AgdaPkt 2002-05-20 - Laserfiche WebLink

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7, ~-El REPORT To the Honorable Mayor and City Council From the City Manager May 20, 2002 Subject Task Order No. 1 - CCS Planning and Engineering, Inc. Recommendation Approve Task Order No. 1 with CCS Planning and Engineering, Inc. for additional Consulting Engineering Design Services for the Redwood Shores Levee Segment III Improvements Project, in an amount not to exceed $31,747. Background On July 9, 2001 (Resolution #14267), the City Council approved an agreement with the CCS Planning and Engineering, Inc. for Consulting Engineering Design Services for the design of the Redwood Shores Exterior Levee Segment Ill Improvements Project, which is located within the South Bayside System Authority (SBSA) facility. The initial scope of work was derived from the Corps of Engineers and Bay Conservation and Development Commission (BCDC) permits that seek to minimize the potential harm to the environment and the endangered species. During the permit stage for the overall levee project, it was proposed that this segment of the levee be built inside of the SBSA facility in order to minimize complications, as a full sized, exterior replacement levee would involve encroachment of the Bay. At that time it was thought that a simple wall could be built around the facility, and the existing levee could be left without further maintenance. This would also take construction activities away from the edge of the Bay. After CCS and their sub-consultant, Bay Area Geotechnical Group (BAGG), got into the design of the project, we quickly realized that the 'simple wall' would in fact be quite massive. The size of the wall is proportionate to the height, which has to be higher than the flood elevation, assuming that the exterior levee is not them. In some areas, this wall has to be 11 feet high. The second challenge for this design is the fact that soil at the project site is composed of soft Bay Mud. This Bay Mud is not capable of supporting much weight, much less the massive wall, and it so happens that the site is underlain by some 70 feet of mud, the deepest in the entire Redwood Shores Area. Using piles to support the wall is one option, but it is not economically feasible. The consultant also looked into replacing the mud with engineered material (enough to support the wall), but that was still very costly. Finally, interconnecting sheet piles were looked at, and because of the soft Bay Mud not being able to effectively counterbalance the force of flood water, it was also not feasible.