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Boring Machines Emerge From Central Subway Tunnel

The sun was scorching at the site of the old Pagoda Palace Theater in San Francisco’s North Beach last Friday morning, but the Central Subway crews were blissfully protected. They were toiling away yards below the surface, at the damp, chilly foot of two recently constructed tunnels.

The tunnels will host the extension of the Muni Metro T Third Line from SOMA to Union Square and Chinatown when the Central Subway goes live in 2019. 

When it begins carrying passengers, the Central Subway will cut travel time in more than half along the busy San Francisco corridor. The 1.5 mile-long tunnels run under the notoriously congested Fourth and Stockton Streets, and the stations are situated in some of the city’s most vibrant neighborhoods and busiest commercial districts.

Workers are now swiftly exctracting and dismantling the tunnel boring machines (TBMs) that were custom-constructed for the Central Subway Project. The mud-caked TBMs – called “Mom Chung” and “Big Alma” – completed their nearly year-long journeys underneath San Francisco’s streets last month.

The identical 750-ton machines were christened in honor of the country’s first Chinese-American female physician and a 19th century San Francisco socialite, respectively. Each machine consists of a rotating cutter wheel, a steel shield and 300 feet of tunnel-building mechanisms that trail behind it. Mom Chung was launched in July 2013 to carve out the southbound tunnel, and Big Alma followed her that November to create the northbound counterpart. 

“You want to learn how the machine’s going to behave,” said project engineer Mun Wei Leong, as he showed a contingent of reporters and photographers around the busy construction site. “We took all those lessons [from Mom Chung] and applied it to the northbound machine.”

The machines were manufactured by the Robbins Company, which assembled them together with Barnard Impregilo Healey Joint Venture. Two crews of seven workers operated them. 

At 40 feet per day, the TBMs moved twice as fast as predicted. In a few months, the machines will be fully dismantled and ready to be sold back to Robbins for a future project. Remaining work on the tunneling front includes removal of utilities — fans, gas — from the 47-foot diameter tunnel shafts. The next construction phase will see tracks laid. 

“This part of the job has gone remarkably well,” said Matthew Fowler, project manager for design. “We got under BART without moving it a quarter inch, if that. A lot of the credit goes to the machine and how they operated it.” At 40-120 feet under the earth, the TBMs caused no noise or vibration above ground.

Next up is station construction. Work has already begun — and in some cases is visible — on the three subway stations in Chinatown, Union Square and by the Moscone Center, and on the surface-level station at Fourth and Brannan Streets. 

MTC is a sponsor of the $1.578 billion Central Subway Project, along with the San Francisco Municipal Transportation Agency, the Federal Transit Administration, the state of California and the City and County of San Francisco. MTC provided $100 million of state population-based transit funds to the project. 

 Natalie Orenstein

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