Shimmick/Skanska crews ready Warm Springs BART extension

First phase, the Fremont Central Park Subway, will break ground in September.

By Greg Aragon

Crews are readying their wet suits as the first segment of the $890 million Warm Springs Bay Area Rapid Transit extension project prepares to cut through an aquifer at Fremont Central Park in the city of Fremont, on the southeast side of the San Francisco Bay.

One of BART’s early designs of the Warm Springs Station.

“The main challenge we face on this project is groundwater conditions,” says Paul Medved, project manager for BART. “We have to go through a drinking water aquifer with no way to dewater it, so we have to construct some very significant temporary works.”

Known as the Fremont Central Park Subway, the design-bid-build project is scheduled to break ground in September and take 43 months to complete. Construction is being led by Oakland-based Shimmick Construction Co., Inc., and Riverside-based Skanska USA Civil West California District Inc. The companies were awarded a $136 million joint venture contract in May.

The project consists of a mile-long subway running north and south beneath the park, and the construction of the at-grade Warm Springs Station. Work also includes two ventilation structures, a 2,000-space parking lot, bike lockers, elevators and escalators, and sustainable features.

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But before trains can traverse the park, a 22-ft-deep by 40-ft-wide cut-and-cover box trench for the subway must be carefully cut directly through the Above Hayward Fault Aquifer, which sits about 10 ft below the surface.

“We have to confine the excavation to keep water out during construction,” says Medved. “So the vertical portions of this confinement will be CDSM (Cement Deep Soil Mix) walls.”

CDSM is a technique in which a drill rig shoots cement deep into the ground where it mixes with soil to create a type of strong “slurry” in which columns can then be inserted to form cut-off walls. On this project the columns will be about 3-ft.-diameter and will go down between 40 ft and 60 ft.

After the CDSM walls are in place, crews will use a pressure grout machine to shoot grout about 30 ft into the ground between the walls, creating a plug floor.

“This makes the soil stronger and keeps the permeability down so it won’t leak,” says Todd Majors, project construction manager with Shimmick. “And it adds weight to the soil and in conjunction with the two cut-off walls will keep everything down and will not float from the water when we dig the soil and open up the trench.”

Majors adds that because of varying soil conditions in the aquifer, the main challenge will be to get the mix the correct combination of cement and soil to “create an impervious plug.”

The subway project is part of the larger Warm Springs Extension project, which will add 5.4 mi of new tracks from the existing Fremont Station south to a new station in the Warm Springs District of Fremont.

The second and final phase of the extension will be the design-build Line, Track, Station and Systems Contract. This $750 million design-build project is scheduled to go out to bid in the fall, awarded in spring 2010, and take approximately four years to complete.