Design approach for the hybrid underground station at Union Square/Market Street in San Francisco / Entwurfskonzept für eine hybride U-Bahnstation Union Square/Market Street in San Francisco

The new Central Subway extension through downtown San Francisco consists of three underground stations and 2.7 km TBM-driven twin tunnel. This paper provides a description of the preliminary analyses and design of the ground support and final lining for the Union Square/Market Street Station (UMS) along Stockton Street. This station will serve the Union Square Shopping District and connect to the BART Powell Street Station. Due to shortage of space above ground and to minimize surface disruption, the UMS station design requires a complex hybrid method consisting of a 20 m deep braced cut-and-cover box with a mined enlargement bulb below it with a height of 9.3 m and a width of 17.8 m.

The majority of the UMS station will be excavated in saturated alluvial deposits. Undifferentiated old bay deposits will be encountered in the invert, underlain by dense marine sands. The groundwater varies from 5 to 10 m below ground level, so uplift of the combined bulb/box structure has to be taken into account.

The Finite Element (FE) analysis of the UMS station cavern reflects the separate construction phases of the station platform box and the bulb to account for soil-structure interaction and load-sharing effects. FE analyses are used to estimate support requirements including ground improvement and to predict surface settlements.