Subsurface Characterization and Geotechnical Design for a Cut-and-Cover Tunnel in Soft Ground in San Francisco

摘要

The proposed Main Post Tunnels are part of the Doyle Drive Replacement Projectin San Francisco, California to improve the existing roadway access south of the GoldenGate Bridge. The tunnels will be a cut-and-cover structure of about 305 m in length and38 m in width involving 4-m deep excavation and 10- to 12-m backfill/embankment dueto aesthetic requirements. Various geotechnical investigations and testing were performedto characterize the subsurface conditions, including borings, cone penetration tests andlaboratory shear strength tests. The geotechnical investigations and analyses indicate thatthe tunnel subsurface consists of a total of 8 to 10 m thick saturated loose sands(including sand of hydraulic fill and marine sand) and soft clay which are subject to largedeformation due to consolidation and potential liquefaction. A number of groundimprovement methods were evaluated and cement deep soil mixing (CDSM) wasidentified as the preferred mitigation method. The CDSM panels will be constructed in agrid pattern and will have 30 to 40 percent of area replacement ratio.