Linking up Silicon Valley

When San Francisco Bay’s rapid transit system (BART) was built in the 1960s, Santa Clara County opted out. Now the 1.7M residents are getting connected, thanks to the Silicon Valley Extension. With 54,600 daily passenger capacity, it will free people from their cars, cutting congestion and pollution as well as speeding up journeys. “It’s more than a transit project,” says our project manager Michael Lehnen. “BART Silicon Valley is an entire programme of improvements that will transform Silicon Valley.” Having completed the first phase, we are now – again in joint venture – hard at work designing the second phase. Comprising 10km of line and three stations, this $9.3bn section is almost entirely underground, and will feature the third-biggest bored tunnel ever constructed. Underground transit systems are traditionally built using twin-bore running tunnels, with cut and cover for stations. But that would have been unacceptably disruptive in busy downtown San José, home to commercial giants like Google and Adobe. The solution was to use a single-bore tunnel for both guideways and stations, something never done before at this scale. And, by opting for a massive 14.6m diameter, we could fit both tracks side by side. Coming up with efficient engineering solutions to implement this ambition has demanded both a range of expertise and ingenuity.