPRETATION OF THE ORDINANCE MAKES SENSE BECAUSE THE SAO SHOULD BE CONCERNED WITH WHAT IS PRACTICAL ---- MAKING SURE THAT THE PROPOSED PROJECT DOES NOT ADD TO THE INSTABILITY OF THE LAND AND SURROUNDING PROPERTIES. EVERYTECHNICAL REVIEW OF OUR PROPOSAL HAS CONCLUDED THAT MY PROJECT WILL DO NOTHING TO ADD TO THE INSTABILITY OF THE AREA, BUT WILL ONLY INCREASE THE STABILITY OF THE AREA BY DEWATERING THE SITE , THER ARE MANY HOMES IN THE AREA WHICH THERE ARE MANY HOMES IN THE AREA WHICH THE HAS NOT SEEN FIT TO REQUIRE PEOPLE TO ABANDON UNDER ANY CONCERN FOR HEALTH AND SAFETY OF THE OCCUPANTS . WHILE EVERYONE AGREES THAT MY PROPOSED HOMES AND THE HOMES IN THIS AREA WILL BE SAFER, MORE STABLE, THE STAFFS INTERPRETATION OF THE SAO IS SUCH THAT IT WILL PROHIBIT ADDING TO THE STABILITY OF THE AREA. THIS MAKE NO SENSE VERY TRULY YOURS : LEROY C C. LOWE A.1 .A CHRONOLOGY OF EVENTS 6 DATES HILLCREST BOUNDARY LINE ADJ. BOUNDARY LINE ADJUSTMENT 5 -17 -90 SITE SURVEY COMPLETED 5 -21 -90 MORITORIUM WAVER REQUESTED 5 -30 -90 Isr GEO' REPORT COMPLETED 7 -2 -90 MORITORIUM WAVER GRANTED 7-0 -90 PRELIM. PLANS SUBMITTED 8 -27 -90 GEO REPORTS COMPLETED 9 -0-90 GEOTECH REPORTS TO CITY 10 -20 -90 SITE SURVEY WITH LOTS 35 & 36 ADDED 1991 ' 4 -24 -91 LETTER FROM CASCADE AT THE REQUEST OF THE CITY RE ; GEOTECH & PLANS REVIEW 9 -6 -91 B.L.A. APPLICATION FILED - 1992 1 -8 -92 A.J. BREDBERG WETLAND STUDY 3 -26 -92 PLANNING COMM. HEARING 4 -20-92 WETLAND STUDY FILED WITH CITY 6 -15 -92 REASONABLE USE EXCEPTION REQUEST FILED WITH CITY 1993 1 -13 -93 LOWE DEFENDS POSITION THAT ADDITIONAL STUDIES ON OTHER PROPERTY', NOT HIS RESPONSIBILITY 3 -30 -93 SLOPE STABILITY ANALYSIS FILED WITH CITY 3 -30 -93 CONTRACT FOR PIER REVIEW OF SLOPE STABILITY ANALYSIS FILED WITH CITY LEROY C. LOWE A.IA. ARCHITECT ' FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB City of Tukwila PRELIM PLANS REJECTED BY DARREN WILSON. LOTS ARE NOT CONTIGUOUS. LOWE DISAGREES. 7 -0-90 DARREN WILSON FINALLY CONCEEDS & ACCEPTS PRELIM. DOC. FOR B.L.A. 10 -20 -90 JACK PACE CANCELS MEETING 12 -31 -91 JACK PACE CANCELS MEETING 1 -10 -91 JACK PACE CANCELS MEETING 1 -16 -91 MAR JACK PACE DEMANDS WETLANO STUDY 6M05 AFTER B.L.A. SUBMITAL NEEDS INVENTORY OF WHAT IS APR TO BE DEWATERED. 3 -26 -92 MAY JUN JUL AUG SEP OCT NOV JACK PACE DEMANDS PIER REVIEW. OF ALL GEO REPORTS 22Mos AFTER GEO REPORTS WERE SUBMITTED 9 -0 -90 DARREN WILSON -NEVER SEEN THESE PLANS BEFORE) 6 -15 -92 PLANNING COMM HEARING 7 -30 -92 SHANNOM & WILSON GEOTECH PIER REVIEW 11 -24 -93 DEC I JACK PACE DEMANDS ADDITIONAL GEOTECH STUDIES BE PREPARED ON PUGET WESTERN'S PROPERTY. 12 -9 -92 JAN FEB MAR SHANNON & WILSON DEMAND SLOPE STABILITY STUDY WITH A PIER REVIEW OF SAME. 3 -2 -93 THIS DEMAND APR IS MADE OVER TWO (2) YEARS AFTER GEOTECH REPORTS ORIGINALLY SUBMITTED. MAY JUN JUL AUG SEP OCT NOV DEC SET 23RD MEETING CANCELED . CCT 14Th PLANNING COMMISSION PUBLIC HEARING HILLCREST. BOUNDARY LINE 'ADJ. BOUNDARY LINE ADJUSTMENT, 5 -17 -90 SITE SURVEY COMPLETED 5 -21 -90 MORITORIUM WAVER REQUESTED 5 -30 -90 1sT GEO' REPORT COMPLETED 7 -2 -90 MORITORIUM WAVER GRANTED 7 -0-90 PRELIM. PLANS SUBMITTED 8 -27 -90 GEO REPORTS COMPLETED 9-0 -90 GEOTECH REPORTS TO CITY 10 -20 -90 SITE SURVEY WITH LOTS #5 & #6 ADDED 1991 4 -24 -91 LETTER FROM CASCADE AT THE REQUEST OF THE CITY RE : GEOTECH & PLANS REVIEW 9 -6 -91 B.L.A. APPLICATION FILED 1992 1 -8 -92 A.J, BREDBERG WETLAND STUDY 3 -26 -92 PLANNING COMM. HEARING 4 -20 -92 WETLAND STUDY FILED WITH CITY 6 -15 -92 REASONABLE USE EXCEPTION REQUEST FILED WITH CITY 1993 1 -13 -93 LOWE DEFENDS POSITION THAT ADDITIONAL STUDIES ON OTHER PROPERTY., NOT HIS RESPONSIBILITY 3 -30 -93 SLOPE STABILITY ANALYSIS FILED WITH CITY 3 -30 -93 CONTRACT FOR PIER REVIEW OF SLOPE STABILITY ANALYSIS FILED WITH CITY LEROY C. LOWE A.I.A. ARCHITECT' MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC P-,ELIM PLANS REJECTED BY DARREN WILSON. LOTS A NOT CONTIGUOUS. LOWE DISAGREES. 7 -0-90 D • RREN WILSON FINALLY CONCEEDS & ACCEPTS PRELIM. DOC. FOR B.L.A. 10 -20 -90 JA JA JA JA B. TO 1 K PACE CANCELS MEETING 12 -31 -91 K PACE CANCELS MEETING 1 -10 -91 K PACE CANCELS MEETING 1 -16 -91 K PACE DEMANDS WETLAND STUDY 6Mos AFTER .A. SUBMITAL NEEDS INVENTORY OF WHAT IS BE DEWATERED. 3 -26 -92 JAIK PACE DEMANDS PIER REVIEW OF ALL GEO REPORTS 22Mos AFTER GEO REPORTS WERE SUBMITTED 9 -0 -90 DA REN WILSON -NEVER SEEN THESE PLANS BEFORE) 6 -1 . -92 PLANNING COMM HEARING 7 -30 -92 SH 1 JA PR SH WI IS ORI SE NNOM & WILSON GEOTECH PIER REVIEW 11 -24 -93 K PACE DEMANDS ADDITIONAL GEOTECH STUDIES BE PARED ON PUGET WESTERN'S PROPERTY. 12 -9 -92 NNON & WILSON DEMAND SLOPE STABILITY STUDY A PIER REVIEW OF SAME. 3 -2 -93 THIS DEMAND ADE OVER TWO (2) YEARS AFTER GEOTECH REPORTS 3INALLY SUBMITTED. T 23RD MEETING CANCELED CCT 14TH PLANNING COMMISSION PUBLIC HEARING nILLCIZ6T 1990 CHRONOLOGY OF EVENTS & DATES HILLCREST BOUNDARY LINE ADJ. BOUNDARY LINE ADJUSTMENT 5 -17 -90 SITE SURVEY COMPLETED 5 -2f -90 MORITORIUM WAVER REQUESTED 5 -30 -90 IsT GEO' REPORT COMPLETED 7 -2 -90 MORITORIUM WAVER GRANTED 7 -0-90 PRELIM. PLANS SUBMITTED 8 -27 -90 GEO REPORTS COMPLETED 9 -0-90 GEOTECH REPORTS TO CITY 10 -20 -90 SITE SURVEY WITH LOTS #5 & N6 ADDED 1991 4 -24 -91 LETTER FROM CASCADE AT THE REQUEST OF THE CITY RE : GEOTECH & PLANS REVIEW 9 -6 -91 B.L.A. APPLICATION FILED r992 1 -8 -92 A.J. BREDBERG WETLAND STUDY 3 -26 -92 PLANNING COMM. HEARING 4 -20 -92 WETLAND STUDY FILED WITH CITY 6 -15 -92 REASONABLE USE EXCEPTION REQUEST FILED WITH CITY 1993 1 -13 -93 LOWE DEFENDS POSITION THAT ADDITIONAL STUDIES ON OTHER PROPERTY., NOT HIS RESPONSIBILITY JAN • FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR APR City of Tukwila PRELIM PLANS REJECTED BY DARREN WILSON. LOTS ARE NOT CONTIGUOUS. LOWE DISAGREES. 7 -0-90 DARREN WILSON FINALLY CONCEEDS & ACCEPTS ' PRELIM. DOC . FOR B.L.A. 10 -20 -90 JACK PACE CANCELS MEETING 12 -31 -91 JACK PACE CANCELS MEETING 1 -10 -91 JACK PACE CANCELS MEETING 1 -16 -91 JACK PACE DEMANDS WETLAND STUDY 6Mos AFTER B.L.A. SUBMITAL NEEDS INVENTORY OF WHAT IS TO BE DEWATERED. 3 -26 -92 JACK PACE DEMANDS PIER REVIEW OF ALL GEO REPORTS 22Mos AFTER GEO REPORTS WERE SUBMITTED 9 -0 -90 DARREN WILSON -NEVER SEEN THESE PLANS BEFORE) 6 -15 -92 PLANNING COMM HEARING 7 -30 -92 SHANNOM & WILSON GEOTECH PIER REVIEW 11 -24 -93 I JACK PACE DEMANDS ADDITIONAL GEOTECH STUDIES BE PREPARED ON PUGET WESTERN'S PROPERTY. 12 -9 -92 SHANNON & WILSON DEMAND SLOPE STABILITY STUDY WITH A PIER REVIEW OF SAME. 3 -2 -93 THIS DEMAND IS MADE, OVER TWO ,(2) YEARS AFTER GEOTECH REPORTS LEROY C. LOWE A.I.A. ARCHITECT P.O. BOX 1141 •EATTLC. WA•HINOTON 90111 %It II JACK PACE SENIOR PLANNER CITY OF TUKWILA 6300 SOUTHCENTER BLVD. TUKWILA, WA. 98188 21 ijJ3 DEAR MR. PACE : JULY 20, 1993 1 HAVE REVIEWED THE IMPACT OF CONSTRUCTING RESIDENCES ON THE HILLCREST PROPERTY WITH THREE PROFESSIONAL ENGINEERS. THEIR OPINION IS THAT THERE WILL BE NO ADVERSE IMPACT TO THE PROJECT OR SURROUNDING PROPERTIES; TO THE CONTRARY, CONSTRUCTION WOULD HAVE A STABILIZING EFFECT IN CONTROLLING ON SITE WATER AND THUS ADD A DIMENSION OF SLOPE STABILITY. WE APPEAL YOUR CONSULTANTS INTERPRETATION OF THE SAO AND DISAGREE WITH HIS OPINION REGARDING THE SLOPE STABILITY ANALYSIS PREPARED BY CASCADE GEOTECH. MR. LAMB OF CASCADE HAS STATED THAT THIS SITE IS STABLE AND THE POTENTIAL FOR LANDSLIDING ON THIS PROPERTY HAS BEEN ADDRESSED. The SAO indicates that "prior to permitting alteration of an area of potential geologic instability, the applicant must demonstrate one of the following: 1) There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no signifi- cant risk to the proposed development or surrounding properties; or RESPONSE : 1) THE HILLCREST PROPERTY HAS NOT BEEN THE SUBJECT OF EARTH MOVEMENT IN RECENT HISTORICAL TIME AND THE HILLSIDE BELOW SLADE WAY HAS BEEN STABLE FOR SOME 32 YEARS. THE QUANTITATIVE ANALYSIS OF SLOPE STABILITY PREPARED BY GEORGE LAMB INDICATES THERE IS NO SIGNIFICANT RISK TO THE PROPOSED DEVELOPMENT OR SURROUNDING PROPERTIES. L•• 2) The area of potential geologic instability can be modified or the project can be de- signed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability." RESPONSE : 2) THE DEVELOPMENT OF THE PROPERTY HAS BEEN DESIGNED SO THERE IS NO IMPACT TO THE PROJECT AND SURROUNDING PROPERTY. SLOPE STABILITY WILL BE INCREASED BY THE CONTROL OF SURFACE WATER WITH THE CONSTRUCTION OF OUR STORM WATER DETENTION SYSTEM. WATER DISCHARGE WILL BE CONTROLED AND SEDIMENTATION ELIMINATED WITH THIS WATER DETENTION SYSTEM. WE ARE INCLUDING WITH THIS LETTER, FOUNDATION DESIGN OPTIONS THAT WILL ELIMINATE ANY IMPACT TO THE PROJECT AS A RESULT OF GEOLOGIC INSTABILITY. ALSO INCLUDED ARE LETTERS FROM MR. TWELKER & .BREDBERG VERY TRULY YOURS LEROY C . LOWE A.1 A.I.A. section elevation detail section elevation detail section elevation •edberg ycs' Associates, Inc. Post Office Box 1337 (206) 858 -7055 Gig Harbor, WA 98335 FAX (206) 858 -2534 July 20, 1993 Leroy Lowe P.O. Box 3972 Bellevue, WA 98009 RE: Wetland creation Tukwila site - --- - Dear Lowe: Class ill wetlands can be created on the Tukwila site by detaining the surface sheet flow to create hydric conditions and supplementing the existing vegetation with wetland species. The wetland can be created with out creating areas of surface water and running water. It will require specific design and implementation, but I feel it can be done with the team of geoengineers and engineers currently working on the project. The intent will be to compact the soils and establish low (less than 1 foot) berms to create hydric conditions. Willow, black cottonwood and westem red cedar will be planted to help stabilize the slope. A conceptual mitigation plan can be provided when needed. If you have any questions please feel free to call me. Si cerel ‘ „01.111111 ,,,,, NSCSS 4110 It , A.J. Bre P :43 t Telephone: (206) 284 2410 Nei I H. Twelker and Associates, Inc. Consulting Soils Engineers 5645 42nd Ave. West Mail to :. P.0 Box 99086 Seattle, Washington 98199 July 19, 1993 Leroy C. Lowe, A.I.A P.O. Box 1241 Seattle, Washington 98111 Re: Residential Foundation Design Hillcrest Residential Development Slade Way near South 160th St. Tukwila, Washington Dear Mr. Lowe: In several recent discussions which we have had (including the meeting with representatives of the City of Tukwila on June 21, 1993, the question of the interpretation of the applicable City Ordinance has arisen. The following paragraphs are intended to explore this matter in greater detail, and to develop a foundation system which would resolve the doubts of the City personnel regarding the feasibility of the project. The part of the Ordinance in question states: The area of potential geologic instability can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability." (City of Tukwila Regulation) The first question to be resolved here is: What is the area of potential instability? While we continue to believe that the loss of Slade Way, which would constitute a threat to all of the properties on the uphill side of Slide Way (some 300 families), is a catastrophe of simply unthink- able proportions which would never be allowed to happen, it is nevertheless true that some people are viewing this as a real possibility. If this were to occur, the potential area of instability could conceivably extend to the base of the scarp on the upper part of the Hillcrest site. In the long run, it would not matter if the ancient slide which has been hypothesized by some members of the Geotech- nical community had actually occurred, or if the extent of previous slide movement is represented by the present location of Slade Way. In either case, the threat posed to uphill property by uphill progression of a major landslide is the same (i.e., damage to improvements and loss of property values). Leroy C. Lowe July 19. 1993 P.ge 2 Although the improvement of drainage on the Hillcrest property would be of some limited value (while producing no adverse impacts on adjacent properties), we must assume that, in the face of a total breakdown of the drainage system which has thus far maintained the stability of the area, nothing within reason could be done on the Hillcrest property to actually stabilize the ground below Slade Way ; consequently, the only recourse here would be to design the structures "...so that any potential impact to the project and surrounding properties is eliminated . .." Now comes the question: What is the nature of the impact? From the behavior of the slope during the past several years, we conclude the impact to the new development would consist of Toss of support of the structures, either in part or in whole, over a period of months, or (probably) years. The effect would not be limited to Hillcrest, however; it would extend to many other properties as well. A significant aspect of the potential threat to all uphill locations is that it would not proceed with such speed that it would be a threat to human life. Can the projects be designed to eliminate the potential impact of future instability? The answer to this question is, "Yes, at moderate cost." The problem is to restore foundation support to those areas of the structure in which support has been lost. This could require either support to certain areas of a building (where support has been lost by progressive breaking up of the soil formation) or it could mean the releveling of an entire building because of a massive rotational slip. In either case, the buildings would be designed to resist the stresses imposed by the loss of support and to be releveled by jacking against concrete pads placed beneath the bearing elements, or by jacking against portions of the foundation. Three specific design proposals have been advanced and have been reviewed by us for use in these projects. These are (1) Separation of the stemwall from the spread footing, with bolted brackets at closely spaced intervals, in which jacks would be placed to relevel the structure, (2) Separation of the foundation plate from the stemwall, with the superstructure to be raised by jacking (as in the above system); and (3) If the structure is to be supported on augercast piles, the jacks would be placed between the grade beams and the piles. Any of these sys- tems (as illustrated in working drawings which you have prepared and forwarded to us) could be employed with every prospect of success, and at comparatively modest cost. Of course, it could be argued that if the slide were to go on and on and on, that it would swallow up any and all attempts to correct for the loss of support. It would appear, however, that the large number of dwelling units which would be affected (nearly all of which would have been built without the accommodating features described above) by uncontrolled extension of 'the slide mass would mandate a restoration of the original 1960's drainage system which proved so effective for so many years. A possible remaining question is that of the interpretation of the wording of the Ordinance: Does " .. potential impact to surrounding properties . . ." refer to the impact on adjacent properties produced. by the proposed development, or does it refer to the impact produced on adjacent properties by the hypothetical landslide? It seems very clear to us that the wording is meant to apply to the impact produced by the development, rather than the landslide. (The "impact" of the devel- opment would be entirely favorable, because of the improved surface and subsurface drainage.) The alternative interpretation (i.e., that the development must address the impact of the landslide on the adjacent properties) would be so outlandish as to assign to a developer the task of guaranteeing eternal life and prosperity to everyone within the distance of a cannon shot. In the discussion of foundation types and possible methods of mitigation of future damages, we feel obliged to point out that at the present stage of the development only the question of lot line Leroy C. Lowe July 19. 1993 Page 3 r ev i s i o n is under consideration; specific design proposals for general outline, substructure type, foundation support, and other details are to be dealt with in future stages of the project. The decision to allow a lot line revision will presumably be based on the perceived practicality of a foundation system which will provide the necessary degree of support for the structures to be constructed here, and the method which would be used to correct any loss of grade which might be occasioned by continued deterioration of the 1961 drainage system. The releveling methods outlined above have been used by us on many occasions; we are able to state unequivocably that they are simple, economical, and effective. We note also that the various Geotechnical consultants whose input has been sought for this devel- opment have recommended a variety of foundation systems, ranging from simple spread footings to augercast piles. While, we continue to believe that this site presents no unusual or difficult prob- lems, and that the development can be executed by spread footings, simple drainage procedures, and a modest amount of regrading, we have no overwhelming objection to the use of ultraconservative foundation systems, nor do we believe that they would interfere with the cor- rection of future damages brought about by a reactivation /extension of the old landslide. Wetlands Substitution We have been in contact with A. J. Bredberg of Bredberg and Associates, Inc. regarding the formation of a wetlands on the slope below Slade Way (as a substitute for the wetlands to be lost in the development of the Hillcrest property). His proposal calls for the for- mation of a Class III wetland, in which hydric soils would be created by compacting the existing soils to reduce their permeability, and surface runoff impeded slightly by very low berms. Exist- ing vegetation would then be supplemented with wetland species. We note that the effect of this program would be to create a local alteration of drainage within the surficial soil layers, producing a perennial increased moisture content in the uppermost layers. To the degree that infiltration of surface water is impeded by compaction of the surface soils, the effect of the wetland addition would actually be beneficial to the stability of the hillside. In any event, we believe that the wetlands aspect of the Hillcrest development could have no adverse effect whatever on the groundwater con- dition which has been responsible for the landslide below Slade Way. We shall be pleased to discuss our conclusions at greater length with you or other interested parties at any time or in any place. -��: eo. ,O. H. T WE, t , 1 � .. z ) ..p il_ 15, .., 5757 _ , • :=t .vi ..acrr- ----.: `.:j' • tiS S .4.; ? i NHT:nt • .• r� I EXPIRES 31131 / q ct'Q- Very truly yours, NEIL H. TWELKER AND ASSOC.. INC. Neil H. Twelker, Pres. L City of Tukwila ..,......._... _. _ ____ — ,,.