San Francisco News

Excavation underway at Transbay Transit Center

Wednesday, July 25, 2012

In downtown San Francisco the massive Transbay Transit Center construction project is well underway. It will be a couple years before the above ground structures start to take shape. For now, most of the work is below street level in the area roughly bordered by Mission St., Beale St., Howard St., and 2nd St.

Officials say the $4.2 billion transit center is currently on schedule and on budget. When finished, it's expected to accommodate 100,000 rail and bus passengers a day. But, for now, it's really just a big dig.

At the western end of the project excavation has reached about 35 feet down with another 20 to go, with pipes in place temporarily for support. Bob Beck with the Transbay Joint Powers Authority says excavation this deep, at a site this size, is a slow process, "About half of our overall construction schedule is excavating down to the bottom of the rail level and bringing that structure back up to grade."

High speed rail and Caltrain will someday enter at 55 feet below street level through a tunnel at the western end. Three streets will soon have to become temporary bridges to accommodate excavation. First Street is completed. This fall, Fremont Street will become a bridge, followed later by Beale Street.

"The high speed rail trains are 400 meters long," Beck said. "So when they pull into the station they're going to extend from Main Street all the way down here west of First Street. And those rail levels will pass underneath First, Fremont, and Beale streets."

At the eastern end, Beck says buttresses are being built and seven foot diameter casings are placed 250 feet down then filled with concrete to help stabilize the silty soil. The eastern end was once covered with water and called Yerba Buena Cove, "We're drilling down to bedrock, excavating out the existing soil and replacing it with concrete. And that's just to improve soil quality here in this area so it resists heave as we excavate the site."

Phase one of the project, which will accommodate buses, is expected to finish by 2017. Then, funding willing -- about $2 billion worth of it still uncertain -- Caltrain and high speed rail could follow in 2019, at the earliest.

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san francisco bay, high speed rail, caltrain, transportation, san francisco news, heather ishimaru
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