VTA Staying With Single-Bore Tunnel for BART Silicon Valley Extension
Written by Jennifer McLawhorn, Managing Editor
Tunnel Boring Machine (TBM)
Courtesy of VTA
SAN JOSE, Calif. - Valley Transportation Authority (VTA) discussed the possibility of switching to twin tunnels. After analysis, it concluded a single-bore tunnel was more financially viable.
The single-bore tunnel design is the preferred design for extending BART to Santa Clara as it is “more cost-effective, quicker to build, already approved, and far less disruptive,” according to the VTA. The single-bore tunnel option was approved in 2018 with the VTA and BART Boards of Directors endorsing this plan. The VTA also says the FTA has issued a Record of Decision to allow for engineering and construction to proceed. If the VTA had switched to the more costly twin-bore design, it would have to restart the process with environmental reviews, engineering work, and purchasing new tunnel boring machines (TBMs).
According to a report from abc7, the VTA board discussed this possibility, but a 78-page report found the twin tunnels approach would cost $13 billion. In addition to the substantial cost, twin tunnels would cause “significantly more construction issues” for the city. The single-bore tunnel will stretch five miles from Berryessa to Santa Clara with a diameter of 53 feet.
VTA Chief Megaprojects Delivery Officer Tom Maguire reportedly said the single-bore approach is already underway and that “It’s an approach that’s been done successfully outside of North America, but it will be the biggest transit tunnel in North America. So we think it’s the right project, and we chose it because of all the positives.”
Outside of cost and construction issues, the twin approach is an older version that would not be suitable for South Bay, the report says. VTA’s Maguire continued, stating groundwater and soil “make the single bore, the innovations of large single bore particularly well-suited to our project, and it’s the right way to go.”