„ ,:::. .. • John W. Rants, Mayor Department of Community Development Rick Beeler, Director July 12,1993 Leroy Lowe P.O.Box 1241 Seattle, Wa 98111 RE: Soil Geotechnical Evaluation of proposed Lowe BLA Dear Mr.Lowe: At the June 21,1993 meeting Mr.George Lamb of Cascade Geotechnical Consultants, disgreed with with our interpretation of the first part of item 2 of Section 18.45.080(e) of the SAO. Attached to this letter is the results of our rewiew of the interpretation. Under the SAO , you have 10 days from the date of this leter to appeal the decision (TMC Section 18.45.125). Should you have any questions, please feel free to call or write. Sincere y, ck Pace enior Planner cc:Ron Cameron, City Engineer ATTACHMENT B 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206).431.3670 • Fax (206) 431 -3665 SHA! ION 6WILSON, Now GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS June 28, 1993 City of Tukwila Department of Public Works 6300 Southcenter Boulevard, Suite 100 Tukwila, Washington 98188 Attn: Mr. Jack Pace, Senior Planner RE: SOILS AND GEOTECHNICAL EVALUATION OF PROPOSED HILLCREST DEVELOPMENT, TUKWILA, WASHINGTON SEATTLE EVERETT KENNEWICK FAIRBANKS ANCHORAGE SAINT LOUIS In our peer review of the slope stability analysis report for the proposed Hillcrest Development we indicated that, in our opinion, the applicant has not adequately addressed the potential for landsliding on this property, nor has he demonstrated that the property meets the requirements set forth in the City of Tukwila Sensitive Areas Ordinance (SAO). Specifically, we indicated that the requirements of Section 18.45.080E of the SAO have not been met. This section of the SAO indicates that "prior to permitting alteration of an area of potential geologic instability, the applicant must demonstrate one of the following: 1) There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no signifi- cant risk to the proposed development or surrounding properties; or 2) The area of potential geologic instability can be modified or the project can be de- signed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability." A meeting was held on June 21, 1993 between the City of Tukwila and Mr. LeRoy Lowe, the applicant, to discuss our review and what steps need to be taken to proceed with application to develop the property. At that meeting the applicant's geotechnical consultant, Mr. George Lamb of Cascade Geotechnical Consultants, disagreed with our interpretation of the first part of Item 2 of Section 18.45.080E of the SAO. As we understand his argument, Mr. Lamb interpreted this clause to mean the following: 400 NORTH 34TH STREET• SUITE 100 P.O. BOX 300303 SEATTLE, WASHINGTON 98103 206.632.8020 FAX 206-M3.13777 JUN 2 9 1993 W- 6367 -02 City of Tukwila Attn: Mr. Jack Pace, Senior Planner June 28, 1993 Page 2 ` "SHANNON &WILSON, INC. The area of potential geologic instability can be modified or the project can be designed - so -that' any. potential impact resulting from the project to the project itself or to the surrounding properties is eliminated. Based on his interpretation of the wording of this clause, Mr. Lamb felt that this condition has been met for the proposed Hillcrest Development and, consequently, that this requirement of the SAO has been fulfilled. In contrast, we interpret this clause to also require that any potential impact to the project from potential geologic instability must be eliminated by design or site modification. Our review indicated that this requirement has not been demonstrated for the proposed development. In order to resolve this difference of opinion, Mr. Jack Pace advised the applicant that the City would review the intent of the wording and consult with the geotechnical consultant who prepared the study on which the SAO was based to determine what their intent had been in this wording. At your request, we contacted Mr. Donald Tubbs of GeoEngineers, Inc., the author of the report that was used in. preparing sections of the SAO pertaining to landslide . hazards. Mr. Tubbs advised us that GeoEngineers, Inc. did not write the clause in question. However, he said that their intent was that for development in an area of potential geologic instability, an applicant should be required to demonstrate that through either site modification or project design there would be no adverse impact to the development as a result of geologic instability. We also reviewed the Sensitive Areas Ordinance for King County to determine what require- ments the county has for developments on areas of potential geologic instability. The King County ordinance specifies that alterations to property in landslide hazard areas may only be allowed in the following circumstances: 1) If the development proposal will not decrease slope stability on adjacent properties; arid 2) If the landslide hazard area can be modified or the development proposal can be designed so that the landslide hazard to the project and adjacent property is eliminated or mitigated and the development proposal on that site is certified as safe by a licensed geologist or geotechnical engineer. W- 6367 -02 City of Tukwila Attn: Mr. Jack Pace, Senior Planner June 28, 1993 Page 3 SHANNON &WILSON, INC. Based on this review, we believe that the intent of the clause in the Tukwila SAO is clearly stated and is consistent with both GeoEngineers' intent and interpretation and with the wording of a similar provision in the King County Sensitive Areas .Ordinance. Therefore, it is our opinion that there is no reason to revise the conclusions presented in our review of the slope stability analysis for the proposed Hillcrest Development. Sincerely, SHANNON & WILSON, INC. Daniel N. Clayton,,C.E.G. Senior Associate DNC:WPG/eet W6367- 02.LT1 /W6367- 1kd /ect W- 6367 -02 Telephone: (206) 284 2410 Ne i I H. Twelker and Associates, Inc. Consulting Soils Engineers 5645 42nd Ave. West Mail to : P.0 Box 99086 Seattle, Washington 98199 June 22, 1993 City of Tukwila 6300 South Center Blvd, Suite 100 Tukwila Wa 98188 Attn: Jack Pace, Dept of Cummunity Development Re: Hillcrest Development (Leroy Lowe) Dear Jack: In reviewing our conference of yesterday (June 21, 1993), it has occurred to me that we seemed to be talking past each other, and that we (all of us) somehow failed to center on the real issues which are involved in this dispute. I would like to take one last shot at a real focus on just what is involved here, not as much for Leroy Lowe as for the entire community; to do this I am taking the liberty of addressing you directly. One remark that Dan Clayton (Shannon and Wilson) made stands out in my mind: "Slade Way is just a peanut on the back of an elephant!" If he is viewing Slade way as a load, and the slope as the beast which must bear the load, I agree with him entirely. In this respect Slade Way is even Tess than a peanut. B ut, it depends on one's point of view; if Dan is thinking of Slade Way as a load which could contribute to a reactivation of that prehistoric slope failure which he has hypothesized, Slade Way is as nothing, as close to nothing as anything could be. On the other hand, to a resident of the area, who must depend on Slade Way for thoroughfare, fire and police protection, sewerage, electricity, water, communications and other vital links to the outside world, Slade Way is a great deal more than a peanut. Let us go back to the facts on which we can all agree: 1. The 1960 slide was instigated by an excavation near the base of the slope. 2. None of the ground which now lies uphill of Slade Way was involved in this activity. 3. For a period of more than 30 years the drainage system installed by WashDOT kept the formerly unstable earth mass in equilibrium. 4. The recent evidences of unstable earth in the vicinity of Slade Way are associated with deterioration of the drainage system installed over 30 years ago. WED ATTACHMENT C JUN 2 3 1993 COMv1rviUNITY DEVELOPMENT City of Tukwila June 22. 1993 Page 2 While I am not entirely certain as to just when it was that the concept of the prehistoric slide (with its 3000-ft long headscarp, with groundwater emerging from its base) made its appearance, I nevertheless have the impression that the potential implications of this feature did not play an important part in any of the developments which have taken place in the area during the decades since the slide occurred. (And if these implications were considered, they certainly appear to have been discounted.) Like Dan Clayton, I can easily visualize a horde of young geologists becoming involved in a never- ending exploration into the mysteries of ice-marginal deposition; it sounds like an absolutely fascinating project, especially when one considers that the answer to the real problem is already known! (Just repair the old 1960 drainage system.) The final truth must be self-evident to everyone: Unless Slade Way moves, the Hillcrest property is not going to move. If the City of Tukwila (or the State of Washington ) is going to maintain the drainage system which has been shown to be so effective in protecting Slade Way and the ground which lies uphill of it, the Hillcrest property is not going to be endangered by a reactivation of either the 1960 slide or a reactivation of the hypothesized "prehistoric slide." During this meeting you refused on several occasions to answer the question as to what the City's intentions are regarding the future of Slade Way. I cannot help but think that if the City of Tukwila intends to let Slade Way fall into disuse it should (out of courtesy to its citizens) so state without delay. . Very truly yours, NEIL H. TWELKER AND ASSOC.. INC. by Neil H. Twelker, Pres. NHT:nt cc: Leroy Lowe CASCADE GEOTECHNICAL INC. 12016 115TH AVENUE N.E., BLDG. H (206) 821 -5080 KIRKLAND, WASHINGTON 98034 FAX: (206) 820.6953 May 20, 1993 Leroy Lowe PO Box 1241 Seattle, WA 98111 SUBJECT: Hillcrest Residential Development Dear Leroy: I have . reviewed the letter dated March 14 from the City of Tukwila and the attached review dated May 5 from Shannon and Wilson. I have also discussed the situation with Neil Twelker and reviewed the draft of his letter responding to the "peer review" by Shannon and Wilson. I have also re- examined all our previous work on the property and the previous reports by others in our files. From these I have reached the following conclusions: • The Shannon and Wilson "peer review" of May 5, 1993 fails to address the stability of your property in itself. The review concentrates on stability downhill from Slade Way. In regard to your property itself, the review does state that "from a cursory review, however, it appears that the stability analyses are conservative with respect to shallow soil conditions and seismic load conditions ". The review also requests further information regarding our methodology. This information is in our file and will be forwarded if needed. • My review reinforces my professional opinion that your property is stable at the present time. • My review also reinforces my professional opinion that the property will be even more stable after it is developed as we recommended. • The only realistic expectation of danger to your property would be if the City of Tukwila fails to protect your property against the loss of lateral support. This could occur if the City permitted Slade Way to fail completely and thus remove lateral support from your property. Barring the possibility of this irresponsibility action by the City, your property is stable and can be developed as planned. CAL:ADE GEOTECHNICAL'INC. I trust that this brief note restates my position clearly enough to resolve most of the confusion. Good luck. Sincerely, CASCADE GE GEL:pg 1EXPIRES 11-21-93 • The steep lope in the dry sand unit on the Hillcrest property is being continually undermined by emerging groundwater. As soil is carried off, the slope retreats, all the while maintaining the . angle of repose of the sand which ,comprises it. ' This is the slope which Shannon and Wilson state is the head - scarp of a prehistoric landslide. i Thisjis the "deep-seated" S /oy�y 200t- slide studied by Cascade Geotechnical. (-7) Ft_ !SA i ✓ofe 'rove_ 744111111ka. brra.. --ter Test borings in the immediate /co- vicinity of the site extend to depths of 200 ft. �► f, • This is the area in which the drainage measures were placed to control the slide on the hillside between Slade Way and Klickitat Drive. This area and the drainage measures within it are believed to be publicly owned (by the State of Washington?) This is the approximate location of the "deep-seated" slides which occurred in 1960-61. sr-% /)-1} 74-4 'moo • This is the area fron material was remove and which resulted it lateral support of th between Klickitat Di Way. OwA,E0 ? r -t 42� 1 This is the approximate position of the deep-seated slip surface which Shannon and Wilson believe to be a threat to the Hillcrest Property. Goss SEG-7i0 T RouGfi •I LL_c ES-r .SLA DEF. \A/Y a- Kt • 5Ga;e.: 111 = 1 • The steep•pe in the dry sand unit on the Hillcrest property is being continually undermined by emerging groundwater. As soil is carried off, the slope retreats, all the while maintaining the angle of repose of the sand which comprises it. ' This is the slope which Shannon and Wilson state is the head - scarp of a prehistoric landslide. OEEL_ ,,a Thus`is the "deep-seated" 26,o.. slide studied by Cascade i Geotechnical. Test borings in the immediate vicinity of the site extend to depths of 200 ft. This is the approximate position of the deep-seated slip surface which Shannon and Wilson believe to be a threat to the Hillcrest Property. This is the area in which the drainage measures were placed to control the slide on the hillside between Slade Way and Klickitat Drive. This area and the drainage measures within it are believed to be publicly owned (by the State of Washington?) This is the approximate location of the "deep-seated" slides which occurred in 1960-61. r\ , a/e pro vC/ �y p✓sue /CL Y •014//v O 2 This is the area from which material was removed in 1960, and which resulted in the loss of lateral support of the hillside between Klickitat Drive and Slade Way. 1 Soo 250 200 /So ioo CFZOSc SEs- 1O N TWROuGH H- t LLC2`S-r,. SLA DE WY a- VLICF<► T- T May 19, 1993 Job No. 9005 -13G Leroy Lowe PO Box 1241 Seattle, WA 98111 SUBJECT: Peer Review of Our Evaluation of Your Tukwila Property Dear Leroy: I have reviewed the letter from Shannon and Wilson which you dropped off yesterday and wish to make a few brief comments. Feel free to share these with the City if you wish. The peer review basically restates the City's position that development of your property will not be permitted until you correct any stability problems on the street and the property below the street. It seems clear to me that the C'ty staff will not willingly issue a permit for the development of this property. The City and Shannon and Wilson indicate that the owner of your property is responsible for maintaining the hillside from the street to the freeway. They also believe that the scarp on your side is unquestionably the head scarp of that movement, although Neil Twelker's analysis indicates that other reasonable possibilities exist. I have enjoyed working with you on this project, but I feel it would be a disservice to you to continue to perform analyses which will continue to be ignored. The peer review starts with the assumption that you must address the stability of the entire region, rather than that portion which now involves your property. During our conversations, and during our work on this matter we have repeatedly emphasized that we are analyzing the stability of the slope on your property. That is, the area over which you have some control. An attempt to dewater a lower aquifer from wells on your property would require that you discharge large quantities of water which the City will not allow to enter the City drainage system. It would also, according to Shannon and Wilson's theory, provide a major advantage to both the City, which maintains the street, and the lower property owner by stabilizing their property at no cost to themselves. I believe that the responsibility for the street and lower property must be established, before any work is undertaken to correct it. I do not see that it is your responsibility to fix a problem brought about by the excavation at the toe of the slope, the failure of the Highway Department's drainage system, and the failure of the City to protect the street from encroachment from below. •re�a:ele:::r::': You have a political problem not a technical problem. Your property can't be used until the City . allows the boundary adjustment and the city staff won't allow that regardless of the technical facts. I will be happy to work with you to evaluate and provide solutions for the technical problems when we get to that point. In the meantime I would be very reluctant to waste your money on additional technical work at the site. Once you have obtained the permits, and I am confident that you will, I will be happy to work with you on the details of developing this property. Good luck Leroy. Keep me posted. Sincerely, CASCADE GEOTECHNICAL, INC. George E. Lamb, P.E. President GEL:pg Telephone: (206) 284 2410 Nei I I-1 Twelker and Associates, Inc. Consulting Soils Engineers 5645 42nd Ave. West Mail to : • P.0 Box 99086 Seattle, Washington 98199 May 17, 1993 Leroy C. Lowe, A.I.A P.O. Box 1241 Seattle, Washington 98111 Re: Clarification of Slope Stability Issues Hillcrest Residential Development Slade Way near South 160th St. Tukwila, Washington Dear Mr. Lowe: We are in receipt of the "peer review" recently made by Shannon and Wilson of the March 22, 1993 report of Cascade Geotechnical. Our comments and further clarification of the issues which are involved in this discussion are presented in the following paragraphs. The Shannon and Wilson position in this latest report is exactly the same as in their previous report (November 23, 1992), namely that the site is threatened by a reactivation of the 1960 slide because of the deterioration of the WSDOT drainage system at the base of the hill. Inasmuch as the terms " deep - seated sliding," deep - seated aquifers," and "confined groundwater tables" have been bandied about without explanatory detail, and because the discussion of the problem will once more take place before persons not totally familiar with those terms or their true significance, we feel obliged to present the following additional clarifying information. Deep - Seated Slide Studies On page 4 of the Cascade Geotechnical Report (March 22, 1993) the approximate positions of the two deep - seated slide circles for the pseudostatic case are described. The report does not state that the static circles were in the same (or nearly the same location); however, we consider it only logical that the positions of the critical circles would not differ greatly for the two conditions of analysis. We note also that the report contains no other direct reference to the location of the circles for the static analysis. The failure surfaces referred to are described as, ...for section A -A' (it) began at the crest of the slope and continued approximately forty (40) feet eastward beyond the toe. The deep - seated, pseudostatic failure surface for section B -B' began forty (40) feet west of the crest of the slope and terminated eastward at the toe. Both deep- seated failure surfaces were, at their deepest point, approximately twenty (20) feet deep." Leroy C. Lowe May 17. 