Evan DuCharme/Special to The S.F. ExaminerWorkers tape off Stockton Street during work on the Central Subway

Following the lead of the first Central Subway tunnel-boring machine that drilled beneath Union Square late last year, the machine nicknamed Big Alma on Tuesday began making its way through the same undercrossing.

To build about 425 feet of new tunnel under the existing Muni and BART infrastructure, Big Alma will turn slightly left beneath Fourth Street, just south of Market Street, and veer right to go north under Market and Stockton streets.

When completed, the new subway tunnels about 10 feet below the existing BART tracks will allow trains on Muni’s T-Third Street line to travel rapidly beneath Stockton and Fourth streets.

“This is another key milestone that puts us a step closer to providing a major transit improvement that will cut travel times by more than half along this very dense and busy corridor,” said San Francisco Municipal Transportation Agency spokesman Paul Rose.

Big Alma’s crossing may take up to six days and cause BART to run at slower speeds in the vicinity.

Meanwhile, tunnel-boring machine Mom Chung is starting to make its way under Nob Hill toward Chinatown. Considering both tunnels, contractor Barnard Impregilo Healy has constructed more than 6,900 feet under Stockton and Fourth streets so far.

Similar to Mom Chung, Big Alma’s crossing includes about 150 monitoring devices installed at the Powell Street station and neighboring buildings to provide live data feeds on the tunneling conditions that the contractor, SFMTA, BART and an independent panel of tunneling experts can view at all times.Bay Area NewsBig AlmaCentral Subway ProjectTransittransportationUnion Square

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