The deeper dig: BART tracks will lay side-by-side in subway

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Valley Transportation Authority (VTA) engineers are working on how Bay Area Rapid Transit (BART) will extend via tunnel under Santa Clara Street from the Berryessa Station through downtown San Jose.

In July, after months of study with BART staff, VTA directed engineers to proceed with a new subway design that widens the tunnel’s diameter from 41 feet to 55 feet, allowing side-by-side tracks and a 24-foot center platform.

Side-by-side tracking is consistent with other stations along the BART lines.  The original 41-foot bore would have required stacking of tracks and separate 22-foot-wide station platforms.

Estimators are busy recalculating the additional cost of the bigger (and deeper) dig and gauging the impacts of approximately 50 percent more earth removal on the project’s environmental review.

Under the new configuration, the top of the tunnel would be 65 feet under Santa Clara Street, and the platform at the station near Market Street would be 93 feet 6 inches underground.  As with the smaller single bore, the larger tunnel would be excavated without cutting open Santa Clara Street.  Earlier this year, engineers discovered that a larger bore with center platform could result in relocating auxiliary BART facilities from street level into the tunnel.

“The extra space in the tunnel can potentially accommodate systems and facilities such as traction power sub-stations and ventilation plants,” said Bernice Alaniz, BART Silicon Valley business operations and communications director.

VTA and BART have been working closely on the design since making the unprecedented decision in March 2018 to build a single-bore tunnel.  The engineering firm hired to design the extension from Berryessa to a system endpoint in the City of Santa Clara is finalizing the design this fall.  Their schedule calls for delivering 10 percent of the final design by December.

Also expected in December is the opening of the 10-mile BART extension from Warm Springs to Berryessa.

Preliminary engineering, a finalized plan and schedule, completed cost estimate and completed environmental update are all expected by summer 2020.  

Construction of the $5.1 billion project is slated to begin 2022 and be finished in 2026.