1993 Paget The positions of these critical circles are illustrated in the attached cross - section of the site, which extends from the westerly margin of the site, through Slade Way to Klickitat Drive at the base of the hill. It is evident that Cascade was concerned only with the possibility of slides on the actual property, whereas Shannon and Wilson are principally concerned with a recurrence of slide activity on the much deeper failure surface of the 1960 episode, an approximate location of which is also shown on the attached cross section. We direct attention to the fact that the known boundary of slide activity stops well short of the Hillcrest property; in fact, it does not actually extend to the pave- ment of Slade Way at this location (although it has crossed Slade Way to the south of the property. The disparity of focus is clearly stated in the Shannon and Wilson "Conclusions" (page 4 and page 5): ". ...the primary concern for stability of the property is related to the potential for a deep - seated failure that could affect much if not all of the property, rather than the smaller more localized type :' of failures which are addressed in this (the Cascade) report." On page 3 of their May 5, 1993, letter Shannon and Wilson state, "The site subsurface data do not define the depth of the slip plane of the ancient landslide that is interpreted from geomorphic expression and previous studies to underlie the eastern half of the property." The slip plane which they have inferred is also shown (approximate position) in the attached cross section. It is of the greatest importance to recognize that the Shannon and Wilson slip surface is not a feature which has been directly observed; it is based (as they state) on an "interpretation of geomorphic expression." This is an interpretation with which we take the strongest exception, even though it appears that the other firms which have reported on the problem (including Cascade Geotechnical) may also have made the same blunder. As we have stated in our letter of May 3, 1993, an abnormally steep slope can either be explained as having been created by massive earth movement, or by differential ero- sion (sapping). The presence of a water table at the base of the slope, and the vigorous springs which exit there can easily explain the presence of the steep slope. To postulate that a massive earth movement has created a feature with a length of some 3000 ft, is to reduce the presence of ground- water throughout the toe of that feature to the status of "mere coincidence." (What an amazing coincidence!) With regard to the City of Tukwila requirements quoted on page 5 of the S and W report, we offer the following comments: "There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no significant risk to the proposed development or surrounding properties." (City of Tukwila Regulation) Evidence of past instability is confined to the area below Slade Way, and to the south of the subject property. While the interpretation of the word "vicinity" might be open to some discussion, it appears to us that the importance of Slade Way (as a corridor for emergency vehicles and vital utilities) •is so great that it is inconceivable that it will be allowed to be seriously disrupted, or even that its "rescue" would be shunted off to the shoulders of a private citizen. We point out once more that the stability of the area for more than three decades is absolute proof that the means of protecting this valuable area are sufficiently understood that remedial efforts can be undertaken at any time (without the elaborate and expensive geotechnical studies so dearly beloved of Shannon and Wilson). ' We call attention to a report prepared by GeoEngineers in 1982 in which they state (page 8), "... . it appears that the overall site stability in the aproxiamtely 20 years since these drains have been constructed has been good and that the drainage system continues to be generally effective in Leroy C. Lowe May 17, 1993 Page 3 : preventing deep - seated movements. It is of prime importance, however, that the system remain fully operational. If the drainage system should deteriorate so that artesian pressures can again increase beneath the hillside, the risk of deep - seated and extensive earth movements can be expected to significantly increase." With the security of Slade Way assured, it is a simple matter to vouch for the safety of the site from deep- seated sliding. The Cascade report covers (and indorses) the stability of the site itself; moreover, we believe that it follows automatically that development of the site would improve local soil and groundwater conditions and would therefore improve the stability of the general area. "The area of potential geologic instability can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability." (City of Tukwila Regulation) Given the safety of Slade Way, the only area of potential geologic instability is the steep slope which trends north -south through the center of the property. The height of this feature varies from 20 to a maximum of 32 feet; to modify it to improve its stability is a comparatively simple exercise in regrading, which when combined with the drainage improvements of the project, would have far- reaching effects on the stability of the neighborhood. The Cascade recommendation for a 45 -ft set- back from the toe of the slope apparently assumes that the slope would be left in its present over - steepened condition; however, we see no reason why such a feature should be allowed to remain. A very modest regrading at the toe of the slope, combined with a simple subsurface drainage system consisting of perforated pipes placed in shallow trenches will provide all the stability which could be asked for (either static or seismic pseudostatic). We note that surface water runoff, groundwater discharge, and sedimentation are at present totally uncontrolled, leaving the steep slope entirely unprotected; a correction of this state of affairs would automatically accompany the development of the property, to the overall improvement of this and neighboring properties. In summary, we point out that the misinterpretation of a Iandform has led Shannon and Wilson to conclude erroneously that the two vital requirements of the City of Tukwila ordinance are not met by this project, whereas, if the correct interpretation is applied, the requirements are met. We welcome the opportunity to discuss this directly with any or all interested parties. Y .. H. T Witt- •, Very truly yours, \WAS \ p -5 ti % ��(Th C• . NEIL H. TWELKER AND ASSOC.. INC. V '*,:.. j ±� Z' 5757 - . ^t=r• �SICVA- - "'":;v' by V—.1 -`%4. Neil H. Twelker, Pres. EXPIRES 31131 1'9 9-�- Ind Cross Section Through the Hillside City of Tukwila John W. Rants, Mayor May 14,1993 Department of Community Development Rick Beeler, Director Leroy Lowe P.O. Box 1241 Seattle, WA 98111 RE: Soil geotechnical evaluation of proposed Lowe BLA Dear Mr. Lowe: Attached to this letter is the peer review completed by Shannon & Wilson,Inc. In their conclusions they do not believe the studies demonstrate that the property meets the requirements set forth in the City of Tukwila's Sensitive Areas Ordinance. Under the SAO, you may appeal this decision (TMC Section 18.45.125). Given the complexity of this issue, I suggest a meeting to review the positions of the consultants and discuss ways in which to resolve the issues. Should you have any questions, please feel free to call or write. Sincerely, Jack ?ace Senior Planner cc: Ron Cameron, City Engineer ATTACHMENT E 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431-3665 ®111 SHANNON MWI�LSON, INC. May 5, 1993 City of Tukwila Department of Public Works 6300 Southcenter Boulevard, Suite 100 Tukwila, Washington 98188 Attn: Mr. Jack Pace, Senior Planner RE: SOILS AND GEOTECHNICAL EVALUATION OF PROPOSED LEROY LOWE DEVELOPMENT, TUKWILA, WASHINGTON SEATTLE EVERETT KENNEWICK FAIRBANKS ANCHORAGE SAINT LOUIS At your request, we have reviewed the slope stability analysis report dated March 22, 1993 by Cascade Geotechnical, Inc. for the proposed Hillcrest Residential Development, Slade Way South of 160th Street, Tukwila, Washington. We understand that this report was prepared to fulfill a requirement of the City of Tukwila Sensitive Areas Ordinance (SAO) for an application for a Boundary Line Adjustment for the subject property. As part of our review, we have reviewed three previous geotechnical reports for the Hillcrest Development dated May 30, 1990, August 27, 1990, and April 24, 1991, and a supplemental letter dated July 27, 1992, all prepared by Cascade Geotechnical, Inc. We also have referred to our geotechnical review of the proposed Hillcrest Development dated November 23, 1992, and our letter of February 1, 1993. These latter documents recommended that slope stability analyses be performed to assess the potential for both shallow and deep seated landsliding, and stressed the need for careful evaluation of the potential for landsliding on this property before proceeding with any development that might endanger human health or safety. It is our opinion that these recommendations are consistent with the requirements set forth in the City of Tukwila SAO. REVIEW COMMENTS The March 22, 1993 report by Cascade Geotechnical, Inc. describes the methodology, soil parameters used, and results of pre- and post - construction stability analyses for both shallow and deep - seated sliding under static and pseudostatic conditions. These analyses were conducted specifically to evaluate the stability of the north -south trending, steep slope that runs through the center of the property. Two cross sections were reportedly developed and analyzed, both 400 NORTH 34TH STREET •SUITE 100 P.O. BOX 300303 SEATTLE, WASHINGTON 98103 206.632.8020 FAX 206.633.6777 l� fl (•■•••• a. L Y WW. I 0 1993 DEViEi OPALiENT W- 6367 -01 City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 2 SHANNON 6WILSON, INC. trending in a northeasterly direction across the property and extending 25 feet beyond the western and eastern property lines. Cascade Geotechnical's report concludes that with respect to deep - seated stability, the site is stable under both pre- and post construction conditions, and that deep - seated slope stability will be significantly improved by the planned drainage. With respect to shallow slope stability, only the pre - construction factors of safety are presented. These show the site to be marginally stable under static conditions and unstable under seismic loading. The report states that shallow failures may still occur under post - construction conditions, both under both static and pseudostatic conditions, and therefore recommends a 45 -foot construction setback from the toe of the steep slope. This setback would appear to require relocation of three of the proposed residences shown on the site plan. In reviewing this stability analysis report, we have identified three main issues which are not addressed, and which in our opinion must be considered in an evaluation of the stability of this property. These issues, as itemized below, revolve around a central issue; the stability analyses were performed for the steep north - trending slope rather than for the property as a whole. ► There does not appear to be adequate subsurface data to define the stratigraphic and groundwater conditions below the site in such a manner that they can be used in analyzing the deep - seated slope stability of the property. It is our understanding that three borings ranging from 26.5 to 31.5 feet deep were used, along with several shallow test pits and hand auger borings, to define subsurface conditions and to develop the cross sections for the site. Our primary concern with this is that there are no borings that extend deep enough to intercept the deep confining strata or the underlying confined aquifer which was a primary cause of landsliding immediately to the east of the site. Consequently, the depth of this confined aquifer is not known, nor is there any information on the hydraulic head within the confined aquifer. ► The report does not address the potential for reactivation of landsliding immediately downslope from the property in the stability analyses. There is also no acknowledg- ment in the report that such a problem might exist. Multiple studies for the area immediately downslope from the proposed Hillcrest Development indicate that the stability of this area is dependent on an extensive hillside dewatering system. This system, which is about 30 years old, is reportedly deteriorating and its capability W- 6367 -01 City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 3 SHANNON 6WILSON, INC. to lower the hydrostatic head acting on the hillside is diminishing. Consequently, the factor of safety for hillside stability can be assumed have dropped below a level that several years ago was considered by the Washington Department of Transporta- tion and Dames & Moore to be unacceptably low for a proposed residential devel- opment. Without a basis to conclude otherwise, it appears reasonable to assume that the factor of safety for deep seated slope stability of the proposed Hillcrest Development would be similar to that which has been determined for the property immediately downslope. In our opinion, slope failure of the area immediately downslope could readily impact the stability of the proposed Hillcrest Development, and must be considered in evaluating the deep - seated stability of the property. ► The site subsurface data do not define the depth of the slip plane of the ancient landslide that is interpreted from geomorphic expression and previous studies to underlie the eastern half of the property. While the stability analysis report indi- cates a deep-seated failure surface at a depth of 20 feet, this appears to be in reference to a hypothetical failure plane below the steep slope west of the proposed building sites. We believe that in order to adequately assess the deep seated stability of this site it will be necessary to further evaluate the evidence of a deep seated slide plane under the site, including a determination of its depth and soil and groundwater conditions above and below it. This information would help to resolve uncertainties regarding the potential for slide reactivation or involvement of the property in a deep - seated failure. Because the stability analysis report only addresses the potential for sliding of the steep north - trending slope in the center of the property, and ignores the potential significance of a deeper seated slide, we believe it would be inappropriate at this time to critically review the stability analyses that have been performed. From a cursory review, however, it appears that the stability analyses are conservative with respect to shallow soil conditions and seismic loading conditions. However, the following additional issues that should be addressed or clarified before a critical review can be accomplished. ► The report does not include all of the assumptions that were used in developing the cross sections or in performing the analyses. Specifically, the following are not provided: a) it is not clear how the cross sections and soil parameters were devel- oped below the maximum depth of soil borings of 31.5 feet; b) assumed slip surfaces are not shown, c) groundwater levels used in the analysis are not shown; and d) it is not clear how the considerable subsurface detail shown in the vicinity of the steep slope was derived. W- 6367 -01 City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 4 SHANNON 6WILSON. INC. ► Cascade Geotechnical's report of August, 27, 1990, indicates that a hydrostatic head which reached the ground surface was noted during drilling of one of the borings. This observation suggests that confined groundwater conditions exist in the relatively shallow strata underlying the site strata. Similarly, the report also indicates that there may be a number of separate, confined aquifers at depth. It would be appropriate to evaluate the potential significance of these confined zones to the stability of the site if this has not already been done. ► The planned drainage enhancements that have been assumed in the post- construction stability analyses are not shown on the copy of the plans that we received. Without knowing what these proposed enhancements are, it is not possible to provide an opinion on the validity of this assumption. However, the report assumes that these drainage enhancements would lower the groundwater to the level of the dense sand. According to the table of soil properties presented on page 3, this level would correspond to depths of 33 to 45 feet below the upper slope and 45 to 65 feet below the lower slope. It is not clear to us what type of drainage has been proposed to accomplish such a groundwater drawdown, but this does not appear to be consistent with our previous understanding that relatively shallow French drains were to be used. ► The slope stability analyses reportedly used groundwater levels measured during the summer of 1990, rather than higher water levels that would be expected during the winter months. While this may not make much difference for the analyses of post - construction conditions, it could be an important factor in the analysis of present -day conditions. A higher water table is often an overriding factor in initiating landsliding sliding because of increased pore -water pressures. ► There is no discussion of the impact of site grading or excavation, either of which could significantly affect shallow slope stability during or after construction. CONCLUSIONS The slope stability analysis report for the proposed Hillcrest Development addresses the stability of the north - trending steep slope that runs through the center of the property rather than the stability of the property as a whole. In our opinion, the primary concern for stability of the property is related to the potential for a deep seated failure that could affect much if not all of W- 6367 -01 City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 5 SHANNON iWILSON, INC. the property, rather than the smaller, more localized type of failures which are addressed in this report. With respect to the stability analyses that have been performed, it appears that these analyses are generally conservative in assessing the stability of the north - trending steep slope. However, this cannot be substantiated without additional documentation of the assumptions that were used in the stability analyses. Depending on what these assumptions were, additional subsurface data may be required to derive a meaningful stability analysis of this slope. In our opinion, the slope stability analyses that have been performed do not adequately address the potential for landsliding on this property, nor do they demonstrate that the property meets the requirements set forth in the City of Tukwila SAO. Specifically, Section 18.45.080E of the SAO states that prior to permitting alteration of an area of potential geologic instability, the applicant must demonstrate either: 1) There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no significant risk to the proposed development or surrounding properties; or 2) The area of potential geologic instability can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability. Our review indicates that the proposed development does not meet the first condition, and because the potential for deep seated sliding has not been addressed, it has not been demonstrated to meet the second condition required for permitting a development in an area of potential geologic instability. W- 6367 -01 .