GEOTECHNICAL CONDITIONS AND DESIGN CONSIDERATIONS FOR CENTRAL SUBWAY PROJECT IN DOWNTOWN SAN FRANCISCO

摘要

The San Francisco Central Subway is a $1.6 billion underground extension of thernThird Street Light Rail Project in the heart of the most densely populated areas of SanrnFrancisco. The project includes 1.7-mile-long, 20-ft diameter twin tunnels, five crossrnpassages, two portal/shaft structures and three deep underground stations. The complexrngeological conditions and urban setting strongly influenced the design of the tunnelsrnand associated structures and pose considerable challenges. Highlights of therndesign include cut-and-cover excavations up to 100-foot in depth; top-down excavationsrnof the TBM Launch box, portal, and stations in soft soils with high water tables;rnsequential excavations of a station in soft ground and weak rock; and TBM tunnelingrnwith mixed face conditions. Key design challenges include the use of deep diaphragmrnwalls and secant piles and extensive ground improvements. This paper briefly describesrnthe geotechnical conditions and challenges for the project, and the design solutions thatrnare being developed.