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First Central Subway Tunnel Boring Machine to Pass Beneath Existing BART and Muni Metro Tunnels, Extending T Third Line Tunnel Into Union Square

Wednesday, November 20, 2013

The San Francisco Municipal Transportation Agency (SFMTA), which manages transportation in the city, including the Municipal Railway (Muni), today announced that the Central Subway’s first tunnel boring machine (TBM) will soon pass beneath Powell Street Station and the existing Bay Area Rapid Transit (BART) and Muni Metro tunnels, crossing from SoMa into Union Square. The tunnel, one of two being constructed as part of the Central Subway Project, will allow T Third Line trains to travel quickly beneath 4th Street and Stockton Street when the Central Subway opens, cutting travel times by more than half along this busy corridor.

The SFMTA has worked in close coordination with BART and an independent panel of top tunneling experts to plan and carry out this key phase of tunnel construction. To pass beneath Market Street and the existing transit tunnels, TBM Mom Chung will turn slightly left beneath 4th Street just south of Market Street and travel partially under 801 Market Street, home to Old Navy. The machine will then veer right to head north under Market Street and then Stockton Street. The new T Third Line tunnels will be about 10 feet below the existing BART tunnels.

Starting today, Mom Chung will be in operation 24 hours a day to build the approximately 425 feet of new tunnel beneath Old Navy and the Market Street tunnels. Mom Chung is expected to begin crossing beneath the Market Street tunnels on Monday, Nov. 25. The tunneling methods used during this key crossing will be the same that are in use along the rest of the tunneling route. As the TBM moves forward, it installs tunnel segments within the section known as the “shield,” a 20-foot-diameter cylinder at the front of the machine. The shield and the newly installed tunnel lining create a watertight barrier that holds the ground outside in place. Using these methods, tunneling contractor Barnard Impregilo Healy (BIH) has constructed more than 2,100 feet of tunnel under 4th Street so far.

“Today we are another big step closer to building the 21st century transportation system our world-class city needs and deserves,” said Mayor Edwin M. Lee. “The Central Subway and the tunnels we are excavating today are essential to our vision, and crucial to expanding our reliable, modern public transportation system for San Francisco residents and visitors.”

“The new subway tunnels 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.”

In preparation for the crossing, the contractor has injected a ground-stabilizing grout underground near the BART tunnels, accessing this subterranean area via a deep shaft they constructed on Ellis Street. They will inject additional grout as needed while Mom Chung is mining below BART.

About 150 monitoring devices installed in the Powell Street Station and on neighboring buildings will provide live data feeds about tunneling conditions to web applications that the SFMTA, the contractor, BART and an independent panel of tunneling experts can view at all times. Similar devices are installed along the entire tunneling path, from 4th and Bryant in SoMa to Columbus and Powell in North Beach. The readings of these instruments and others on the TBM itself allow the tunneling contractor to respond rapidly to ground conditions around the machine as it moves forward.

“The Central Subway will bring together neighborhoods of our city that have long been in need of improved public transit,” said Tom Nolan, chairman of the SFMTA Board of Directors. “With the Central Subway, the T Third Line will provide a rapid north-to-south connection into the heart of the city, from our eastern neighborhoods to SoMa, Union Square and Chinatown.”

“We’re thrilled at the progress our tunneling contractor and tunnel boring machines are making to build this essential new subway line for San Francisco,” said SFMTA Director of Transportation Edward D. Reiskin. “We are using state-of-the-art technology and the country’s top expertise to ensure the entire tunneling process safeguards the city as well as the region’s critically important transportation infrastructure. We thank BART for the excellent partnership in this process.”

“SFMTA has worked closely with BART as it proceeds with this landmark project,” said BART General Manager Grace Crunican. “We greatly appreciate Muni’s willingness to provide expertise and information, including real time data, and their commitment to minimizing any potential impact to our riders.”

Tunneling may cause BART to run at slower speeds in the area. You can get BART Service Advisories (BSA) on your phone. To sign up for service alerts, please visit BART on the web at or call 511 to get up-to-date service information.

The crossing under BART may last up to six days, with the machine moving forward more slowly than usual due to the tunnel’s curves. The second TBM, Big Alma, is expected to cross under Market Street in January 2014.

For more information about tunneling and the Central Subway Project, visit