✓ "�". �� City of Tukwila Attn: Mr. Jack Pace May 5, 1993 Page 6 Please call if there are any questions regarding this letter. Sincerely, SHANNON & WILSON, INC. Daniel N. Clayton, C.E.G. Senior Associate Reviewed by: W. Paul Grant, P. E. Vice President DNC:WPG /dnc W6367 -01.LTR/W6367- 1kd /lkd SHANNON EIWILSON, INC. W- 6367 -01 ti MEMORANDUM TO: RON CAMERON / JACK PACE DATE: MAY 4, 1993 PROJECT: HILLCREST BLA SUBJECT: PIER REVIEW SLOPE STABILITY STUDY ENCLOSED FOR YOUR REVIEW & COMMENT IS A PIER REVIEW OF GEORGE LAMB'S SLOPE STABILITY STUDY PREPARED BY NIEL TWELKER P.E. YOUR EARLY RESPONSE WILL BE APPRECIATED. THANK YOU. L. LOWE LEROY C. LOWE A.I.A. ARCHITECT ...0. •OK 1841 O<•TTI.[. WASMINGTON ••111 ATTACHMENT F tt 1 r+ ©51993 Telephone: (206) 284 2410 Ne i I H. Twelker and Associates, Inc. Consulting Soils Engineers Leroy C. Lowe, A.I.A P.O. Box 1241 Seattle, Washington 98111 5645 42nd Ave. West Mail to : P.0 Box 99086 Seattle, Washington 98199 May 3, 1993 Re: Review of Slope Stability Analysis by Cascade Geotechnical Hillcrest Residential Development Slade Way near South 160th St. Tukwila, Washington Dear Mr. Lowe: At your request we have performed a review of an analysis of slope stability prepared by Cascade Geotechnical Inc. (March 22, 1993) for your proposed residental development near the intersecton of Slade Way and S. 160th St. in Tukwila, Washington. We present herewith a report of our findings, conclusions, and recommendations. Site Description The site, which has 50 ft of frontage on S.160th St. and 300 ft of frontage on Slade Way, is an irregular - shaped tract having overall dimensions of 300 ft east -west by 500 ft north - south. It consists of three principal topographic elements: (1) an upper very gently sloping area along the west boundary of the tract, the width of which varies from 20 ft at the north to approxi- mately 180 ft at the south; (2) a steeply sloping central area whose topographic relief varies from 20 ft at the south to 32 ft at the north; and (3) a gently sloping, poorly drained area occupying the westerly half of the property, whose total relief is 20 to 24 ft, with an average declivity of 15 to 20 per cent. Two well - defined streams cross the property from west to east, fed by springs at the base of the steep slope. The site is devoid of structures, with the exception of a sewer which has been constructed in an easement along the south property Tine, to a manhole at the southeast corner of the property,. and thence northerly in Slade Way. The property is covered by scattered deciduous trees, a few small cedar trees, and a sparse understory of native brush. Soils within the site are known (from subsurface explorations performed by Cascade Geotechnical Inc.) to consist of the following principal soil units: 1. The upper (i.e., most recent) unit consists of soft sedimentary peat covering much or most of the poorly drained lower area to depths of Tess than a foot to as much as five feet. Leroy C. Lowe. A.I.A May 3. 1993 Page 2 2. The underlying unit consists of moderately dense to dense sand, the upper foot or two of which is somewhat loosened from weathering. 3. A thin unit of stiff silt was found between 23 and 28 ft in one test boring (located in the low area). The lowermost unit encountered consists of dense sand. Groundwater was encountered at depths varying from 1 to 2 ft in the low area and at depths of 25 to 30 ft in the westerly upland. An important feature of the site is the emergence of groundwater at the base of the steep slope. Cascade Geotechnical Inc. Stability Analyses In their report of March 22, 1993, Cascadce Geotechnical begins by mentioning that they had reviewed their own previous reports and also, a report prepared by Shannon and Wilson dated November 23, 1992. They go on to outline their methodology, soil values, and the results of their studies, including both static and pseudostatic (seismic) reviews of stability. The method employed was the "Simplified Method of Slices," developed by Alan Bishop (based on the original "Method of Fellenius "). Two cross sections were selected, to be representative of the most critical areas of the slope, and because of their proximity to locations at which soil conditions had already been determined. The parameters for the strength and density of the soil were given for 6 classifications of inorganic soils found at the site. No soil parameters were assigned to the peat, presumably because it was expected to play no part in the scenario of a hypothetical slope failure. The Cascade Geotechnical studies showed the preconstruction site to be very stable for static and pseudostatic (seismic at 0.2 times the acceleration of gravity) conditions in the deep - seated failure mode. The deep- seated failure mode was taken from 4 ft west of the crest of the steep slope and extending 40 ft east of the toe of the slope, while the shallow zone study addressed only the actual steep slope itself, an area which is very near the "angle of repose" of a granular soil. The shallow failure mode was calculated to be only barely stable for static conditions, and not at all stable for the pseudostatic seismic review. With the site drained in accordance with the Cascade Geotechnical recommendations, the stability of the deep- seated failure mode is improved somewhat; the results of the shallow failure mode are not reported. Cascade Geotechnical concludes its report with a recommendation to exclude "any building develop- ment within 45 feet of the toe of the slope," in order to avoid the consequences of a shallow zone failure. Although the Cascade study included a review of the Shannon and Wilson report of Nov. 23, 1992, they did not comment on any of the conclusions presented in that report. Conclusions On the basis of our review of the available information (including an examination of the site) we draw the following principal conclusions: 1. The soil parameters used in the Cascade Geotechnical report are believed to be conservative and generally representative of the conditions which prevail at the site. 2. The method of analysis employed is an accepted one, and the results of their study compare very well with our intuitive appraisal of the site and with our shorthand methods of calcu- lation. We note in particular that the most serious failure mode (deep - seated movement) is very unlikely in any event (whether drained or undrained, static or seismic). • • Leroy C. Lowe. A.I.A May a. 1993 Page 3 3. That the safety factor in shallow zone mode is low is -not all surprising, inasmuch as the declivity of the steep slope is very near the angle of repose (i.e., the declivity or pitch at which any additional material would simply roll or slide to the base of the slope). The amount of material involved in a failure of this kind is comparatively small, and the consequences correspondingly minor. Moreover, it is a fairly simple matter to reinforce those parts of the slope which are critical to structures placed at or near the toe, or to provide catchment for the displaced material, a fact which would obviate the need for the 45 ft set -back recommended by Cascade Geotechnical. 4. No analysis was applied specifically to the gently sloping lower area, presumably on the grounds that an area with a declivity as low as this, and with a dense soil structure, must be inherently stable, even if it does have a high ground water table. We concur with this view; we see no problem with the stability of the lower area, whether in the static condition or in the seismic condition. 5. The peat which covers much of the lower area is totally incapable of support of any major or settlement - critical structure; all major improvements must be founded on the underlying moderately dense sand (a comparatively simple undertaking). The simplest and most econom- ical site preparation procedure will be to conduct an initial program of shallow drainage, after which the peat may be removed from the building area and the structure placed on spread footings on the native sand formation. We see no need for the use of piling foundations for structures in any part of the site. We believe that any drainage of the soil units, or expediting of the escape of surface water, will have a favorable effect on the stability of the entire area. 6. We note that the poorly drained lower area, besides having outstanding capabilities for the breeding of mosquitoes, possess a special status under the designation of "Wet Land," making it necessary to find a way to execute the project to the satisfaction of the regulating agencies who sit in judgment of such matters. Two methods may considered for the treatment of the wetland problem: (1) removal of the peat only from the building footprint, with the remainder of the site covered with organic materials such as wood chips or bark (to allow foot traffic), or (2) stripping of the entire site and regrading with imported inorganic materials. Inasmuch as the law requires that "lost" wetlands be replaced with a 50 per cent increase in wetland, it would be necessary to provide a poorly drained area of an appropriate size at some other location in either case. You have advised us that you own or have access to property below Slade Way, to the north of the 1960 slide mass which can be converted to "wetland" by altering its topography so that it becomes poorly drained. 7. The Shannon and Wilson Report Cascade Geotechnical reviewed the Shannon and Wilson report but did not comment on it; we, on the other hand, feel compelled to note two important points of disagreement with that report, as presented in the following paragraphs. 8. Indications of Prehistoric Landslide On page 3 of the S and W Nov. 23, 1993, report they state, "The steep slope that separates the upland and lowland portions of the site is interpreted to be the southern end of the headscarp of an ancient extensive landslide. This landslide escarpment extends about 3000 feet northwest of the site and typically displays about 30 to 40 feet of topographic relief. At the base of the escarpment, the ground is typically hummocky and poorly drained and commonly is the site of springs and boggy conditions, as it is on the Lowe property. This disrupted drainage probably has contributed to the numerous smaller slides within the ancient slide mass." At this juncture we direct l Leroy C. Lowe, A.I.A May 3. 1993 Page 4 attention to the fact that the steep slope (or scarp) which traverses the Lowe Property is a feature whose declivity exceeds that which could be expected in a "normal" landscape whose contours are developed from erosion from sheet flow and surface weathering. When one encounters a feature of this kind, an explanation for its presence is called for. The two most common reasons for the occurrence of an abnormally steep slope are (1) it is the head scarp of a large scale landslide, or (2) it has been developed by "sapping," (i.e., the result of the emergence of groundwater at the toe of the slope). The conditions reported by Shannon and Wilson describe the scarp throughout its length as having springs and seeps at its base; nevertheless, they unhesitatingly attribute the presence of the scarp to an "ancient landslide." We are unable to dispute (on the basis of direct evidence) the existence of an ancient landslide; however, we can point to the presence of the springs and seeps at the base of the scarp as a "smoking gun," easily capable of having caused the feature in question. The great length of the steep slope, together with the presence of groundwater emergence at its toe throughout its length, lead us to conclude that sapping (rather than mass earth movement) has been responsible for the formation of the scarp. It appears that in their search for an explanation of a physical feature, Shannon and Wilson have overlooked an obvious and ongoing natural process in favor of an unsupported hypothesis whose implications are somewhat sinister (in that they cast doubt on the history of the Hillcrest property). 9. Shannon and Wilson call attention to the presence of "bowed and leaning tree trunks" in the easterly part of the Hillcrest area as evidence of shallow soil creep. While shallow creep could very well be taking place in the upper organic soil units, it should be pointed out that the trees in this area consist mainly of alders, whose growth direction is influenced primarily by the search for light and air. 10. Influence of Recent (1960) Landslide Shannon and Wilson allude (at considerable length) to the large landslide which occurred between Slade Way and Klickitat Drive in 1960. We were present almost at the moment when that slide commenced; it was instigated by large - scale removal of material from the slope (for use in preparing the Andover industrial site). A very high groundwater table also played a prominent part in the loss of hillside stability. The slide was eventually brought under control by an elaborate network of drainage which included horizontal drains and deep well pumps. From the early 60's to the mid 80's the system appears to have been effective in preventing noticeable movement within the slope; however, a number of small slides and set -downs in the area (even extending into the right -of -way of Slade Way) have led to a series of studies by WSDOT, Dames and Moore, and Geoengineers, all of which concluded that the drainage system was undergoing an "ongoing deterioration," and that ".... without a functional drainage system there would be an unacceptable risk of deep - seated slope failure at the site." Shannon and Wilson then proceed to attach the feasibility of the Hillcrest development to the restoration of the drainage system which had demonstrated so well its ability to maintain stability for several decades. They imply that i f the 1960 landslide were to become reactivated (because of the deterioration of its drainage system) and i f the slide mass were to extend laterally into Slade Way to the boundaries it has been known to occupy, and if the slide mass were extended even more to the west (involving earth which had not been previously disturbed, and if the new boundaries of the unstable mass somehow were to "join up with" the those of prehistoric slide which they have postulated (we believe mistakenly), then the improvements of the Hillcrest development would be at risk. While we are able to understand and sympathize with the reluctance of a responsible engineer to acquiesce to the construction of a new development on ground which has fallen under the Leroy C. Lowe. A.I.A May 3. 1993 Page 5 cloud of potential instability, we nevertheless feel compelled to call attention to the following points which we believe to over -ride and outweigh all other considerations: a. Slade Way, which lies between the Hillcrest property and the 1960 landslide, is a very important (if not absolutely vital) thoroughfare in this area. In addition to providing access for local traffic and emergency vehicles, it also contains important buried utilities without which any number of households in the area could not survive. b. The unfolding of the scenario outlined above would threaten a considerable number of existing residences in the same manner that the Hillcrest development would be threatened. Barring a fortuitous intervention by a "public spirited angel," it appears that the City of Tukwila must either (1) undertake the restoration of the deteriorating drainage system or (2) have on hand a contingency program to include the condem- nation of existing residences (and relocation of their occupants), and the rerouting of the utilities which serve residents located beyond the area which would be directly affected by the contemplated reactivation of the 1960 landslide. Likewise, it also appears that the residents whose lives would be disrupted should be concerned with their own agenda: (1) If forced to move, where would they go? (2) What recourse could they pursue following the loss of Slade Way? (3) What steps could be taken to prevent the loss of Slade Way? and (4) How can they force the City of Tukwila to perform the tasks ordinarily accepted by city government? In all this confusion, it is obvious that the least affected of all parties would be the owners of the Hillcrest property (which, after all, is nothing more than poorly drained vacant land). c. The success of the means of stabilizing the 1960 landslide area has been well demonstrated over a period of three decades. Continued stability simply requires that proper maintenance and /or replacement of the drainage installations be performed in a timely manner. This is a civic responsibility which, because of its great extent, the large number of residential properties involved, and the potential impact on the community, cannot be placed on the shoulders of a private citizen. In summary, we believe that whatever stability problems the proposed Hillcrest development might have are of very minor importance, that they are confined to the actual site, and that they can be dealt with at the time of development by fairly straightforward methods of drainage and site preparation. We shall be pleased to discuss our conclusions with you or other interested parties, at your convenience. N HT: nt .4. t3h EYPinE3 31131 1 q q'4- Very truly yours, NEIL H. TWELKER AND ASSOC.. INC. by Neil H. Twelker, Pres. •t MEMORANDUM TO: DATE: PROJECT: SUBJECT: RON CAMERON / JACK PACE APRIL 9, 1993 H1LLCREST B.L.A. SLOPE STABILITY ANALYSIS GENTLEMEN : ENCLOSED IS THE FOLLOWING DOCUNENT SLOPE STABILITY ANALYSIS WITH CROSS SECTIONS PREPARED BY CASCADE GEOTECH INC . THIS DOCUMENT IS PROVIDED TO YOU FOR YOUR REVIEW AND COMMENT. LEROY C . LOWE A.1 A.I.A. 7 C.C. NEIL TWELKER P.E. - ..P_.H LEROY C. LOWE A.I.A. ARCHITECT P.O. BOX 1241 SEATTLE. WASHINGTON GI 0111 • MEMORANDUM TO: DATE: RON CAMERON / JACK PACE MAR . 30 , 1993 PROJECT: HILLCREST BLA SUBJECT: SLOPE STABILITY ANALYSIS GENTLEMEN : RECEIVED MAR 3 11993 COMMUNITY DEVELOPMENT ENCLOSED ARE THE FOLLOWING DOCUMENTS : 1 . SLOPE STABILITY ANALYSIS PREPARED BY CASCADE GEOTECH INC . 