The Union Square-Market Street stop will be located in one of three underground stations to be constructed for the extension of San Francisco’s T-THIRD light-rail line. The extension will pass under some of San Francisco’s most congested and populated streets. Courtesy of the San Francisco Municipal Transportation Agency
It’s been more than 10 years since the Federal Transit Administration approved the preliminary engineering for San Francisco’s Central Subway project. Tunnel preconstruction started last March, and a nearly billion-dollar grant last fall paved the way for construction to ramp up this spring.
April 9, 2013—In a few short years, San Francisco’s $1.6-billion Central Subway project will reestablish north–south electric light-rail service along the city’s busy Fourth Street corridor after a gap of more than 50 years.
A project of the San Francisco Municipal Transportation Agency (SFMTA) and the City and County of San Francisco, the Central Subway is the second phase of the SFMTA’s Third Street Light Rail Project. Phase 1, a 5.1 mi light-rail surface line called T-THIRD, which runs along the Third Street corridor, opened in April 2007.
Parsons Brinckerhoff’s San Francisco and Cleveland offices, along with San Francisco–based PGH Wong Engineering, Inc., provided preliminary engineering and design for the Central Subway project. Final design of the tunnels was carried out by PB Telamon, a joint venture of the San Francisco office of Parsons Brinckerhoff and San Francisco–based Telamon Engineering Consultants, Inc. The Central Subway Design Group, a joint venture of the San Francisco office of Parsons Brinckerhoff and two local architecture firms—MWA Architects and Kwan Henmi Architecture/Planning, Inc.—is responsible for the final design of the underground stations.
When completed, in 2019, the Central Subway will extend the Third Street line from the Caltrain station at Fourth and King streets to Chinatown, an area that is one of the most densely populated locations in the United States. The project includes construction of three new underground stations—one in Chinatown, one at Union Square and Market Street, and one at Yerba Buena Gardens and the Moscone Center—as well as a new surface-level station at Fourth and Brannan streets.
The new subway will pass under some of San Francisco’s most congested and populated streets, dense commercial areas, and high-end shopping locations. To minimize disruption to traffic and commerce, the project will take advantage of the latest engineering and construction technology for the two 8,300 ft concrete-lined tunnels and the three underground stations.
Construction of the tunnels will begin in earnest later this month with the arrival of two custom-built, state-of-the-art tunnel-boring machines (TBMs). As the TBMs bore through the ground 40 to 120 ft below the surface, the only significant aboveground disturbance will be at the entry and exit portals for the equipment.
The same technology has been used in New York City, Los Angeles, and Seattle, as well as in many European cities, according to the Central Subway’s program director, John Funghi, P.E. “The TBM is an earth pressure balance machine with a pressurized head to hold back the water and soil at the tunnel face,” he explains.
Each TBM consists of a rotating cutterhead and 300 ft long trailing gear. The cutterhead presses against the tunnel face, and the material it excavates is transferred to a conveyor inside the trailing gear. While the steel shield retains the water and soil, 5 ft wide precast-concrete tunnel segments will be installed. “Recent technological advances have made concrete precast segmental liner the state of the art for tunnel construction,” Funghi notes.
When a section of tunnel is completed, the TBM’s hydraulic jacks will press against the concrete lining and propel the machine forward. “The challenge is to ensure only the minimal amount of soil needed is being removed,” Funghi says.
Although using a TBM minimizes settlement, it is still a concern, and precautions will be taken in high-risk areas along the tunnels’ length. Crews will place pipes under existing buildings and under Market Street where the Central Subway will travel beneath existing SFMTA and Bay Area Rapid Transit (BART) tunnels so that compensation grouting can be carried out after the tunnel liner has been installed.
Work will progress at the rate of 40 ft of excavation and liner installation per day, and excavation will continue through 2014. In June 2011 the $233-million tunneling contract was awarded to Barnard Impregilo Healy Joint Venture, comprising Barnard Construction Company, Inc., of Bozeman, Montana; the Italian firm Impregilo S.p.A.; and S.A. Healy Company, of Lombard, Illinois.
The SFMTA will award the station construction contract within weeks, and simultaneous construction of all stations will begin by the middle of this year. The Chinatown station, situated in a densely populated area, will be constructed using the sequential excavation method (SEM) to avoid interruptions to local business and tourism. “We have a narrow right-of-way and businesses out onto the sidewalk in this area, so SEM is an environmental mitigation here,” Funghi says.
Excavation for the Chinatown station will begin with an 80 ft deep shaft and then proceed horizontally under the street using a series of arches to support the face of the excavation, he says. “A pipe pile canopy is constructed, and excavation takes place under the canopy, which allows the soil and water to be held back, Funghi says. “Excavation of the heading occurs in a predetermined sequence.”
As each section is exposed, the surface will be reinforced with shotcrete and other supporting elements, and each section will be tied in to the previous section until the excavation is completed and stabilized. Waterproofing will be installed over this first layer, and the waterproofing will be covered by a final layer of cast-in-place concrete.
The other two subterranean stations will be constructed with conventional cut-and-cover techniques from the top down using a box frame structure in an excavated trench.
The track for the trains will be installed aboveground and belowground beginning in 2016, and this process should be completed in 2017. The Central Subway will be ready for passengers early in 2019.
Funding for the project includes $942.2 million from the Federal Transit Administration’s New Starts program and $41 million from the Federal Highway Administration’s Congestion Mitigation and Air Quality Improvement Program. The balance of the funds will come from the State of California, the Metropolitan Transportation Commission, the San Francisco County Transportation Authority, and the City and County of San Francisco.