Work proceeds on Silicon Valley BART tunnel, while VTA seeks ways to curb spiraling costs

THE WHIR AND THUNKS of heavy equipment hauling clay and soil from underground could be heard throughout the Newhall Maintenance Yard on Monday, as VTA begins construction on the BART expansion through Silicon Valley.

Contractors have been working in the yard 22 hours a day since June 9 pulling bentonite — a clay-like material — from more than 80 feet below ground and installing reinforced concrete to create a launch structure for the tunnel boring machine. The agency plans to use a single-bore tunnel for the 5-mile stretch beneath downtown San Jose.