2. CONTRACT WITH NEIL TWELKER P.E. TO PROVIDE A PIER REVIEW OF CASCADE'S SLOPE STABILITY ANALYSIS THESE DOCUNENTS ARE PROVIDED TO YOU FOR YOUR REVIEW AND COMMENT. LEROY C . LOWE A.1 A.I.A. C .0 . NEIL TWELKER P. LEROY C. LOWE A.I.A. ARCHITECT •.O. sox 1241 S<•TTLt. W *••I•OTON emu" ATTACHMENT G RECD. ;Fr:� MAR 3 11993 DEVELOPMENT "Telephone 284 2410 Ne i I H. Twelker and Associates, Inc. Consulting Soils Engineers 5645 42nd Ave. West Mail to : P.O Box 99086 Seattle, Washington 98199 March 26, 1993 LeRoy C. Lowe, AIA, Architect P.O. Box 3972 Bellevue, Wash. 98007 Re: Slope Stability, Proposed Hillcrest Residential Site Slade Way, Tukwila, Washington Dear Mr. Lowe: On numerous occasions during the past 6 months we have discussed with you the various factors involved in the appraisal of stability of your forthcoming residential development ( Hillcrest) on Slade Way in Tukwila, Washington. We present herewith (at your request) our proposal for a geotechnical report containing a review of the slope stability studies which have been per- formed by Cascade Geotechnical (including independent slope stability calculations if neces- sary). All work will be performed by Neil H. Twelker (personal history statement attached). We propose to invoice you at our regular rate for consulting services, $100 per hour. We estimate that our review of the Cascade Geotechnical studies can be accomplished with an expenditure of 4 hours. If independent slope stability calculations are required, an additional 4 hours could conceivably be expended. The report containing our findings and conclusions will require 2 to 3 hours, making the maximum anticipated amount 51100. We will be in a position to commence our studies within 5 working days of your verbal authorization to proceed. LeRoy C. Lowe, AIA March 26, 1993 Page 2 We trust this proposal meets with your approval, and look forward to this opportunity to be of assistance. aeevetp3 ;f 110 if., 4_ NHT:nt Ind: Personal History Statement of Neil H. Twelker a///2_ 30 / 9�3 Very truly yours. NEIL H. TWELKER AND ASSOC.. INC. Neil H. Twelker, Pres. PERSONAL HIS ORY STATEMENT NEIL H. TWELKER BS (Civil Engineering) AM (Soil Mechanics) PhD (Soil Mechanics, w /Geology Minor) Member American Society of Civil Engineers Geological Society of America Structural Engineers Associat'on of Washington United States Committee on La ge Dams Tau Beta Pi Sigma Xi Lecturer, Univ. Wash., College of rch. & Urban Planning Registered Professional Engineer Washington (Civil) Oregon (Civil) Experience Forty -eight years total experience to engineering works, including: University of Washington Ha yard University 1942 1954 Ha yard University 1958 Nine years with the Seattle with responsible charge of s projects, including design Joseph Dam and Albeni Falls numerous dam sites throughou levees, breakwaters, and buil Two years with Shannon and W Engineers, Seattle, Washingt investigations for Wells Dam River Dam, Upper Baker River for open pit copper mines in foundation and landslide inve Three years of combined priv study, including work on the on Waterton Lakes Dam, Albert Thirty -four years in present neering for buildings, brid Over 6000 professional assign nia, and Alaska, including w opments for Port of Seattle, tal and private organizations Adak Air Stations, and Puget on design and execution of n ground and hard rock tunn Numerous remedial and underp tions, and foundation and s Alaska (Civil) in the application of soil mechanics istrict of the Corps of Engineers, ils engineering on major and minor and construction phases of Chief Dam, preliminary investigations on the Pacific Northwest, airfields, ing foundations. lson, Soil Mechanics and Foundation n, with responsible charge of site (Columbia River, Washington), Tolt Dam, slope stability investigation Utah and Nevada, and miscellaneous tigations. to practice, teaching, and graduate St. Lawrence Seaway and consultation ractice. Soils and foundation engi- es, roads, dams, and breakwaters. ents in Washington, Oregon, Califor- terfront and major industrial devel- Port of Tacoma, and other governmen- , and U.S. Navy at Whidbey Island and Sound Naval Shipyard. Consultation erous large and small diameter soft ls, deep trenches, caissons, etc, nning projects for building founda- te preparation for residential and commercial structures of all sizes. Soil and foundation studie River). In Seattle: Crown P Hotel, Market Place North, s for Tri- cities Bridge (Columbia laza Hotel, Westin Building, Warwick Seattle Aquarium, Pacific Museum of Flight and restoration of Pike Place Market. In Portland, Ore.: Portland General Electric He dquarters Complex, KOIN Plaza, Marri- ott Hotel, Ben Franklin, and others. LEROY C. LOWE A.I.A. ARCHITECT P.O. BOX 1241 BEATTLE. WABHINGTON Dtl111 RON CAMERON P.E . CITY ENGINEER CITY OF TUKWILA 6200 SOUTHCENTER BLVD TUKWILA, WA. 98188 HILLCREST BLA DEAR SIRS : REC VED. MAR 261993 COMMUNITY PFVELOPMENT JACK PACE SENIOR PLANNER MAR. 25 , 1993 THANK YOU FOR YOUR CORRESPONDENCE OF MAR. 15, 1993 AND A COPY OF DAN CLAYTON'S CORRESPONDENCE OF MAR, 2. WE TAKE EXCEPTION TO SEVERAL OF MR. CLAYTON'S COMMENTS IN HIS MAR. 2ND LETTER AND HAVE INCLUDED THEM WITH OUR CORRECTIONS; THEY ARE AS FOLLOWS A meeting was held at the request of Mr. Lowe to discuss the recommendations presented in our geotechnical review of the proposed Hillcrest Development, dated November 23, 1992. The primary point of contention during this meeting was Shannon & Wilson's recommendation that additional geotechnical studies are needed to evaluate the potential for shallow and deep- seated landsliding on the proposed property. 0• RESPONSE: THE JAN. 13, 1993 MEETING WAS HELD AT MY REQUEST BECAUSE THE CITY SENIOR PLANNER INSISTED THAT IT WAS MY RESPONSIBILITY TO PROVIDE ADDITIONAL GEO- TECH STUDIES ON PROPERTY OTHER THAN THE HILLCREST SITE. THIS DEMAND WAS MADE AT OUR DEC. 9, 1992 MEETING, BETWEEN PACE, FRASER, SCHULZ , AND LOWE. IT IS MY OPINION THAT 1 AM RESPONSIBLE FOR STUDIES AND REPORTS ON MY PROPERTY ONLY. Mr. Lowe's representatives acknowledged that there was a potential for a slope failure on the proposed Hillcrest Development property, but maintained that it was not Mr. Lowe's responsibility to evaluate this potential as a prerequisite to obtaining a Boundary Line Adjustment. Specifically, his consultants, Mr. George Lamb of Cascade Geotechnical Inc. and Mr. Neil Twelker, indicated that because a large slide that might affect the Lowe property would also involve adjoining downslope properties (owned by Puget Western and the City of Tukwila), it was someone else's responsibility to evaluate, or accept the responsibility for, the safety of Mr. Lowe's property with respect to the potential for deep- seated landsliding. RESPONSE : MR . TWELKER & MR . LAMB HAVE NEVER ACKNOWLEDGED NOR STATED THAT THERE WAS A POTENTIAL FOR A SLOPE FAILER ON THE PROPOSED HILLCREST PROPERTY. TO STATE OTHERWISE IS FALSE. THE JAN, 13, 1993 PACE AND COPIES AND RECEIVED ON MEETING WAS TAPE RECORDED BY JACK OF THOSE TAPES WERE MAILED TO ME FEB. 13, 1993 . THESE TAPES HAVE SIGNIFICANT OMISSIONS OR ERASURES SO AS TO ELIMINATE MAJOR ELEMENTS OF THE DISCUSSION DURING THAT JAN. 13, 1993 MEETING. THESE ERASURES ARE OF GREAT CONCERN TO ME AND MY CONSULTANTS. THESE TAPES CHALLENGE THE ACCURACY OF MR . CLAYTON'S COMMENTS, FOR IN TRUTH THE TAPES SPEAK FOR THEMSELVES. MR. LAMB & MR. TWELKER BOTH STATED "LOWE'S PROPERTY IS OUTSIDE THE DEEP SEATED SLIDE AREA." MR . LAMB & MR . TWELKER STATED THAT IT WAS NOT LOWES RESPONSIBILITY TO MAINTAIN SLADE WAY OR THE DOWN -SLOPE PROPERTY, BUT RATHER THE CITY AND WASH. STATE DEPT. OF TRANSPORTATION HAD TOTAL RESPONSIBILITY TO MAINTAIN THE SLOPE AND SLADE WAY FOR THE HEALTH WELWARE AND SAFETY OF THE CITIZENS OF TUKWILA. TO SIT BY AND WATCH SLADE WAY SLIDE DOWNHILL THEREBY REMOVING THE LATERAL SUPPORT FOR THE HILLCREST SITE IS "CORP. NONFEASANCE" NEIL TWELKER QUOTE THE TAPE RECORDING CLEARLY INDICATES THAT ALL PARTIES AGREE THAT DEWATERING THE LOWE PROPERTY WOULD ADD STABILITY TO THE SITES SHALLOW STRUCTURE. MR. CLAYTON WAS RELUCTANT TO STIPULATE THAT DEWATERING THE LOWE SITE WOULD ADD STABILITY TO THE HILLSIDE THO MR . LAMB & MR . TWELKER WERE NOT. SIMPLY STATED, MR . LOWE'S PROPERTY IS OUTSIDE THE DEEP SEATED SLOPE STABILITY PROBLEM & DEWATERING THE LOWE PROPERTY WILL ADD TO THE GENERAL AREAS STABILITY, AS A RESULT OF THIS MEETING�WE ARE PREPARING A SLOPE STABILITY ANALYSIS FOR\Y`q R R VIq AND COMMENT. VERY TRULY YOURS OY( C LOVI E A .1 .A CASCADE GEOTECHNICAL INC. 12016115TH AVENUE N.E., BLDG. H (206) 821.5080 KIRKLAND, WASHINGTON 98034 FAX: (206) 820.6953 March 22, 1993 Job No. 9005 -13G Mr. Leroy C. Lowe, A.I.A. P.O. Box 1241 Seattle, Washington 98111 Reference: Slope Stability Analysis Hillcrest Residential Development Slade Way, South of South 160th Street Tukwila, Washington Dear Mr. Lowe: As requested, we have completed a slope stability analysis for the above site. The following letter summarizes our analysis and presents conclusions regarding the present and post - construction stability of the north -south trending, east facing slope. We understand that this analysis will be used for a Boundary Line Adjustment application. SCOPE Per the proposal for services letter dated February 24, 1993, our scope of work consisted of: • reviewing existing geotechnical reports previously prepared for this project • analyzing the present stability of the north -south trending slope on your property by constructing two cross sections oriented perpendicular to the slope. • preparation of this letter stating the results of our analysis. PROJECT DESCRIPTION Slope profiles were derived from an undated topographic site plan titled "Hillcrest," that was prepared by Leroy C. Lowe. The site plan is scaled at 1 " =30' and topography is shown in two foot intervals. Property lines and adjoining roads Slade Way and South 160th Street are clearly delineated. Based on our understanding of the project, development will consist of the construction of five (5) single family residences, with driveway access from Slade Way and South 160th Street. All five residences are to be located in the gently sloping area at the base of the 2(H):1(V) slope. Several areas on the slope are moderately steeper than 2(H):1(V). March 22, 1993 Job No. 9005 -13G We reviewed two geotechnical reports previously prepared for this project by Cascade Geotechnical Inc.' We also reviewed a report prepared by Shannon & Wilson, Inc.2 which addresses geotechnical implications for this project, Slade Way, and Klickitat Drive. SLOPE STABILITY Methodology We analyzed the stability of two sections of the north -south trending slope, for both pre - construction and post- construction conditions. These sections were chosen based on their proximity to previously obtained subsurface information, as well as their ability to represent the most critical areas of the slope. We performed both shallow and deep seated failure analyses. Cross section A -A' begins in the southwest property corner and extends northeasterly across the entire property. Cross section B -B' begins approximately 200 feet north of the southwest property corner and also extends northeasterly across the entire property. Both cross sections extend twenty -five (25) feet beyond the eastern and western property lines. For more details, please refer to the site plan and cross sections contained in the appendices. Utilizing the computer program SB- SLOPE, we have determined minimum factors of safety against slope failure under both static and pseudostatic (seismic) loading conditions. SB -SLOPE incorporates Bishop's Simplified method of slices. To mimic a severe seismic event, a horizontal inertial force equal to 0.2 times the total weight of the potential sliding mass was used in the pseudostatic analysis. This inertial value was selected based on Seattle D.C.L.U. recommended values', which are generally accepted as conservative. Parameters Soil parameters were conservatively estimated based on previously obtained subsurface soils information. Six soil types were chosen that we believe accurately characterize actual soil conditions. The typical soil strengths selected are based on our experience with similar soils in this area. The pre - construction analysis incorporated a ground water surface based on elevations ' Cascade Geotechnical Inc., May 30, 1990, Preliminary Geotechnical Investigation for Hillcrest Slade Way, South of South 160th Street, Tukwila, Washington; and addendum report dated August 27, 1990. 2 Shannon & Wilson Inc., November 23, 1992, "Geotechnical Review of Proposed LeRoy Lowe Development, Tukwila, Washington." 3 Seattle Department of Construction and Land Use Director's Rule 3 -93, March, 1, 1993. 2 r CAg ADE GEOTECHNICAL \.-4C. March 22, 1993 Job No. 9005 -13G encountered during our subsurface investigation in May and August of 1990, and springs, which were observed at the toe of the slope. The post - construction analysis incorporated the benefits of the drainage enhancements shown on the plans. The phreatic surface is assumed to be lowered to the dense sands after drainage is installed. The following table summarizes the soil parameters used in our analysis. SOIL PROPERTIES Soil Type & Classification Approx. Depth from Surface Dry Unit Weight (pcf) Cohesion (psf) 0 1, (SP) dry sand, loose 0 to 12 feet: upper slope 96 0 31 2, (SP) dry sand, medium dense 12 to 25 feet: upper slope 101 0 34 3, (SP /ML), wet sand and silt, med. dense to dense 25 to 33 feet: upper slope 38 to 45 feet: lower slope 108 200 30 4, (SP), wet sand, dense 33 to 45 feet: upper slope 45 to 65 feet: lower slope 110 0 37 5, (SP /SM) wet silty sand, loose 0 to 25 feet: lower slope 96 500 31 6, (SP /SM) wet silty sand, medium dense 25 to 38 feet: lower slope 101 500 34 Pre - construction RESULTS With respect to deep failures, our results show that the slope is presently stable under static and pseudostatic (with seismic loading) conditions. With respect to shallow failures, our results L L CA. ADE GEOTECHNICAL iti ,IC. March 22, 1993 Job No. 9005 -13G show that the slope is stable under static conditions, but unstable during pseudostatic conditions. The following tables summarize the results of our analysis. Pre - construction Factors of Safety Cross Sections Deep Seated Shallow Static Pseudostatic Static Pseudostatic A -A' 1.87 1.20 1.16 0.75 B -B' 1.66 1.10 1.04 0.69 The shallow, pseudostatic failure surface for section A -A' began four (4) feet west of the crest of the slope and extended approximately forty (40) feet eastward beyond the toe. The shallow, pseudostatic failure surface for section B -B' began and ended at the crest and toe of slope, respectively. Both shallow failure surfaces were, at their deepest point, only approximately three (3) feet deep. The relatively low factors of safety for shallow, pseudostatic conditions are a result of the oversteepened slope created during the past ancient Landslide, which left the fine to medium grained sands on the slope face very near their angle of repose. The deep seated, pseudostatic failure surface for section A -A' began at the crest of the slope and continued approximately forty (40) feet eastward beyond the toe. The deep seated, pseudostatic failure surface for section B -B' began forty (40) feet west of the crest of the slope and terminated eastward at the toe. Both deep seated failure surfaces were, at their deepest point, approximately twenty (20) feet deep. Post- construction With respect to deep failures, our results show that the slope will be significantly improved by the planned drainage. The slope will be stable under static and pseudostatic conditions. Shallow failures may still occur on the slope due to the slope being very near the angle of repose. The following tables summarize the results of our analysis. 4 CA._ ;ADE GEOTECHNICAL March 22, 1993 Job No. 9005 -13G Post - construction Factors of Safety Cross Section Deep Seated Static Pseudostatic Section A -A' 2.18 1.35 Section B -B' 1.85 1.25 The failure surfaces occurred at approximately the same location as in the preconstruction analysis. CONCLUSIONS Under pre - construction conditions, we conclude that the slope will be stable except for shallow instability during a maximum probable seismic event that may result in mass wasting of several feet of soil directly on the slope face. This shallow mass wasting can be characterized by localized slumping and not movement of the entire slope. With post- construction improvements, we conclude that the slope will be generally stable for deep failures, under both static and pseudostatic conditions. The post construction improvements include the French drains shown in the plans. Shallow failures may still occur under strong seismic loading. We conclude that the proposed Boundary Line Adjustment for this site will not affect the suitability of the site development provided our recommendations contained in the Recommendations Section of this report are carefully followed. RECOMMENDATIONS Building Setbacks To protect property from shallow failures (localized sloughing) during a maximum probable seismic event, we recommend the following: • The exclusion of any building development within forty -five (45) feet of toe of the slope. 5 CA.4. ;ADE GEOTECHNICAL. March 22, 1993 Job No. 9005-13G GENERAL We recommend that we engaged to review the final plans when they become available to confirm that our recommendations have been properly interpreted and to provide any additional or alternate recommendations as necessary. We expect the on-site soil conditions to reflect our findings; however, some variations may occur. Should soil conditions be encountered that cause concern and/or are not discussed herein, Cascade Geotechnical Inc. should be contacted immediately to determine if additional or alternate recommendations are required. This letter has been prepared for the exclusive use of Leroy Lowe, for specific application to the proposed Hillcrest Residential Development located at Slade Way, South of South 160th Street, Tukwila, Washington, in accordance with generally accepted soils and foundation engineering practices. No other warranty, expressed or implied, is made. Thank you for this opportunity to work with you on this project. If you have any questions, feel free to contact us at any time. Respectfully submitted, CASCADE GEOTECHNICAL INC. George E. Lamb, P.E. President 6 Larry Jackson Project Manager HILLCREST SLOPE STAE:51LITY ANALYSIS SITE PLAN TEST PIT (05/15/90) HAND AUGER (05/23/90) X TEST BORING (07/24/90 & 07/25/90) FROM A LANDSCAPE SITE PLAN BY LEROY C. LOWE, ALA. ARCHITECT 03/18(93) AND FROM TEST HOLE LOCATION MAP BY CASCADE GEOTECHNICAL (05129/90) I CASCADE GEOTECHNICAL INC. 1/01b11S111AV11111111i 111114 11 1.11014,111.WA)11114■411.)1.1.9t1014 111.1 1/.11.1e, Job No 9303•05G I SCALE r = 50' LOCATIONS ARE APPROXIMATE 00 4/08,93 Own 117.111A Eno-Gaol F A 330 — 300 270 — 240 — 210 180 z i HILLCREST SLOPE STABILITY ANALYSIS CROSS SECTION A - A' EXISTING 2 3 4 SURFACE EXISf/A/G 5 6 -...._ 4 PROPER T Y TINE A' — 330 300 — 270 — 240 — 210 0 30 60 90 120 150 180 210 240 270 300 330 360 Feet 180 rprit•CASCADE GEOTECHNICAL_ INC kks. 1101611s1. .41,lJI 1 1 &CC 14 SCd0 100a LAND v.a.,....3)4C)10Iso 5803a Fa •2• :"E9' 1 • Job No. 9303-05G Scale 30' LOCATIONS ARE APPROXIMATE ch." 03/17/93 °""' BY Eno Goal To 0 350 320 — 290 — 260 — 230 — 200 PROP FRIT I INT 1 2 3 4 EXISTING HILLCREST SLOPE STABILITY ANALYSIS CROSS SECTION B B' ss. 5 6 4 0 30 60 90 120 150 180 Feet 210 PROPERTY LINE 1 1 240 270 300 330 360 B' — 350 — 320 — 290 — 260 — 230 200 CASCADE GEOTECHNICAL INC .1 21 I i° NT 4V, IP.t.,1 C T. t SB " 4206: a:, scoo Job No. 9303-05G sow. 30. LOCATIONS ARE APPROXIMATE Onto 0307193 Own. BY HLA eoircoos City of Tukwila John W. Rants, Mayor Department of Community Development Rick Beeler, Director March 15, 1993 Leroy Lowe P.O. Box 1241 Seattle, WA 98111 Re: Soil and geotechnical evaluation of proposed Lowe Development Dear Mr. Lowe: At the January 13, 1993 meeting, you asked for clarification on the basis for Shannon E. Wilson, Inc. recommendations for additional geotechnical studies. Attached to this letter is their clarification for the additional studies. Under the Sensitive Areas Ordinance, you may appeal this decision. The ordinance states: "18.45.125 Appeals. (a) Any aggrieved party who objects to or disagrees with Department of Community Development (DCD) decisions or