Annual review 2022
Linking up Silicon Valley
The world’s third-biggest bored tunnel is being built to close a gap in the Bay Area’s rapid transit system, with minimal disruption to residents and businesses.
Project
Bay Area Rapid Transit, Silicon Valley Extension Phase II
Client
Santa Clara Valley Transportation Authority (VTA)
Location
San José, California, USA
Expertise
Preliminary design for tunnel and track, oversight of progressive design-build contractor, digital delivery, advanced design for underground and at-grade stations, yard, maintenance facility, and systems
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BART Silicon Valley Extension will provide 1.7M residents of Santa Clara County with access to transit services
The San Francisco Bay Area is one of the most populous regions of the US, home to 7.7M people. From 1940 to 1970, the population surged from 1.7M to 3.6M, leading several Bay Area counties to join in the 1960s to form the Bay Area Rapid Transit system. BART is now the fifth-busiest heavy rail rapid transit system in the US, with 210km (130 miles) of track serving an average of 118M passenger journeys each year.
Santa Clara County originally chose not to join the transit system. This gap in the rail network around the Bay forces many commuters to travel by car.
Two projects for the Santa Clara Valley Transportation Authority (VTA) are closing the gap and transforming transport for millions of workers at the heart of the US tech industry.
BART Silicon Valley Extension will give more choice to travellers. It is expected to accelerate a 20-year-long rise in the number of journeys made by public transportation. Together, phases I and II will provide access to transit for 1.7M residents of Santa Clara County, with capacity for 54,600 passengers per day, helping relieve heavy congestion on two north-south commuter highway routes.
We managed phase I in joint venture with Bechtel. On phase II, we led the preliminary design and continue with advanced design in a joint venture with San Francisco-based PGH Wong Engineering.
“It’s more than a transit project – BART Silicon Valley is an entire programme of improvements that will transform Silicon Valley,” says Michael Lehnen, our BART project manager.
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Station innovations
Underground transit systems in urban environments are traditionally built using twin-bore running tunnels (train guideways), with cut and cover methods used for the stations.
However, the new stations for BART Silicon Valley phase II are in some of the busiest parts of San José. Cut and cover construction would mean months of disruption in the bustling commercial centre, as sites are cleared for excavation, roads are closed, utilities are relocated, and traffic is diverted.
San José is a key commercial centre, home to globally important companies including Adobe and Google, which plans to add almost 700,000m² (800,000yd²) of office space and 4000 units of new housing. Popular hockey and football teams also bring large crowds to downtown San José and Santa Clara – which in turn brings revenue to local restaurants and hotels.
The challenge was to come up with the best alternative to cut and cover construction that would have caused major disruption.
“Our work includes a tunnel 8km (5 miles) long, to be constructed by a tunnel boring machine (TBM). There will be 1.6km (1 mile) of street level track, three underground stations, an above ground station, and a train yard and maintenance facility near the end of the extension,” says Michael.
Early station designs