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 the Third Street Light Rail Project in the heart of the most densely populated areas of San Francisco. The project includes 1.7-mile-long, 20-ft diameter twin tunnels, five cross passages, two portal/shaft structures and three deep underground stations. The complex geological conditions and urban setting strongly influenced the design of the tunnels and associated structures and pose considerable challenges. Highlights of the design include cut-and-cover excavations up to 100-foot in depth; top-down excavations of the TBM Launch box, portal, and stations in soft soils with high water tables; sequential excavations of a station in soft ground and weak rock; and TBM tunneling with mixed face conditions. Key design challenges include the use of deep diaphragm walls and secant piles and extensive ground improvements. This paper briefly describes the geotechnical conditions and challenges for the project, and the design solutions that are being developed.