Central Subway Moves Closer to Completion

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Project Update by Hilary Schaadt

Civil construction on the Central Subway Project is nearing completion, with revenue service scheduled to go online in mid-2020. Central Subway is Phase 2 of the San Francisco Municipal Transit Authority’s (SFMTA) Third Street Light Rail Project, which will link the South of Market neighborhood with downtown San Francisco (Financial District and Chinatown). The estimated total cost for Central Subway is $1.6 billion. The project scope consists of 1.7 miles (2.7 km) of tunnel trackwork and three new underground stations for light rail service: Yerba Buena/Moscone, Union Square/Market Street, 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.

Crossover track installation in progress . ©Robert J. Pierce/SFMTA. Used by permission.

As subcontractor to the AECOM/EPC joint venture, McMillen Jacobs has provided both preliminary design and construction management, primarily focusing on tunneling and station-related work. The designer is WSP. 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 rock. Major design and construction issues include minimizing community disruption, managing tunneling risks, complex utility relocations, and extensive building protection measures.

Rail installation at southbound tunnel. ©Robert J. Pierce/SFMTA. Used by permission.

Upon completion of the tunnel contract by Barnard/Impregilo Healy, work began in June 2013 on the three underground stations, surface station, and tunnel trackwork with Tutor Perini. Station construction was achieved via 60- to 100-foot-deep (18.3–30.5 m) open-cut methods using braced slurry, secant, and tangent pile wall construction as ground support, as well as 600 feet (183 m) of cavern constructed using the sequential excavation method (SEM). A comprehensive compensation grouting program was also installed as mitigation for settlement for station excavations, and extensive jet grouting was installed to limit permeability. Final paving on Stockton Street was completed prior to its closure for the Holiday Moratorium. Before going online in 2020, many activities will need to be finished, including track installation and station finishes.