Beginning Wednesday night, Mom Chung started working 24/7 instead of its usual 10-hour shift. The Central Subway tunnel-boring machine is making its way from South of Market, underneath the Powell BART and Muni stations, to Union Square.
The Mom Chung machine isn’t resting in order to get through the 425-foot section as quickly as possible and minimize potential delays for BART riders.
In the 2,100 feet of tunnel contractor Barnard Impregilo Healy has completed so far, the average was nearly 40 feet per day, but it has exceeded 100 feet on some days, said the project’s program director, John Funghi. Mining will be slower in this part due to the curves — left beneath Fourth Street, just south of Market Street, and right from underneath Old Navy to head north under Market and Stockton streets. About 150 monitoring devices have been installed in the Powell Street station and neighboring buildings to provide live data on tunneling conditions.
“We all have remote Web access to the instrumentation monitoring system so I can wake up in the middle of the night and look at how any instrument has moved in response to the tunneling operation,” said resident engineer Sarah Wilson.
The goal is to leave city infrastructure in the same state, and a panel of three independent tunnel experts has been providing oversight, Funghi said.
“Thus far everything has been great,” he said. “All this inspires our next major milestone, which is the BART crossing. We’ve been planning this for six years.”
In preparation of the crossing, the contractor injected a ground-stabilizing grout underground near the BART tunnels and will inject additional material as needed.
Crossing under BART, 10 feet above, may take up to six days. Starting Sunday, train speeds will be reduced from 36 to 25 miles per hour, which is a “relatively small reduction,” according to BART spokesman Jim Allison.
“We don’t expect to see any delays but we wanted to notify customers that there may be some,” he said.
The second tunnel-boring machine, Big Alma, is expected to cross under Market Street in January. When complete, the subway project will allow Muni’s T-Third Street line to cut travel times by more than half along the busy corridor.
Muni Transportation Director Ed Reiskin said Tuesday the project is “working as designed and happening better than anticipated as far as schedule.”