Mayor Lee Celebrates Start of Central Subway Tunneling

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Mayor Edwin M. Lee today celebrated the start of tunneling for the Central Subway Project, the major extension of the Muni Metro T Third Line through SoMa, Union Square and Chinatown. A 350-foot-long tunnel boring machine (TBM), now nearly assembled under 4th Street, will soon begin tunneling its way north to construct one of the two tunnels included in the Central Subway Project.

“Among the most critical challenges we face in our world-class City is to plan, build and maintain a reliable public transportation system. Today we are moving one step closer to meeting those demands and providing the 21st century transportation system that our world-class City needs and deserves,” said Mayor Lee. “As we start tunneling construction on the Central Subway, I thank our federal, state and local partners for their continued commitment to improving public transit in San Francisco.”

“The next phase of the Central Subway means our city is one step closer to more jobs for workers, less congestion for commuters, greater safety in our public transit system, and reduced pollution in our air,” said Democratic Leader Nancy Pelosi. “From start to finish, this project demonstrates once again how San Francisco is leading the way – investing in our infrastructure, protecting our environment, bringing our city closer together, promoting commerce and strengthening our economy.”

The TBM, named Mom Chung after Dr. Margaret “Mom” Chung, the country’s first female Chinese-American physician and a surrogate mother to thousands of veterans in World War II, is being assembled within an excavation under 4th Street in SoMa. It will launch in June, traveling beneath 4th Street, Stockton Street and Columbus Avenue to excavate and construct San Francisco’s first new subway tunnel in more than 30 years.

An identical TBM, named Big Alma after 19th century San Francisco philanthropist and socialite “Big Alma” de Bretteville Spreckels, will arrive this summer and launch soon after, constructing a tunnel parallel to Mom Chung’s. The state-of-the-art machines, fabricated specially for the Central Subway Project, weigh approximately 750 tons. It will take them approximately 10 months to construct each 1.5-mile-long tunnel.

“The tunnels that Mom Chung and Big Alma will build will connect major job, retail and cultural centers to rapid transit and speed up transportation through two of the City’s most congested corridors,” said Board of Supervisors President David Chiu. “With the Central Subway, we will finally realize the decades-long vision of bringing fast, efficient transit to the 4th and Stockton corridors.”

“As we build the Central Subway tunnels, we can look forward to significant improvements to our public transit network and to the quality of life of the thousands of people who travel the crowded Stockton and 4th Street corridors every day,” said Tom Nolan, Chairman of the SFMTA Board of Directors.

The Central Subway will significantly decrease travel times along 4th Street and Stockton Street, two of San Francisco’s most congested corridors, while accommodating job and population growth anticipated in SoMa, downtown, Chinatown and along the existing T Third Line in eastern and southeastern San Francisco. Three subway stations will be built along the route the TBMs will follow:

  • The Chinatown Station, a subway station with an entrance at Stockton and Washington streets;
  • The Union Square/Market Street Station, a subway station with entrances at Stockton and Geary streets and Stockton and Ellis streets;
  • The Yerba Buena/Moscone Station, a subway station with an entrance at 4th and Clementina streets.

In addition, the project includes a surface-level station at 4th and Brannan streets.

Each TBM consists of a rotating cutter wheel (the cutter head), a cylindrical steel shell (the shield) and a 300-foot train of tunnel-building mechanisms (the trailing gear). The TBMs arrive in several parts and are assembled in about six weeks underneath 4th Street between Harrison and Bryant streets, the site where tunneling will begin. The tunnels will end in North Beach, at the site of the former Pagoda Palace Theatre on Powell Street.

The Central Subway is expected to open to the public in 2019. For more information, visit

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Mayor Lee Celebrates Start of Central Subway Tunneling (PDF)