EVAN DUCHARME/SPECIAL TO THE S.F. EXAMINERWork continues Wednesday at the Transbay Transit Center site downtown.

Transbay site work reaches milestone

Sixty-five feet below ground level between Mission, Howard, Second and Beale streets, crews working on the Transbay Transit Center project reached a quiet milestone.

A 10- to 12-hour concrete pour completed Saturday was the last one necessary until excavation wraps up early next year. That makes 10 mat slab pours to date, each between 4,000 to 6,000 cubic yards of concrete. Rail tracks will eventually sit on the slabs.

“This is achieving a big milestone for our project,” said project superintendent Jordan Smith of Webcor/Obayashi, the general contractor, at the site Wednesday. “Essentially half of the project has mat slab. What that allows us to do is continue on schedule on the walls and columns and rebar on one side.”

Construction has made good time, with the foundation for the western half of the 600,000-cubic-yard hole in the ground already set with mat slab.

The multilayer process started in early September and involved drilling 65-foot-long micropiles on the dirt base to anchor the coming structure, four inches of concrete slab, a waterproofing layer, another 4 inches of concrete slab, rebar reinforcement and finally 5 feet of mat slab that Caltrain and high-speed rail will arrive on.

Dirt is still visible in the easternmost portion of San Francisco’s biggest hole in the ground, with excavation slated to end in mid-January. The final mat slab pours will occur in mid-February.

In the western section and portions that already have mat slab, work has begun to focus on solidifying the walls, which also require waterproofing, rebar and a concrete finish. With construction materials, timing is paramount.

“It’s kind of a dance down there in terms of how things have to move forward,” said Monique Hawn, superintendent for Turner Construction, another contractor.

Hitting this milestone also enables another contractor to insert structural steel starting in the middle of the site and moving outward in both directions. The recent cold snap has had no bearing on the construction process and less-than-normal rainfall has “immensely” helped the project stay on schedule, Hawn said.

There has been “an amazing safety record,” added Transbay Joint Powers Authority spokeswoman Stephanie Reichin.

The first phase of the $4 billion project involves creating a five-story transit center with one above-grade bus level, a ground floor, concourse and two below-grade rail levels and a rooftop park. The project broke ground in August 2010 and is scheduled to be completed by fall 2017. It is expected to accommodate more than 100,000 passengers each weekday.Bay Area NewsTransbay Transit CenterTransittransportation

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