Central Subway

LOCATION:  San Francisco, California   |   OWNER:  San Francisco Municipal Transportation Agency
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The Central Subway Project is Phase 2 of the Third Street Light Rail Project, which will link the South of Market neighborhood with downtown San Francisco. The project cost is $1.6 billion and consists of 1.7 miles (2.7 km) of tunnel and three new underground stations for light rail service to Union Square and Chinatown. By providing a direct, rapid transit link between downtown and the existing T Third Line route on 3rd Street, the Central Subway Project will vastly improve transportation to and from some of the city’s busiest, most densely populated areas.

McMillen Jacobs Associates was awarded roles during both the preliminary design and construction phases, primarily focusing on tunneling and station-related work. The project has required tunneling and station excavation, by top down and sequential excavation methods, in both soil and rock through downtown San Francisco. Materials ranged from saturated alluvial soils to cemented sand and weathered to fresh bedrock. Major design and construction issues have included minimizing community disruption, managing tunneling risks, coordinating complex utility relocations, and actively managing extensive building protection measures.

Twin bore tunnels ($234M; Barnard Impregilo Healy) were excavated using earth pressure balance tunnel boring machines between a launch shaft supported with slurry walls under Interstate 80 and a retrieval shaft constructed by cutter soil mixing across the street from Washington Square Park in the North Beach neighborhood. Tunnel cross passages were built using ground freezing and conventional mining methods. The project required an extensive network of compensation grouting arrays to be installed and preconditioned beneath critical buildings and facilities along the tunnel alignment, including one location where the new tunnels passed directly beneath existing, in-service BART tunnels. The tunnels were completed in mid-2014.

Upon completion of the tunnels, construction began on three underground stations ($839M; Tutor Perini Corporation) at Yerba Buena/Moscone Center, Union Square/Market Street, and Chinatown. The stations are located in highly urban, congested areas, surrounded by high-rise, historic structures. Station construction is being achieved via 60- to 100-foot-deep open-cut methods using braced slurry, secant, and tangent pile wall construction as ground support, as well as 600 feet of cavern constructed using the sequential excavation method (SEM). An elaborate compensation grouting program was also necessary for the excavation of these stations, and extensive jet grouting was installed to limit permeability. Revenue service is expected to begin in the fall of 2021.

Related: Central Subway Moves Closer to Completion