Central Subway

LOCATION:  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 (2.7km) 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 requires tunneling and station excavation in both soil and rock through downtown San Francisco, with materials ranging from saturated alluvial soils to cemented sand and weathered to fresh bedrock. Major design and construction issues include minimizing community disruption, managing tunneling risks, complex utility relocations, and extensive building protection measures.

Twin bore tunnels ($234M; Barnard Impregilo Healy) were excavated using earth pressure balance tunnel boring machines between a launch shaftsupported with slurry walls under Interstate 80 and a retrieval shaft constructed by cutter soil mix across the street from Washington Square Parkin 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. The tunnels were completed in mid-2014.

Upon completion of the tunnels, three underground stations ($839M; Tutor Perini Corporation) began construction 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 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 be on line in 2019.