The San Francisco Municipal Transportation Agency (SFMTA) announced the Central Subway’s second tunnel boring machine (TBM) will soon pass beneath existing Bay Area Rapid Transit District (BART) and Muni Metro tunnels, while passing under Market Street from SoMa into Union Square.

When the Central Subway opens, the new tunnels will allow T Third Line trains to cut 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.

Beginning Tuesday, Big Alma was set to 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.

The crossing under BART may last up to six days, with the tunneling also possibly causing BART to run at slower speeds in the area.