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

The proposed Main Post Tunnels are part of the Doyle Drive Replacement Project in San Francisco, California to improve the existing roadway access south of the Golden Gate Bridge. The tunnels will be a cut-and-cover structure of about 305 m in length and 38 m in width involving 4-m deep excavation and 10- to 12-m backfill/embankment due to aesthetic requirements. Various geotechnical investigations and testing were performed to characterize the subsurface conditions, including borings, cone penetration tests and laboratory shear strength tests. The geotechnical investigations and analyses indicate that the 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 large deformation due to consolidation and potential liquefaction. A number of ground improvement methods were evaluated and cement deep soil mixing (CDSM) was identified as the preferred mitigation method. The CDSM panels will be constructed in a grid pattern and will have 30 to 40 percent of area replacement ratio.