San Francisco Central Subway

As of January 2023, riders of the San Francisco Municipal Transportation Agency’s (SFMTA) Central Subway now have a rapid, underground rail alternative to the often-crowded buses and congested city streets. The Central Subway is a 1.7-mile extension of the Third Street light rail line, known as the T-Third. T-Third Phase I opened in 2007 with 5.1 miles of new surface track and 18 platforms through the Mission Bay and Bayshore neighborhoods ending in Visitacion Valley at the southern boundary of the city.

The Central Subway project includes one surface station and three underground stations. Twin, 1.6-mile-long bored tunnels were completed in April 2015 in advance of the three underground stations using two, 20.6-foot diameter tunnel boring machines. It is San Francisco’s first major subway with underground stations since the Market Street subway and stations were completed in the 1970s.

Four stations serve the Central Subway:

• Fourth and Brannan Station: At-grade station with center-boarding platform along Fourth Street on the north side of Brannan Street near the Caltrain Station.
• Yerba Buena/Moscone Station: Located in the South of Market neighborhood, the off-street portion is designed to support future transit-oriented development.
• Union Square/Market Street Station: Located in Union Square, with the city’s highest concentration of jobs and many hotels, restaurants and retailers, the station platform had to be constructed to a depth of 90 feet for the new tunnels to pass below the existing BART and Muni tunnels along Market Street. Top-down, cut-and-cover construction methods using inclined and vertical secant and tangent pile walls for temporary and permanent ground support were necessary to accommodate the desired center-platform layout within the narrow, 65.5-foot-wide public right-of-way.
• Chinatown-Rose Pak Station: Located in Chinatown, an off-street headhouse extending to a depth of 115 feet below ground provided access for construction of the mined platform and crossover caverns under Stockton Street and houses traction power and emergency ventilation facilities among its other station functions. The 60 foot-span platform and crossover caverns along with the connecting cavern to the headhouse were constructed in soft-ground and weak rock using mined, sequential excavation methods. The top of the headhouse features a public plaza for the Chinatown community.

The twin tunnels were driven 2000 feet beyond the Chinatown station to an alternate retrieval shaft location near Columbus Avenue in the North Beach neighborhood. The tunnels lie ready to be fitted with rail and services and incorporated into a further extension to the north waterfront when San Francisco eventually embarks on Central Subway Phase III.

The logistical challenges posed by working in the dense urban setting included boring the Central Subway tunnels immediately below the BART and Muni tunnels along Market Street. This required extensive three-dimensional soil-structure interaction modelling to assess potential settlements and assure BART and SFMTA of its technical feasibility.

Other challenges included:

• The maintenance of vehicular traffic and pedestrian movement, and truck access to the Moscone Center conference facilities during construction.
• Relocation and maintenance of utilities into joint utility trenches under each sidewalk, including design and construction of building closure walls where needed.
• Instrumentation, monitoring and mitigation of settlement effects on adjacent utilities, BART tunnels, and buildings.

Creating rapid north-south rail transit will bring quicker rides and safer, more efficient transit service from the core of the city to its fast-growing southeast neighborhoods. The Central Subway will reduce to seven 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 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.

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

• Evaluation of tunnel construction methods
• Relocation of underground utilities
• Preliminary design of the stations
• Environmental studies and report preparation, which led to the issuance of a Record of Decision from the Federal Transit Administration

Services included:

• Preliminary engineering and final design
• Environmental assessments
• Public outreach support
• 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

For the final design phase, WSP lead the joint venture and team of 14 subconsultants responsible for the final design of two advance utility relocation contract packages and for the tunnels and portal construction package, which was completed in 2015.

The Central Subway Design Group joint venture, with WSP in the lead, was responsible for the final design and design support during construction for the three underground stations - Yerba Buena/Moscone, Union Square/Market Street and Chinatown-Rose Pak completed in 2023.