conditions for development in a sensitive area shall appeal to the Planning Commission. Any such appeal shall be made in writing within ten days of the interpretation, condition or decision being appealed, and shall set forth the basis for the appeal.. (b) In considering appeals of decisions or conditions, the following shall be considered: (1) The intent and purposes of the sensitive areas ordinance from which this chapter derives; (2) Technical information and reports considered by the Department of Community Development; and (3) Findings of the DCD Director which shall be given substantial weight." ATTACHMENT J 6300 Southcenter Boulevard, Suite #100 • Tukwila, Washington 98188 • (206) 431-3670 • Fax (206) 431 -3665 .Leroy Lowe March .15, 199.3 ,Page 2 As .I mentioned on the phone, another letter will be sent to you to respond to Cascade Geotechnical, Inc., proposal for slope stability analysis dated February 24, 1993. As of this date, Shannon E. Wilson, Inc., have not given the city a time frame for response. Should you have any questions, please feel free to call or write. Sincerely, 0g4/ ck Pace Senior Planner JP /dg attachment cc: Ron Cameron, City Engineer X111 SHANNON IC AND GEOTECHNICAL CONSULTANTS March 2, 1993 City of Tukwila Department of Public Works 6300 Southcenter Boulevard, Suite 100 Tukwila, Washington 98188 Attn: Mr. Jack Pace, Senior Planner RECEIVED MAR - 41993 TUKWiLA PUBLIC WORKS RE: SOILS AND GEOTECHNICAL EVALUATION OF PROPOSED LEROY LOWE DEVELOPMENT, TUKWILA, WASHINGTON SEATTLE ' ETT (4 rC ' ,KENNEWICK {v- FAIRBANKS `■N9HOR{iOEr ., • The purpose of this letter is to summarize the results of a meeting held between the City of Tukwila and Mr. LeRoy Lowe and his consultants on January 13, 1993, and to provide clarifica- tion on the basis for our recommendations for additional geotechnical studies for the subject property. Issues pertaining to wetland development or geotechnical design considerations were not discussed at depth in the meeting and are not addressed in this letter. A meeting was held at the request of Mr. Lowe to discuss the recommendations presented in our geotechnical review of the proposed Hillcrest Development, dated November 23, 1992. The primary point of contention during this meeting was Shannon & Wilson's recommendation that additional geotechnical studies are needed to evaluate the potential for shallow and deep - seated landsliding on the proposed property. Mr. Lowe's representatives acknowledged that there was a potential for a slope failure on the proposed Hillcrest Development property, but maintained that it was not Mr. Lowe's responsibility to evaluate this potential as a prerequisite to obtaining a Boundary Line Adjustment. Specifically, his consultants, Mr. George Lamb of Cascade Geotechnical Inc. and Mr. Neil Twelker, indicated that because a large slide that might affect the Lowe property would also involve adjoining downslope properties (owned by Puget Western and the City of Tukwila), it was someone else's responsibility to evaluate, or accept the responsibility for, the safety of Mr. Lowe's property with respect to the potential for deep - seated landsliding. Our report concluded that additional site - specific geotechnical evaluations, including both static and dynamic slope stability analyses, are necessary to evaluate the potential for both shallow and deep - seated landsliding that could involve the proposed Hillcrest Development. This conclusion is based on a review of the geotechnical investigations on the Hillcrest property and adjoining areas which indicated the following: 400 NORTH 34TH STREET •SUITE 100 RO. BOX 300303 SEATTLE, WASHINGTON 98103 206.632.8020 FAX 206.633.6777 W- 6367 -02 City of Tukwila Attn: Mr. Jack Pace March 2, 1993 Page 2 SHANNON bWILSON, INC. ► The proposed development is situated on an old landslide deposit, with the scarp of that slide crossing the property. ► A steep slope exists on the property (greater than 25 degrees), there is some evidence of instability in the site surficial soils, and there is considerable seepage on the property. ► Surficial soils in several areas downslope from the site appear to be unstable, including an area which is affecting Slade Way to the southeast of the proposed development. ► Slope indicator data from the vicinity of Slade Way immediately downslope of the property provide questionable evidence of minor, relatively deep - seated slope movement since the 1960 landslide. ► A large deep - seated slope failure occurred in 1960 on an adjoining property a short distance downslope. Recurrent movement on this landslide, if it were to occur, might threaten the stability of the site. ► Three static and dynamic slope stability analyses conducted during the 1980s indicate that the slope stability of the area immediately downslope from the proposed Hillcrest Development is marginal at best, and likely to diminish over time if the hillside dewatering system operated by WSDOT continues to deterio- rate. The findings listed above indicate to us the need for careful evaluation of the potential for land - sliding on this property before proceeding with any development that might endanger human health or safety. This conclusion is consistent with the requirements set forth in the City of Tukwila Sensitive Areas Ordinance (SAO). Specifically, the property fits the description of a Class 4 area of potential geologic instability as described below from the Draft Codified SAO October 18, 1991, Section 3, 18.45.020E: "Class 4 areas, where landslide potential is high, which include sloping areas with mappable zones of ground water seepage, and which also include existing mappa- ble landslide deposits regardless of slope." Section 18.45.080E of the Sensitive Areas Ordinance states that prior to permitting alteration of an area of potential geologic instability, the applicant must demonstrate either: W- 6367 -02 City of Tukwila Attn: Mr. Jack Pace March 2, 1993 Page 3 SHANNON i;WILSON, INC. 1) There is no evidence of past instability or earth movement in the vicinity of the proposed development, and quantitative analysis of slope stability indicates no significant risk to the proposed development or surrounding properties; or 2) The area of potential geologic instability can be modified or the project can be designed so that any potential impact to the project and surrounding properties is eliminated, slope stability is not decreased, and the increase in surface water discharge or sedimentation shall not decrease slope stability. Our review indicates that the proposed development does not meet the first condition, and has not been demonstrated to meet the second condition required for permitting a development in an area of potential geologic instability. The Sensitive Areas Ordinance further states that a geotechnical report must be submitted for areas of potential geologic instability, and specifies that a detailed slope stability analysis must be done for all Class 4 areas. It is our opinion that in order to comply with the requirements and the intent of the SAO, such slope stability analyses should be accomplished for both static and dynamic (earthquake ground motion) conditions. In the case of a proposed development which would significantly modify the drainage or ground configuration, stability analyses also should be accomplished for pre -and post - construction conditions. Obviously, a stability analysis for post - construction conditions cannot be accomplished until specific development plans are completed. It is our recommendation, and the stated intent of the SAO, that the geotechnical study evaluating the current site stability be accomplished early in the design stages of the a project. This study should be performed by a geotechnical engineer and in accordance with the requirements of the SAO. We would recommend that the geotechnical engineer selected to perform this study be experienced in slope stability analysis and familiar with the geologic conditions and slope stabilization efforts that have been accomplished in the vicinity of the proposed development. Upon completion of the geotechnical report documenting the findings of this study, we recom- mend that an independent review be performed by a geotechnical engineer. This recommenda- tion is consistent with our interpretation of the requirements of the Tukwila Municipal Code 21.04.140. W- 6367 -02 J rn,rn..a.en..A..uur..w..n.naaU, • ∎• M 1 /1M «n. ••••,•..rn,fwrY.f..sw City of Tukwila Attn: Mr. Jack Pace March 2, 1993 Page 4 Please call if there are any questions regarding this letter. Sincerely, SHANNON & WILSON, INC. Daniel N. Clayton, C.E.G. Senior Associate Reviewed by: EXPIRES 11/10/ 9 ¢ W. Paul Grant, P. E. Vice President DNC:WPG /dnc W 6367- 02.LTR/ W6367 -1kd /e et SHANNON 6WILSON, INC. W- 6367 -02 • .. . Mechanical & Civil Engineering 17815 S.E. 146th Renton, WA 98050 (206) 2284244 FAX (206) 228.4292 Leroy C. Lowe, A.I.A. P.O. Box 3972 Bellevue, Wa. 98009 February 17, 1992 RE: Hilicrest Boundary Line Adjustment Dewatering and stability Dear Leroy: EVOCR `` FEB 18 1992 LITY OF TUKWILA PLANNING DEPT. At your request I once again reviewed your proposed above referenced project. My review included a site visit and review of the information supplied by other consultants. There can be no doubt in anyone's mind that Slade Way is threatened by a potential landslide downslope of your property if the present conditions are not corrected. Your property and that portion of Slade Way East of it are saturated by seeps. The saturated soil is very heavy and viscous and hence, at present, lead to a very unstable situation. Slade Way is presently serving as an earthen dam holding the heavy saturated soil in place. Slade Way was clearly not built or designed to withstand the potential lateral loads due to the upland saturated soils. The most elementary Geotechnical calculations would demonstrate that Slade Way is not stable within reasonable safety limits. The other consideration is the presence of a wetland on your property. While I am not a sensitive areas or wetlands expert, it is clear from yr. Bredberg's report and my observations, that there are some wetlands on the property. It appears to me that there is clearly a question of balancing the stability of Slade Way and the utilities in it along with the corresponding public health and safety against the value of the wetland. It would seem obvious to me that the logical and most _expedient solution to the Slade Way stability problem would be the "controlled responsible" development of your property. This would include street frontage, and stormdrain improvements to Slade Way along with on -site drainage, dewatering and stability measures that would assure stability over the entire affected area. While I realize that I have not said anything here that you do not already know, I hope that my comments will be of some help to you. Please call me if you have any questions or need additional information. Sincerely, Richar• . Stu h, P.E. Washington State registered mechanical and civil engins.rin^ corporation CASCADE GEOTECHNICAL INC. GEORGE E. LAMB, P.E. 'President/Principal Engineer EDUCATION - B.E. Civil Engineering, Yale University, 1954 M.S. Civil Engineering, University of Washington, 1959 Graduate Courses in Environmental Protection, Law, Real Management, Finance and Technical Specialties. Property, Project REGISTRATION - Registered Civil Engineer: Washington, Alaska, California, D.C., Idaho, and Oregon PROFESSIONAL EXPERIENCE - Mr. Lamb has directed major environmental, geotechnical, and construction management projects for a wide variety of private and public clients throughout the Northwest and world -wide. He has over 30 years of increasingly responsible experience in engineering and environmental work including 25 years as a principal in Seattle Area engineering and environmental firms.• He has also taught undergraduate and graduate courses in geotechnical engineering at the University of Washington and Seattle University. SELECTED PROJECT EXPERIENCE - Directed EIS for Alderwood Mall Regional Center, Lynnwood, Washington, including EAR, DEIS and FEIS addressing traffic, socioeconomic studies, employment studies, air quality, wildlife and aesthetic studies, and tax and public facilities impacts. Marine impacts included potential effects on migrating species, including salmonids. Trans Mountain Pipeline Company, Port and Pipeline, Washington to Minnesota via Calgary. Directed the U.S. portions of the Siting Study and EAR /EIS. The reports were designed to fulfill the requirements of EFSEC in Washington and FERC and other Federal agencies. Assessment of oil spill, air quality, socioeconomic, and transportation impacts. Marine impacts included potential effects on migrating species, including salmonids. Northern Tier Pipeline, Washington Coast to Minnesota. Directed EIS /EAR for this project. Studies were designed to fulfill the requirements of FERC, EFSEC and all the state regulatory agencies along this 1,800 mile route. Everett Middle School No. 5. Directed, developed and delivered back -up information and testimony regarding potential groundwater effects of construction of a middle school in the rapidly developing, but still largely rural area. Directed Foundation study and Environmental Assessment Study for proposed supertanker port and shore facilities, Shell Oil Company, Anacortes, Washington, including addressing soil and bottom conditions, marine risks, air quality, socioeconomic, and transportation impacts, and providing recommendations for protection of intertidal zone. Directed habitat and access enhancement studies for anadromous fish on several major tributaries to the Columbia River, under contract for the BPA salmon /steelhead enhancement efforts on Deschutes /White, Yakima, Lemhi, Similkameen, Wenatchee, and Columbia Rivers. CASCADE GEOTECHNICAL INC. George E. Lamb, P.E. Resume - page 2 Directed investigations at the Ferndale /ARCO Refinery, for Trans - Mountain Pipeline, Olympic and Northwest pipelines, and directed numerous studies for refinery facilities, including tank farm foundations and the foundations for a large coke calciner on sensitive clays. Contaminant Plume Modeling, Hanford Washington. Directed the characterization and modelling of the groundwater regime under the Hanford Reservation. Developed a transient state multi- aquifer groundwater model from empirical data. The model reflected past groundwater behavior and predicted future conditions, including rate of contaminant transport. Work performed under contract to the Seattle District, Corps of Engineers in connection with the proposed Ben Franklin Dam. Access Road Routing and Design, Hanford Reservation. Directed the evaluation of alternative routes for transport of the WPPSS No. 2 Reactor Vessel from the barge dock to the plant site. Evaluated existing pavements and structures and the feasibility of construction of an alternate road for the distance. Designed pavements for new approach areas and monitored progress and pavement conditions throughout the route before and during transport. Work performed for Rigging International. Erection Frame Foundations, WPPSS Plant No. 2. Directed evaluations and design of foundations for the "Green Giant" Lift Frame. The main mast was over 300 feet high, and several back braces and anchors also required foundation designs. Work performed for Rigging International. Excavation Slope Stability and Main Feed Line Backfill Evaluations. WPPSS Plant No. 2. Work performed for Rigging International and several other contractors. Directed Geotechnical, Environmental and Siting Study for proposed supertanker ports at three locations in Washington State and pipeline routes from Pacific to Stillwater, Minnesota, including marine risks, oil spill risks, port siting, underwater crossings in marine and fresh water, pipeline routing across five states, geotechnical, socioeconomic and regulatory studies, traffic and transportation studies, air quality, wildlife impacts and other regulatory and environmental concerns. Tailings Dam Design and Groundwater Regime Characterization. Directed engineering for raising the tailings dam at the Union Carbide Uranium Mill at Uravan, Colorado. Final dam height is over 300 feet. Groundwater effects were complicated by the presence of old mine workings in the area to be covered and by discontinuous impervious layers in the underlying formations. "Zero Pass" Dam Design, Sherwood Mine, Wellpinit, Washington. Designed the dam face and containment system for the tailings pond at the Sherwood Uranium Mine. Techniques used were accepted, and later adopted by the then AEC. Plutonium contamination identification and modeling, Mound National Laboratory, Moundsville, Ohio. Directed the tracking of the contaminant plume and the tracing of the source of Pu20, contamination which had contaminated surface and shallow ground water in the area. Work performed for Monsanto Chemical Company, the Laboratory manager. Nuclear Power Plant Siting, Oregon and Washington. Technical and regulatory contributor to siting studies for the Northwest Power Planning Council. CASCADE GEOTECHNICAL INC. George E. Lamb, P.E. Resume - page 3 Numerous run -off control and erosion studies in the Puget Sound Region and hydropower irrigation and highway hydrology studies world wide have addressed surface water hydrology concerns, including extensive experience in groundwater modelling and contamination, and the development of a number of programs which estimate and predict groundwater flow and contamination. Directed geotechnical and groundwater studies at the Mobil Oil (now B.P.) Refinery at Ferndale, Washington. Studies included foundations for storage tanks and a cracking towe, as well as analysis and design for the phenolic waste detention pond. Served as expert witness in Washington State courts and in Federal District Courts in cases concerning geotechnical engineering, hydrology, construction delay claims, and business valuation. Specific water - related litigation support includes work for Suffolk County, New York, on two -mile underwater sewer outfall; for Panama Canal Commission on widening project near Pedro Miguel Locks; and for Rockland County, New York, on major sewer project. Other litigation support included: Washington State DOT on construction delay claims on the Tacoma Spur, and McFarland Skylights on the evaluation of a contracting business. Barge Dock, Port of Hanford. Directed dock evaluation and strengthening design and barge grounding pad design for barge carrying the reactor vessel for WPPSS plant No. 2, a 1,300 ton load. Work performed for Rigging International, transport and erection contractors. Directed the preparation of geologic hazards mapping for the Bothell- Auburn Corridor for the Legislative Joint Transportation Committee. CSWO Studies, Northern Region, METRO. Directed computer simulation and modelling for the combined storm water overflow studies in the northern portion of King County. This involved establishing a high speed modem connection with the main METRO computer and developing suitable models to reflect storm flows and allow extensive "what -if" manipulation of the model. Directed numerous major geotechnical studies for waterfront structures and in hydraulic fill