Second Central Subway TBM to pass under Market Street, BART this week
Tunnel boring machine Big Alma, shown here, will cross under BART and Market Street this week.
This week the Central Subway’s second tunnel boring machine (TBM) will pass beneath Market Street and the existing Bay Area Rapid Transit (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, cutting transit 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 Big Alma will turn slightly left beneath 4th Street just south of Market Street. 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 second TBM, Big Alma, is making this crossing this week.
Starting today, Big Alma will be in operation 24 hours a day to build the approximately 425 feet of new tunnel beneath the Market Street tunnels. Late last year the first TBM, called Mom Chung, safely and successfully completed the same undercrossing, using the same techniques and interagency coordination. Mom Chung is now beginning to tunnel under Nob Hill, heading toward Chinatown. Including both tunnels, tunneling contractor Barnard Impregilo Healy (BIH) has constructed more than 6,900 feet of tunnel under 4th Street and Stockton Street so far.
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. 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 TBMs themselves allow the tunneling contractor to respond rapidly to ground conditions around the machines as they move forward.
The crossing under BART may last up to six days. Tunneling may cause BART to run at slower speeds in the area.
More information is available in this press release from the SFMTA.
