• LinkedIn
  • Twitter
  • Facebook
  • Youtube
  • Google+
  • Instagram
 
Global  
 

SFMTA Central Subway

Providing light rail transit to the most densely populated area in the US without a modern rail transit system

By late 2019, riders of the San Francisco Municipal Transportation Agency’s (SFMTA) Central Subway will have a rapid, emission-free alternative to the crowded buses and congested city streets. The Central Subway is a 2.7-kilometer (1.7-mile) extension of the Third Street light rail line, known as the T-Third. The Central Subway project includes one surface station between Brannan and Bryant Streets and three underground stations: Moscone, Union Square/Market Street and Chinatown. Two tunnels are being constructed using 20.6-ft diameter tunnel boring machines (TBMs). At Market Street, the tunnels cross under BART with less than 12 feet of clearance.

Disassembly of tunnel boring machine "Mom Chung"

Four stations will serve the line:

  • Fourth & Brannan Station (FBS): At-grade station with center-boarding platform along Fourth Street on north side of Brannan Street.
  •  

  • Yerba Buena/Moscone Station (YBM): Located at the northwest corner of Fourth and Folsom streets in the SoMa neighborhood, the off-street portion is designed to support future transit-oriented development.
  •  

  • Union Square/Market Street Station (UMS): Located in Union Square, with the city’s highest concentration of jobs and many hotels, restaurants and retailers, the station extends from Ellis Street to Geary Street and will be constructed to a depth of over 100 feet, using the top-down cut-and-cover method with inclined and vertical secant and tangent pile walls, large-diameter pipe struts, as well as steel and concrete floor framing.
  •  

  • Chinatown Station (CTS): Located at the southwest corner of Stockton and Washington streets, the station headhouse will extend over 100 feet below ground. The platform and connection to the headhouse are constructed in soft- and mixed-ground using mined sequential excavation method. The top of the headhouse will be capable of supporting future TOD or public park/open space.

 

CLIENT OBJECTIVE

The extension runs along Fourth and Stockton Streets from the existing Third Street light rail platform at Fourth and King Streets to a new terminal in Chinatown. It transitions to subway operations at a portal under the I-80 Freeway, and continues along Fourth Street in a twin-tunnel configuration following a deep profile crossing under four Market Street subway tunnels used by Muni Metro and the Bay Area Rapid Transit (BART) system.

A particular engineering challenge, in addition to the many logistical challenges posed by the dense urban setting, was the Central Subway tunnels crossing immediately below the BART tunnels. This required extensive three-dimensional soil-structure interaction modelling to assess potential settlements and assure BART and SFMTA of its technical feasibility.

Other challenges include:

  • The maintenance of vehicular traffic and pedestrian movement, and access to and operations of adjacent facilities during construction;
  •  

  • Relocation and maintenance of utilities during construction, including design and construction of building closure walls where needed;
  •  

  • Instrumentation, monitoring and mitigation of potential settlement impacts on adjacent utilities, BART, and buildings;
  •  

  • Construction of entrances and connections at various high traffic locations without interruption of service.
  •  

APPROACH

During the Central Subway’s preliminary engineering phase, we led the joint venture that provided:

  • Evaluation of various tunnel construction methods;
  •  

  • Relocation of underground utilities;
  •  

  • Preliminary design of the four stations;
  •  

  • and tie-in to the surface line at Fourth and King Streets.
  •  

The joint venture provided engineering, cost estimating and environmental studies, which led to the issuance of a Record of Decision (ROD) from the Federal Transit Administration (FTA). Also key to the project’s success was a strong public involvement program.

Leading the PB Telamon joint venture team, WSP | Parsons Brinckerhoff carried out the final design stage and provided design support during construction for the twin tunnels, the tunnel-boring machine (TBM) launch and retrieval shafts, five cross passages, ground improvement at station interfaces, settlement instrumentation, portal construction, and mitigation measures for controlling tunnelling-related settlement. Tunnel construction was completed in 2015 on schedule and under budget.

The Central Subway Design Group joint venture, again with WSP | Parsons Brinckerhoff in the lead, is currently responsible for the final design and design support for the three underground stations now under construction at Yerba Buena/Moscone, Union Square/Market Street and Chinatown.

Portals in TBM launch box beneath 4th Street

OUTCOME

Creating rapid north-south rail transit is not the only benefit of the Central Subway, which is scheduled for December 2019. In addition to providing light rail transit to Chinatown – the most densely populated area in the nation without a modern rail transit system – the Central Subway will bring quicker rides and safer, more efficient transit service from the core of the city to its fast-growing southeast neighborhoods.

The subway will reduce to 7 minutes what is now a 20-minute bus trip from Muni's station at Fourth and King Streets to Chinatown. Its two-car trains will be able to carry up to 250 passengers, as compared to the 60 or so carried on a 40-foot (12-meter) bus.

The Central Subway is also key to the economic revitalization of Chinatown, which was weakened economically after the 1989 Loma Prieta earthquake closed the nearby Embarcadero Freeway. Upon opening, the projected ridership along the entire T Third Line will be 43,700 daily boarding, and by 2030, it is projected to reach 65,000 customers per day.

SERVICES PROVIDED

  • Preliminary engineering and Final Design
  • Environmental Assessments
  • Alignment Evaluations
  • Tunnel and Station Construction Method Studies
  • Portal and Station location evaluations
  • Construction cost estimating and scheduling
  • Tunnel lining design
  • Soil Structure Interaction Analysis
  • Cut-and-cover station design
  • Seismic design
  • Safety and security
  • Fire/ Life safety and ventilation