areas, including: Foundation investigation and design, Union Oil Company Dock, Edmonds, Washington; Evaluation of soils conditions, Bethlehem Steel Ingot Storage area, Harbor Island, Washington; Foundation investigation and design, Sears Catalog Building, First and Lander, Seattle, Washington; Barge Dock and ground pad design, Port of Richland for WPPS #2, Reactor Vessel, Columbia River, Washington; Foundation investigation and design, Barge Dock and grounding pad, Shell Oil Co., Tern Island, Alaska; Foundation studies for proposed Harbormaster development, Juneau, Alaska; Crane track foundation and apron pavement design, "K" Line Container Facilities, Port of Long Beach, California. As President of ERTEC Northwest, directed major geotechnical studies in the Pacific Northwest including: Reservoir Stability Study and Seismic Evaluation of Dvorshak Dam, Clearwater River, Walla Walla District Corps of Engineers; Reservoir Stability and Geotechnical Feasibility Study, Proposed Copper Creek Dam, Skagit River, Seattle City Light; Engineer of Record, Proposed Ghost Camp Dam (180 -foot high rockfill dam), Condon, Oregon, U.S. Soil Conservation Service. • CASCADE GEOTECHNICAL INC. George E. Lamb, P.E. Resume - page 4 State Soils Engineer, Oregon Highway Department, in charge of geotechnical work of six District Soils Engineers and provided recommendations and remedial design on all major landslides in the State, including the massive (1 x 1.5 mile) Hooskenaden Slide, and for major landslides on the Deschutes River caused by reservoir filling. District Soils Engineer, Division 1 - California Division of Highways, responsible for recommendations and remediation of more than 100 landslides in northwestern portion of State. PROFESSIONAL SOCIETIES - University of Washington Engineering Alumni Board, First Chairman of Nominating Committee. Association of Ground Water Scientists and Engineers -- Member American Society of Civil Engineers -- Member, Former Chairman National Engineering Management Committee on Professional Activities. Project Management Institute -- Member. International Society for Soil Mechanics and Foundation Engineering -- Member. Security Clearance - U.S. Army Strategic Intelligence Division, Secret Publications - In addition to numerous consulting reports, Mr. Lamb is senior or co- author of six professional papers and publications on technical and programmatic issues. A list is available upon request. PERSONAL HISTORY STATEMENT NEIL H. TWELKER BS (Civil Engineering) AM (Soil Mechanics) PhD (Soil Mechanics, w /Geology Minor) University of Washington Harvard University 1942 1954 Harvard University 1958 Member American Society of Civil Engineers Geological Society of America Structural Engineers Association of Washington United States Committee on Large Dams Tau Beta Pi Sigma Xi Lecturer, Univ. Wash., College of Arch. & Urban Planning Registered Professional Engineer Washington (Civil) Alaska (Civil) Oregon (Civil) Experience Forty -eight years total experience in the application of soil mechanics to engineering works, including: Nine years with the Seattle District of the Corps of Engineers, with responsible charge of soils engineering on major and minor projects, including design and construction phases of Chief Joseph Dam and Albeni Falls Dam, preliminary investigations on numerous dam sites throughout the Pacific Northwest, airfields, levees, breakwaters, and building foundations. Two years with Shannon and Wilson, Soil Mechanics and Foundation Engineers, Seattle, Washington, with responsible charge of site investigations for Wells Dam (Columbia River, Washington), Tolt River Dam, Upper Baker River Dam, slope stability investigation for open pit copper mines in Utah and Nevada, and miscellaneous foundation and landslide investigations. Three years of combined private practice, teaching, and graduate" study, including work on the St. Lawrence Seaway and consultation on Waterton Lakes Dam, Alberta. Thirty -four years in present practice. Soils and foundation engi- neering for buildings, bridges, roads, dams, and breakwaters. Over 6000 professional assignments in Washington, Oregon, Califor- nia, and Alaska, including waterfront and major industrial devel- opments for Port of Seattle, Port of Tacoma, and other governmen- tal and private organizations, and U.S. Navy at Whidbey Island and Adak Air Stations, and Puget Sound Naval Shipyard. Consultation on design and execution of numerous large and small diameter soft ground and hard rock tunnels, deep trenches, caissons, etc. Numerous remedial and underpinning projects for building founda- tions, and foundation and site preparation for residential and commercial structures of all sizes. Soil . and foundation studies for Tri- cities Bridge (Columbia River). In Seattle: Crown Plaza Hotel, Westin Building, Warwick Hotel, Market Place North, Seattle Aquarium, Pacific Museum of Flight and restoration of Pike Place Market. In Portland, Ore.: Portland General Electric Headquarters Complex, KOIN Plaza, Marri- ott Hotel, Ben Franklin, and others. Dennis Joule, P.E. 32729 S.E. 44th Street Fall City, Washington 98024 (206) 222 -46661 Abbreviated Professional Resume 1975 - Pre. ent Consulting Geotechnical Engineer Consultin . geotechnical engineer working in Western and Central Washington. Primary ield of work involves the geotechnical aspect of suburban and rural engineeri g projects. Typical projects include settlement analysis, slope stability analysis, geotechnical investigations for houses, subdivisions, commercial /industrial buildings, schools, roads, utilities, machinery, and retaining walls. Design projects include foundations utilizing piles (auger cast, timber, pin), specialty foundations for steep hills de and soft soils, and earth retaining structures (reinforced concrete, timber, soldier pil- ). In addition, about 1000 on -site sewage disposal systems (single family and commercia1), and about 200 small community water systems have been designed and inspected. 1971- 19 Staff & Project Geotechnical Engineer During th s time worked for Terratech, Inc., a consulting geotechnical engineering firm with offic: s in Northern California. Work included earthwork field control, laboratory testing, field exploration and site reconnaissance, computer programming, engineering analysis, eport writing, project supervision, and client liaison. Projects included settlemen analysis, slope stability analysis, geotechnical investigations for houses, commercial /industrial buildings, schools, roads, utilities, machinery, and retaining walls. 1966 - 19 1 U.S. Navy Pilot Current chnical Positions KING COU BOARD OF HEALTH TECHNICAL ADVISORY COMMITTEE Represen ative for the Washington State Professional Engineers Chairman - Technical Subcommittee INSTRUCTO - CONTINUING EDUCATION PROGRAM Washingt n State Health Department Educatio M.S. Civil Engineering - 1976 Geotech cal Engineering San Jose ' tate University B.S. Agri « ltural Engineering - 1966 Californi . State Polytechnic University Licenses Professio al Civil Engineer - Washington State Professio al Civil Engineer - California Individual Profile ROBERT S. LEVINSON, P.E. PROJECT ASSIGNMENT: Geotechnical Engineer, Principal REGISTRATION: Civil Engineer - Washington Civil Engineer - Oregon, Idaho, Alaska, California, Hawaii, New Jersey Geotechnical Engineer - California EDUCATION: B:S., Civil Engineering, California State University, Los Angeles, 1961 A..A.S., Structural Technology, New York State Technical, 1955 Soil Mechanics, University of Southern California /U.C. Berkeley, 1966 -68 BACKGROUND: Mr. Levinson has provided a wide -range of design, review, and management services on thousands of projects over the past thirty years. He has worked on several continents with other firms and has gained a wealth of geotechnical engineering experience and judgement. Project types he has worked on include highways, airfields, industrial and commercial buildings, high- and mid -rise buildings, schools, shoring, transmission lines, soil stabilization, slopes, embankments, dams, landfills, port facilities, sewage treatment facilities, bridges, stadiums, pipelines, underwater outfalls, and many others. EMPLOYMENT HISTORY: 1975 - Present Earth Consultants, Inc.; President, Principal Engineer 1973 - 1975 Geolabs- Washington, Inc.; Principal Engineer 1972 - 1973 Dames & Moore, Korea; Project Manager 1958 - 1972 Army Corps of Engineers and other firms in Monrovia; Ghana; Liberia; and San Francisco, Santa Barbara, and Los Angeles, California RELATED PROJECT EXPERIENCE: KoII Center - North Creek, Bothell, Washington Provided field investigation and design studies for this major office park founded over peat and soft soils. Inglewood Plaza Shopping Center, King County, Washington Provided geotechnical investigation and design studies for this group of commercial buildings. Circle -K Market, Everett, Washington Provided field investigation and geotechnical design for this commercial building. Woodinville Town Center, Woodinville, Washington Provided geotechnical investigation and design for this commercial development. COM - R.; -1w� • • RESCO INC. NATURE AND HISTORY OF BUSINESS Resco Inc. was founded by Richard E. Stuth in October, 1977, as Stuth Engineering, Inc. practicing in the areas of both civil and mechanical engineering. Mr. Stuth had been practicing as a sole proprietor in the area of civil engineering, however, since 1966. The name of the company was changed to Resco Inc. in 1979. In the years between 1979 and 1985, Resco was predominantly involved in the construction business, primarily in the areas.of developing plats, parks and commercial site work. Resco liquidated its construction division in late 1985 and returned to full time engineering. With the addition of architectural services in 1990, Resco Inc. became a full Architectural & Engineering (A & E) office. Since its founding, Resco Inc. has grown to become a technological leader in both the engineering and architecture professions. Resco Inc. has taken a 'multi - disciplinary' approach to production by combining into one organization all the professions necessary for the design of successful building projects. The multi - disciplinary concept was developed as an answer to the growing diversity and scope of needs of its clients. Increased awareness of environmental concerns, community involvement, restrictive governmental regulations, and ever increasing design and construction costs prohibit the inefficiencies inherent in non - integrated design teams. The architects and engineers at Resco take pride in the years of experience which allow them to produce excellence in design. Being true artists, no effort is spared in the development of sensitive and unique design concepts which still meet the. unyielding demands of conciseness and cost - effectiveness. The primary goal of Resco is to assist its customers in any way to achieve their goals. From the assessment of the requirements of community goals and needs, through the design process, to the creation and administration of contract bid documents Resco remains dedicated to the timely completion of each project. Resco Inc. represents the modern approach to the operation of an A &E office. The firm utilizes the latest computer technology, software, and analysis techniques which allow the most efficient use of design and production time. Direct conversion of design layout drawings to production drawings, automated analysis, parametric drafting techniques, and an extensive library of pre - drawn symbols and specifications all contribute to the reduction of 1 required production time. Computerized billing and management techniques allow all employees to operate productively, minimizing overhead costs. MASTER PLANNING At the master planning level computer design techniques allow for the ability to collate, document and verify vastly differing information mediums into a cohesive whole. Analyses techniques allow for the efficient study of the impacts of differing design alternatives. For example, Resco's master planning for the Preston Industrial Area provided the following services: 1. A hydrologic study for the 500 acre drainage basin surrounding and including the subject property; 2. Preliminary stormwater and temporary erosion/ sedimentation control planning for non - developed sites; 3. Preliminary architectural design for 180,000 sf of office /retail space for two of the parcels; 4. Architectural Design for Building 'C' at the Preston Retail Center; 5. Actual stormwater control /grading designs for four of the developed parcels; 6. Design and construction management of the Preston Industrial Area water system reservoir and conveyance system; and 7. Comprehensive planning and guidelines for on -site sewage disposal systems that exceed King County Rules & Regulations. COMMERCIAL DESIGN Resco Inc. has considerable experience in the design and layout of commercial building and building sites including: 1. Architectural (building design- circulation, zoning, landscaping and parking); 2. Sensitive Area Mitigation; 3. Traffic; 4. Stormwater Control Design and Grading; 2 5. Sewage Disposal Systems; 6. Water Systems (including fire protection components); 7. Contract Preparation and Management; and 8. Project management. Projects: (The following is a parcial list of recent projects completed or in progress) 1. Bremmeyer Logging Office 27202 Kent Kangley Road Ravensdale, Wash. 98051 Duties: Building Design - 4900 SF office Building Interior Design - Structural Design Thermal Design Stormwater & Grading Design (28 acres site) Landscape Design Class 1 Water System Design Sewage Disposal System Utility planning 2. Bremmeyer Logging Shop 27202 Kent Kangley Road Ravensdale, Wash. 98051 Duties: Preliminary Building Design - 18,000 SF Tilt -up .3. Preston Retail Center Building 'C' 30365 SE High Point Way Issaquah, Wa. 98027 Duties: Building Design - 2400 SF Restaurant Interior Design - Structural Design Thermal Design Stormwater & Grading Design Landscape Design Sewage Disposal System 4. San Mar Distribution Center 30110 SE High Point Way 3 Issaquah, Wash. 98027 Duties: Stormwater Control and Grading (20+ acres site) Utilities (Water, domestic & fire protection) Sewage Disposal system 5. Stonefelt & Company Warehouse /Office 30110 SE High Point Way Issaquah, Wash. 98027 Duties: Stormwater Control and Grading (20+ acre site) Utilities (Water, domestic & fire protection) Sewage Disposal System LONG AND SHORT PLATS Resco Inc. is involved in all phases of land development design including short and long plats. Once again the multi- disciplinary approach to plat design provides some real advantages: 1. Survey information can be imported directly into the existing computer database; • 2. Computer aided design techniques allow the quick comparison of design alternatives; 3. Broad experience base allows for comparison of layout alternatives for technical feasibility, engineering efficiency and architectural /marketing desirability concurrently. 4. Background in working with sensitive areas allows for realistic assessment of the impacts of site issues on project goals. (WHAT THE HELL DOES THIS MEAN ? ?) Projects: 1. Paradise Estates - Long Plat Paradise Lake Road King County, Wash. Owner: Glenco Development Duties: Stormwater Control and Grading Utilities Road Design 4 2. Gatewood Court - Long Plat 130th Ave N.E. Bothell, Wash, Owner: Castlewood Homes Duties: Stormwater Control and Grading Utilities Road Design 3. Hall Short Plat Lake Joy Road Carnation, Washington Owner: Maxine & Jerry Hall UTILITY DESIGN Resco Inc. has been involved in various utility designs across the state. Resco has earned a niche as the engineering firm to call for difficult situations. It has done so by developing state of the art water and sewage disposal systems for the 'impossible situation.' Not being content to accept the research of others, the engineers at Resco conduct there own physical experiments for the research and development of new design concepts. Project Examples: 1. Class 1 Water System Island Utilities Bainbridge Island, Washington Owner: Port Blakely Tree Farms 2. Class 1 Water System Preston Industrial Water Association Preston, Washington Owner: Preston Industrial Associates 3. Larger On Site Sewage Disposal System Belfair Foodmart Belfair, Washington Owner: Keith Uddenberg, Inc. 4. Larger On Site Sewage Disposal System I -90 Industrial Park Preston, . Washington Owner: Bernard Development /Pacific Bank KEY PERSONNEL RICHARD E. STUTH, P.E. Dick was born in Seattle and grew up in the Issaquah area, graduating from Issaquah High School in 1959. He graduated from Seattle University with a degree in Mechanical Engineering in 1966. Also in 1966, he began evaluating soils for plats and designing residential septic systems and, to date, has designed over 2000 septic systems, both residential and commercial. He has held a State of Washington professional engineering license as a mechanical engineer since 1969 and as a civil engineer since 1973. Between graduation from college and the founding of Resco Inc., Dick worked for Pacific Car and Foundry, Saurwein Engineering, and The Robbins Company. His work for these companies was primarily in the area of mechanical engineering and more specifically in the design of tunneling machines. While working for The Robbins•Company, he was the project engineer on a number of projects in Europe which involved 'a great deal of travel to primarily Italy and France. He prided himself on picking up enough of both the Italian and French language to be able to communicate with the workers on the job and others with whom he came in contact. In the years between 1979 and 1985, Resco was predominately involved in the construction business, primarily in the areas of commercial site utility work, developing plats and building parks.While the construction endeavors were discontinued in 1985, the hands on experience gained by Dick during this period carries over into the present engineering firm. Dick not only has the respect of contractors and inspectors traditionally extended to the engineer but he has the additional respect given to an experienced contractor. He also has the ability to look at each project through the eyes of a contractor and hence avoid designing the project that is impractical, overly expensive or difficult to build. After returning to full time engineering in 1986, Dick was retained by The Robbins Company to help them with their design of the tunneling machine used to dig the English Channel tunnel. In late 1986 Dick became a believer in the computer age and to that end bought his first computers and took classes to become an effective operator. Today Dick along with all of his staff pride themselves at being at the leading edge of computer technology. ANTHONY JAY BREDBERG P.O. Box 1337 Gig Harbor, Washington 98335 (206) 858 -7055 PROFESSIONAL EXPERIENCE Wetlands Biologist, Soil Scientist, Botanist. BREDBERG AND ASSOCIATES, Inc. Specializing in wetland delineation, soil survey, mitigation design and monitoring, site evaluation, permitting, and expert witness testimony. Established 1986. Senior Soil Scientist. NH SOIL CONSULTANTS, Inc. Founded and directed soils and wetland division. Consultant. BREDBERG SOILS. Agronomic consulting on crop plantings, soils, production, and conservation. College Instructor. BLACK HAWK COLLEGE. Focused on plant identification, soil management, and conservation. EDUCATION MS. North Carolina State University, Raleigh, NC. Studied soil science and plant science. 