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Central Subway Project

Engineering and Technical Opportunities

The Central Subway Project is Phase 2 of the Third Street Light Rail Project, which will link the South of Market neighbourhood with downtown San Francisco. The project cost is $1.6 billion and consists of 1.7 miles (2.7 km) of the 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.

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 neighbourhood. Tunnel cross passages were built using ground freezing and conventional mining methods. In addition, 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 completing 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 20-30m deep open-cut methods using braced slurry, secant, and tangent pile wall construction as ground support and 190m of cavern constructed using the sequential excavation method (SEM). An elaborate compensation grouting program was also necessary for excavating these stations, and extensive jet grouting was installed to limit permeability. Revenue service is expected to begin in the fall of 2021.

Project

Central Subway Project

Client

SFMTA / JACOBS

Location

San Francisco, USA

Time Frame

2011-2014

Budget

$1.57 Billion

Project Highlights

Approach and Innovation

JDK's Principal Engineer aided in the tunnel design review and instrumentation monitoring for the rapid transport link between SOMA, Union Square and Chinatown districts in San Francisco.

Equipment supplied and installed include:

  • Piezometers
  • Inclinometers (manual and automated)
  • Extensometers (magnetic, rod & wire)
  • Water Level monitoring
  • Settlement profile systems
  • Seepage monitoring
  • Pendulum Systems
  • Automatic-Alarm messaging technology 

Value Added to Project

JDK effectively engaged with multiple contractors to safely and effectively deliver the geotechnical reporting and instrumentation requirements of the project. JDK’s professional holistic approach with relevant stakeholders and extensive geotechnical expertise proved highly successful and addressed the needs and requirements of all project stakeholders. Precise inferences of ground conditions allowed the client to adhere to their deliverable schedule and utilise their resources efficiently.

JDK's experience in monitoring other similar projects helped the client deliver industry-best technology to safely mitigate the risk parameters of the project. Lessons learned from the Central Subway Project have allowed JDK Geotech to enhance solutions to the tunnelling industry.