Reg 2018-09-17 Item 3G - Contract Amendment #2 - Riverton Creek Flap Gate Removal with Otak Inc for $37,064

Browse Search

Exhibit A-2 Scope of Work Riverton Creek Flapgate Removal Project City of Tukwila August 7, 20I8 Otak Project No. 32740 Amendment No. 2 A 75-percent design for the Riverton Creek Flapgate Removal Project was completed previously by a different consultant. Under the original Scope of Work, Otak performed an Intake Review of the previous design and found that on-site geotechnical evaluation, cultural resources, and hydraulic modeling and scour analysis had not been provided with the previous design, even though stability concerns had been documented. Further, based on a site investigation of existing conditions,Otak's team recommended that the proposed channel section be revised to address slope stability and erosion concerns. The previous Amendment No. 1 added geotechnical services and some additional scope to revise the channel section to address long-term stability concerns. Once on-site geotechnical explorations and analysis were performed, the team met to review the geotechnical recommendations and identify necessary measures to address global stability, since, the adjacent Tukwila International Blvd.bridge abutment sheet pile wall and private parking lot will be supported by the proposed channel side slopes after flapgate and culvert removal and breaching of the revetment. Options considered included further stabilization measures for the existing sheet pile wall such as a new sheet pile wall at the base of the existing wall or the installation of a lower row of tie-back anchors nearer the base of existing wall. Options for a new retaining wall along the east side of the channel were also considered to maintain the existing private parking lot. Constructability and cost were compared between optional design elements for stabilization. In addition to the channel alignment and pedestrian bridge being shifted east, further away from the existing sheet pile wall, the team recommends that a second row of tie-back anchors be installed in the existing sheet pile wall, and a new, taller soldier pile wall be installed on the east side of the channel to minimize disturbance and excavation of the existing soils and parking lot. After constructability review, the team also recommends revising the construction sequencing shown in the 75% design by others in order to reduce the risk for mobilization of sediment into the Duwamish River during channel excavation. Sediment mobilization will likely be a concern to the environment review agencies. The recommended revisions to the previous design and the addition of new stabilization elements are beyond the final design scope. At the request of the City, Otak also met with WSDOT in the field to review the project and a general permit will be required from the WSDOT Local Agency and Development Services group. They have requested to review all 65