1979. BS. Western Illinois University, Macomb, IL. Studied plant science and botany. 1977. Graduate Studies Soil Science University of Illinois Forestry University of New Hampshire Wetlands University of Massachusetts PROFESSIONAL AFFILIATIONS National Society of Consulting Soil Scientists (NSCSS) Board member since January 1990. Soil and Water Conservation Society of America International Society of Soil Scientists Soil Science Society of America Society of Wetland Scientists QUALIFICATIONS AND EXPERTISE Registered Professional Soil Scientist Expert Witness Testimony Recognized Speaker Site Evaluation Wetlands Biologist/Botanist Wetland Delineator Waste Management Educator/Trainer A. J. Bredberg has cocperated with over 1,500 individual surveys and wetland evaluations. He is a Botanist and a Soil Scientist, and has been involved with the environmental field for 15 years and with wetland delineation for 7 years John Wesley Jennings P. O. Box 9635 Tacoma, WA 98409 206 - 474-47113 This resume documents my background, qualifications, and work experience .1�, a soil scientist. I ani a certified soil scientist under the American Registry of Certified professionals in Agronomy, Crops, and Soils (ARCPACS). GENERAL WORK EXPERIENCE IN SOIL SCIENCE I have worked continuously as a soil scientist since 1974. Most of this work experience has been as the forest coil scientist, Olympic National Forest, Olympia, WA. Since 1990 have worked as an consultant in the private sector. My work has been in a great variety of applications that relate to soil science. 1 have a philosophy and systematic approach to my work. I start from principle, of professional knowledge and understanding and aim toward giving my best counsel in specific_ work applications. I seek the truth in all things, and have a system of logic to guide me in this task. SOIL SURVEY My experience working on soil surveys has proved to be invaluable. I have worked on the following surveys. * 2arembo and Kiui Island Soil Surveys, 1974, Alaska * Soil Survey of Lewis County, Washington * Soil Survey of Grays Harbor County Area, Pacific County, and Wahkiakum County, Washington * Satsop Block Planning Unit Soil Survey, unpublished, Olympic National Forest, Washington * The Olympic National Forest Soil Resource Inventory Update, 1982, in service publication, Washington * Olympic National Forest Cooperative Soil Survey, in progress, Washington. I was project leader on the last three soil surveys. This work experience includes the mapping, description, and interpretation phases of a soil survey. Since 1974, approximately 40 percent of my work experience has been doing soil surveys. LAND USE PLANNING It is both a reward and challenge to properly interpret soil data for land use planning. I have been involved in the following planning efforts. • Canal Front Planning Unit, Olympic National Forest * Satsop Block Planning Unit, Olympic National Forest 4 Land and Resource Management Plan, Olympic National Forest My £Oi1 interpretations for the Cana] Front Planning !;nit followed then current procedures. With the Satsop Block Planning Unit 1 worked closely k the planning unit leader. Our goal was to upgrade the soils data and enhan, the Satsop Block Plan's forest productivity and ero6ian hazard ratings. Th,: results were gratifying. We significantly increased our understanding of th, environment and produced more realistic soil interpretations for the plannii., unit. The creative approaches we used on the Satsop Planning Unit were uscd ..t, a guide for the current Olympic National Forest Land and Resource Managem::c:- Plan. Since 1974, about 10 percent of my work experience has been in land management planning activities. SOILS AND PROJECT ANALMS Relating soils to man's past, present and future activities drives at the ve;y basics of soil science. In project analysis, I start from known information: soil survey data, aerial photos, proposed activity objectives, resource issue.:: and concerns, applicable legal and administrative guidelines, etc. Next I design data collection procedures: variables to sample, critical site locations to visit, individuals to involve. I follow data collection with a systematic analysis directed toward logical conclusions. I present my conclusions in a document geared to my reading audience and targeted to inform and influence individuals in a truthful and honest manner. I have served on environmental analysis teams, and understand the logic and value of this work process. I have conducted numerous site specific analysis during my career, literally 100's, of a variable nature. Since 1974, approximately 50 percent of my work experience has been spent doing soil and project planning analysis. I have a strong background and interest in soil erosional processes and land rehabilitation. It flows from a strong commitment to promote sound land stewardship. I have extensive experience in preventative analysis of erosional processes. For example, if we place a road across a slope, or harvest an area of timber, are we likely to initiate mass movements? What management alternatives will minimize the possibilities for starting surface erosion? (that will be the impacts upon natural resources? What corrective measures can be utilized? These are typical questions I have addressed numerous times. I feel I understand well the forces that drive and restrain * all forms of soil erosion. EDUCATION and PROFESSIONAL BACKGROUND I received a B.S. degree in forest science from the University of Washington and have completed two years of graduate studies in soil science at Oregon State University. I have also attended six other colleges. I view education, both formal and informal, as a continuing process of personal development. I started my professional work as a forester. My last forester position was on Smith River Ranger District, Reedsport, OR. I was the timber staff. assistant to the District Ranger. It was during my work period as a forester that I started to use soils information. I was impressed with the usefulness of soils data to forestry, and realized it had great utility into a variety of man's other activities. SPECIAL ABILITIES /HOBBIES Here I will mention some individual abilities not previously captured. On oI my strong points is the ability to see the broad perspective. I just don't describe soils in a pit. 1 desire to understand how soils tie to forestry, agriculture, urban development. etc. Conversely, I relate to how a former views soil, an engineer views soil, and a geologist views soil. I have developed special abilities in the use of aerial photos. I read well how and land£orin fit on a landscape. I consider myself to be a creative individual. I strive to find new and :superior work procedures. My hobbies include chess, horticulture, and choral singing. Lastly, I would mention I have considerable experience training other individuals. I have received recognition for this from co- workers, superiors, and former trainees. I get a great deal of. satisfaction in helping others learn about soils. This resume is organized to help other individuals understand my abilities and work experience as a soil scientist. I have not made a long list of my former employers and supervisors. I judge the use of this functional resume tells ' more about my abilities as a professional soil scientist. If you have any questions, or feel you need additional information, please contact me. I will meet your needs expediently. Wes Jennings April 6, 1992. December 20, 1993 PROFESSIONAL RESUME Dick Osborne WORK HISTORY 36 years of real estate associated work U.S.Army: European Occupation Forces post W.W. II Family life Insurance Co.: Owned by Merrill Lynch Executive V.P. responsible for facilities and property management Tower Properties: V.P. and partner ownership and management of residential and commercial pro- perties G &O Properties: Partner ownership and management of residential properties ' Overlake Realty: President and owner. General sales and listing of residential,commercial and invest- ment properties CURRENTLY: I sold Overlake Realty to John L. Scott Real Estate and am presently associated with John L. Scott Real Estate Current license expires 4 -24 -95 Ken Eikenberry ATTORNEY GENERAL OF WASHINGTON 7th FLOOR, HIGHWAYS-LICENSES BUILDING . OLYMPIA, WASHINGTON 98504-8071 February 7, 1989 Mr. Joel E. Haggard Suite 1515, IBM Building 1200 Fifth Avenue Seattle, WA 98101 RE: SR 5 MP 154.0 Valley View Estates Dear Mr. Haggard: This is in response to your letter transmitting the instrumentation and monitoring action *plans for the Valley View Estates development to Mr. Ron Bockstruck, District Administrator for District 1 of the Washington State Department of Transportation (WSDOT). Mr. Bockstruck has asked me to reply since it would appear that this is as much a matter of legal liability as it is a matter of technical feasibility. Your description of the deep drainage system along and above Klickitat Drive fails to address the presence of an algae -like organism that is present in the clay soil of this hillside. The air admitted by the weep drains that comprise the horizontal drainage system stimulates the growth of this organism with the result that the pipes become clogged. WSDOT's maintenance crews have been flushing these pipes once a year for the past several years to keep these drains open. Under present circumstances, this level seems to be sufficient to maintain the stability of the slope and, thus, the integrity of the roadway system in the area.' We are very concerned about the conc..usory statements in your letter that the proposed Valley View E:-tates project will have no impact on the functioning or effectiveness of our deep drainage system. As we have indicated in the past, the hillside for your proposed development is within a landslide zone and the proposed complex would be located on clay -type soil over a sliding -block failure surface and partially encompassing the zone of generated critical failure surfaces. It is a very unstable area. Our drainage system was not designed nor intended to withstand high density development on your proposed site nor was our drainage system designed to accommodate any increased soil loadings or OFFIr" " "•) OF. THE ..TTORNEY Joel E. Haggard February 7, 1989 Page 2 changes in drainage which would stem from your development. Furthermore, we decline from accepting the increased liability should our system fail due to.your client's proposed development nor will we accept the burden of additional monitoring and stand- by services for the life of the proposed condominium development. Regarding the surface drainage, your preliminary drainage plans have been reviewed; however, the WSDOT reserves comment at this time. The WSDOT has suspended all plan review on this project until the deep drainage issue can be resolved. Regarding the instrumentation monitoring action plan, the WSDOT has the following comments: 1) WSDOT does not have the legal authority, the funds, or the personnel to provide what are, essentially, "on- call" maintenance personnel to benefit'a private development. 2) WSDOT will not assign responsibility for determining what "remedial measures" are necessary and who should perform them, to a Geotechnical Professional representing the interests of a private developer. 3) WSDOT has no plans nor funds programmed to restore full functioning of the drainage system, reinstall all the pumps in the vertical well system, install new horizontal drains, nor implement localized slope stabilization measures. They would be unable to program funds to accomplish those tasks for the benefit of a private development. If a change in the water level occurs during or after construction of this development, remedial action must be the responsibility of the developer. Sincerely, /14.1./-.4=-",/ / ';✓"ireJ �: MARJORIE T. SMITCH Assistant Attorney General (206) 753 -4961 MTS:vf cc: Mr. Rick Beeler, City of Tukwila Ms. Dharlene West, President, T -MAC Mr. Ron Eockstruck, WSDOT 'Mr. Peter Briglia, WSDOT f•. oio 1:5 342 =Yr.r..sf •• 0' 74. -1_,1 I • roadway section DETAIL .der.outi. (-4.110.41 Zirstw, airw A.a•1 F-C•v.r.a 1 \ \\ \ !7N \ Mr' 1.111.11.4 rata - A .? CSM.. ?CA.', 115 • 2 2 —7— CUL [Mo. 24 • /7 22f2. 54. 4.1. /e• N. Z05. NO,,ar.wc. Aare 2.4. .4.=0ry .000'L limmool" ye C.44.1/.4c7- .4'00C Y.024.5 =0 W. it' "kr el It. AU/ ":4-IL7 X.,PTC.L. 207.0 • IN ZS. CCIT DRAIN/ME SITE PLAN 1"-= 30 n=rir • • czcw Ory 41' .‘o • itt, 4P ,p •••••9.0. heiCar2,72.4 - 2700 trv,,,, 6 2 /40•104 - 74/5 0,=.0. 2:0' W. :LOCK ,ftW,C.WC.0 roe, /4"... REC.1“.V.E.0 1800 ca.arc sesasrre.. •••••::: . , SITE PLAN i= ,Gc , VICINITY MAP 0 ; 0 ao OD. 3 2 4 SITE PLAN. Q .tie: I ..7o' L. 4% 5 planting list •t .'oo; Suol .ham YG.�J.7 .2..1544,s //AC I' G' r..,R ♦. 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ITS 1r LJr rosna.• 1z o�•rstf;; <? 0 N a G ; 40 900 1.•Ir• 15) ON 299052 299052 901. Orar[ [N R �+3� er �3 U +� er 01'src rA fa) 11142' A, 001' 3'5C[ N.05' --�� s1155.5e• k_ for 5'614 Or SHADE 14/4);- P NOIldId3S30 1V031 SUf?VE Y PROFESSIONALS SW f 1.151 ft. 1 , WM" I-109 50rw5�01MLU1wa5 l i1 LLYS 1.s OW f1 UH 11A DRAWN: 6D 5/1ip90 APPRDVED•7EC 171790 SURVEYORS CERTIFICATE 7915 MAP CORRECTLY REPRESENTS A SURVEY MADE BY ME OR UNDER NY DIRECTION 10 CONFOR00 iCC 01TH 111E RC0411RMENI5 OF THE SURVEY RECORDING ACT AND STATE STANDARDS AT 1144: REQUEST OF i,5 nr_ L..e.544;1990 544;1990 _ __._-- ......I'I 690 /n sh,.., E.• •r.a.1.•.r� 00 16898 TO?OGR ,r, HY SURVEY FOR LE600 LOW( A.I.A. A'CHITECT PO BOX 1711 SEATTLE WA. 98111. IN THE NW 1 OF THE NW 1/4 OF SEC. 26, T. 239., 6.40, W.Y., KING OUNTY, WASHINGTON. NO1'03'56-E qi N00 t1'tYM (P) N yN N0,' 0]'SrCN 34w 9r N u 9S a of E 8 170.00' CO (C0., rCO ` V 170,00' V N '0 N N01'03'56 "E 354.92' 184.92' 0 \ 0 L1.% CA V \ Q CO C vJ - 8 N • • 184.92' 154.92 (0 Y3 =O ,x, ONY O r 0 I C Z Fri o t0 Z - 0C Z - •. r \CI A C) m 0 I— C m? (mjZ • N-C 11 ED mm m z Z H Cl (.7 N CO I Z m OA > In 0• or rsrt 153.01 ,9100• 03 33�sovr� -- Ev S0103'9r9G IN3IVJSfIIV ¶NI7 1WVUNflOU Willifi(f !I UMW!! Rpii�fi;f� =ti ;i:'IIsiiit1111! ill Inn f i 1'111141 li S i jlliiii'i SSiii! -i tII p •II 1r111'' i- ill 0 1L GC 3 i /I.i9 C_fl ' R_ Il.. Ming 3 a_Lal :Ii Ij K �:ii 'c a 1! l WIi! 'Ii 9fei ii. =e•= ip iii, D iikill ..4.4 4. 9499 55 ; 5i l I,49 iL `1S T•:•:•:!: i 1•! r iijiiit i1 If i ?_! it i lj Q :lit:: iii£ rIil:Lll_ .T - ° : i1" Li i 1 G 13v- _ :9..901: fi 'L• 1 1111 ° q !f d11 A. L 1. i I -i;l . L._ff s 1 ^ i iLic i i- !i fi l i- f !I l l �I itff i I.!...ti 11, it N GIl3l i L li lffi G L l!� f! I LL 'i !f?fG i f 11 iii= I f iA 1 P. Ii, ! le i! �i i; i'- L i it X111 f i If T !!: c 3f i i , 1 !f ;S+ .Z7 s. 1T 11 i. sif - 3 r i 1 i I ? 1 s 11 3j e f hi! f s -� 9 :^ f 9 i 1�i r l` f i fi t i 1 5 E 11 —I i i fii i ii f I• :4- i I 3 i AA• 3 i; i- L 1 1 �t O z SURVEY PROFESSIONALS APPwa40 JOHN CRAMLR P.L.S. out A 19is3 Pt. = 0339,. R.:.LE.C. 11.121n1031 ((.1 ii,=aii RMS[t. 301 49 90004 o.GC101L -90 SCALE: 1 - -30' SM. 1 00 1 SURVEYOR'S CERTIFICATE THIS MAP REPRESENTS A SURVEY MADE BY ME OR UNDER MY DIRECTION IN CONFORMANCE WITH 111E REQUIREMENTS OF THE SURVEY RECORDING ACT AND STATE STANDARDS AT THE REQUEST 00' LEROY LOWE IN OCTOBER, 1990. P 9 111898 -: sIIILLCPEcT Rl LEROV C. LOWE, .a+.1/1 ARCHITECT 4a NpP Altw (P) NOT. CJSCC _L �n!7 PPy I N NO1'O3'56 "E 54.92' 5 id is • NCI' 3.0r U _��• YOr I.CR' a g N01'03'56'E LS q E WAY 17103'56 "E 1.99' ND /07 311.11 121t'M315 Y1O / SNOL t • S NOIld1210S3O SURVEY PROFESSIONALS N���C JOHN CHANCR P.LS. 1ul a. IHln PL o�a ..e.. e.,I,E.L. urt ww+.. w.. 11:41::".111 .... w,. erAUn yyurin_ 9D➢O.Lurt1Il =1L-9U SCALE: or 1 SHT. 1 or 1 SURVEYOR'S CERTIFICATE THIS MAP REPRESENTS A SURVEY RUDE BY ME OR UNDER MY DIRECTION IN CONFORMANCE WITH THE REQUIREMENTS OF THE SURVEY RECORDING ACT AND STALE STANDARDS AT THE REQUEST OF LEROY LOWE RY DOTOBER. 1990. I'Y S 1flH9H IIILLCJ Ec T LEROV C. LOWE. ARCHITECT pra & ASSOCIATES Inc. • tCUNDA2Y AND -TORXRAPNIC aiRVEY VALLEY VIEW ESTATES FRC:OE-GT F°2 Patti- WESTERN, INC tELLEVUE,NA (gown er awe •A ■•• 1,14,4 1 *1 .11, —1.11 I PI 4, gjl . 44441 — 0\c3\3\ ocz --a :Ks\ \\\ SITE PLAN 4\H H.A.2 T.P.1 H.A.4 ► _�1 T.B.2 w 0 w s / N =MI 11= .7 H.A.5 N \ :\\ \ H.A.8 \ Ca ° o �'c' `p `'� �� � '<3 v. A ca k ' �' f",„? 6' o TEST PIT (05/15/90) HAND AUGER (05/23/90) X TEST BORING (07/24/90 & 07/25/90) FROM A LANDSCAPE SITE PLAN BY LEROY C. LOWE, A.I.A. ARCHITECT (03/18/93) AND FROM TEST HOLE LOCATION MAP BY CASCADE GEOTECHNICAL (05/29/90) CASCADE GEOTECHNICAL INC. 12016 115T11 AVENUE N.E., BLDG. II (206) 821 x(1811 KIRKLAND, WASI IINGTON 98034 FAX (206) 820 69`,3 Job No. 9303 -O5G I SCALE : 1" = 50' LOCATIONS ARE APPROXIMATE Date 04/08/93 Dwn. By HLA'Eng -Gaol HILLCREST SLOPE STABILITY ANALYSIS SITE PLAN • • • • 330 — 300 — 270 — 240 — 210 — 180 PROPERTY LINE EX/STING 1 2 HILLCREST SLOPE STABILITY ANALYSIS CROSS SECTION A - A' 3 4 SURFACE 30 60 90 120 150 180 210 Feet EXISTING 240 SURFACE 5 6 4 PROPERTY LINE 270 300 330 360 A' — 330 300 — 270 — 240 — 210 180 CASCADE GEOTECHNICAL INC. 12016 1157H AVENUE N E-. BLCG H (206) 821-5080FAX 12�b) 820,E953 KIRKLAND. WASHINGTON 98034 Job No. 9303-05G Scale 1" = 30' LOCATIONS ARE APPROXIMATE Date 03/17/93 own. By HLA Eno -Gaol 0 30 60 90 HILLCREST SLOPE STABILITY ANALYSIS CROSS SECTION A - A' SURFACE 120 150 ` _ SURFACE N -_` _ N -_____ 5 4 \ �� 180 Feet 210 240 270 300 330 ; CASCADE GEOTECHNICAL INC. 12016 115TH AVENUE N E.. BLDG H (206) 821.5080 KIRKLAND. WASHINGTON 98034 FAX, (206) 820 6953 Job Dati B 350 — 320 — 290 — 260 — 230 — 200 PROPERTY LINE HILLCREST SLOPE STABILITY ANALYSIS CROSS SECTION B - B' N 0 30 60 90 120 150 5 6 4 PROPERTY LINE B' — 350 — 320 — 290 — 260 — 230 180 Feet 210 240 270 300 330 360 200 Cc CASCADE GEOTECHNICAL INC. 12016115TH AVENUE N.E.. BLDG H (206) FAX. (206) 821-5080 5080 KIRKLAND. WASHINGTON 98034 Job No. 9303-05G Scale 1" = 30' LOCATIONS ARE APPROXIMATE Dete 03/17/93 Own. By HLA Eng-Geoff B 350 — 320 — 290 — 260 — 230 — 200 PROPERTY LINE HILLCREST SLOPE STABILITY ANALYSIS CROSS SECTION B - B' N 6 4 0 30 60 90 120 150 180 Feet 210 240 270 300 33C CASCADE GEOTECHNICAL INC. 12016115TH AVENUE N.E.. BLDG H (206) 821.5080 KIRKLAND. WASHINGTON 98034 FAX, (206) 820.6953 priim • • • e- N Ancient Landslide Scarp . 1960 -1968 SLOPE IMPROVEMENTS •••••••••• ** • OF Original Test Drain Horizontal Drain Large Diameter (5') Deep Well Piezometer Installed During Previous Investigations Slope Indicator Critical Section Location Approximate+ Extent :.pf Slide 4/20/ .T'• SEC. STATIC DYNAMIC A -A F.S. =1.24 1.08 B -B F.S. =1.18 0.86 Fig. 1 1960 Landslide Location & Remediation 11 OD 300 250 200 150 100 50_ 0_- Hillcrest Development s Puget Western Ancient Landscape 1 3 Scarp - y nor 'Deep- seated slip surface studied by Cascade Geotech. Approximate Location Publicly Owned Y of 1960 -61 Slip Surface Infered Location of postulated deep seated slip surface Area where material removal triggered 1960 -61 sliding between Kickitat Drive and Slade Way CROSS - SECTION THROUGH HILLCREST, SLADE WAY AND KLICKITAT DRIVE