First Central Subway tunnel boring machine to pass under existing BART, Muni Metro tunnels next week
In the operating cabin of TBM Mom Chung, two TBM operators monitor tunneling conditions provided from a variety of state-of-the-art instruments.
Next week our first tunnel boring machine, Mom Chung, will pass beneath Market Street and the existing BART and Muni Metro tunnels, crossing from SoMa and 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.
This graphic shows the path the Central Subway TBMs will take when they pass under Market Street. The first TBM, Mom Chung, is expected to begin this crossing on Monday.
Starting yesterday, Mom Chung is 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.
“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.”
With the help of TBM Mom Chung, Central Subway crews have now built a new subway tunnel under 4th Street from Harrison and Stevenson.
Underground, two Central Subway tunnels are now under construction. The tunnel on the left will pass beneath BART and Market Street in the coming days.
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.
BART may run at lower speeds between Powell and Montgomery stations while tunneling is in progress under BART. Please visit BART on the web at www.bart.gov/alerts or call 511 to get up-to-date service information.
For more about this major tunneling milestone, check out this press release from the SFMTA.
