BART station design (Credit Foster + Partners)

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

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A two-phase extension

Phase I extended the BART service 16km from Alameda County into Santa Clara County, with stops at Milpitas and Berryessa/North San José. The project broke ground in 2012 and opened for passenger service on 13 June 2020.

Michael explains that the project has brought wider benefits including transit-oriented communities, multimodal transportation connectivity, plus roadway, utility and environmental improvements.

Phase II will add an extra 10km (6 miles) to the route. Of this, 8km (5 miles) of the alignment will consist of a large single-bore tunnel. It will include three new underground stations in San José and terminate at an above ground station in Santa Clara, with connections to Caltrain Commuter Rail, Amtrak, VTA’s light rail, and the future California High-Speed Rail route.

At an estimated cost of $9.3bn, phase II is the largest transportation project in the history of Santa Clara County. Construction is expected to start in 2023 to 2024 and be completed in the early to mid-2030s.

BART Silicon Valley Extension phase II route

BART Silicon Valley Extension phase II route

It’s more than a transit project – BART Silicon Valley is an entire programme of improvements that will transform Silicon Valley.”

Michael Lehnen, project manager, Mott MacDonald

Phase II

10km

of line

underground stations

8km

tunnel

Underground options

Over the course of almost 20 years, designs for three tunnel configurations had been developed to varying degrees of completion:

Twin-bore tunnels and cut and cover stations
Single-bore tunnel 13.1m (43ft) in diameter with tracks in stacked and side-by-side configurations, and stacked station platforms
Single-bore tunnel 15.2m (50ft) in diameter with tracks side-by-side and with centre island station platforms

The single-bore solution enables stations to be built avoiding the above ground disruption of cut and cover in the public right-of-way. But using a single-bore tunnel for both guideways and at underground stations has only been done once before, on Barcelona Line 9 in Spain. That is a comparatively small metro system, with fewer than 750 passengers per train. This project is for heavy rail, with larger trains carrying more than 2000 passengers.

Michael outlines our involvement: “We began in 2019 by examining the concept designs to assure VTA that a single-bore tunnel was possible,” he says. “The smaller, 13.1m (43ft) option, would have been less expensive to build than the larger bore option from a pure tunnelling perspective, but the changing internal configuration, from side-by-side guideways switching to stacked stations, brought complexity.”

Kiewit Shea Traylor (KST) – a joint venture of Kiewit Infrastructure West, JF Shea Construction, and Traylor Brothers – is a progressive design-build contractor for the tunnel and trackwork components of the BART Silicon Valley phase II project.

KST proposed a larger 14.6m (48ft) tunnel and side-by-side track configuration throughout the tunnel and stations, making it the third-biggest bored tunnel in the world. Although the larger diameter increases the cost of tunnelling, it reduces risks and standardises operations associated with tunnel construction, improving the probability that the project will be delivered to cost and schedule.

BIM model to advance the detailed design

BIM model to advance the detailed design

The right engineering solutions

Turning the large-diameter tunnel concept into an efficient engineering solution has involved a range of expertise. This includes the following:

Geotechnical investigation and reporting – including preparation of a geotechnical baseline report, with a detailed characterisation of ground conditions used as a reference point for detailed design of the tunnel, procurement, and development of the contractors’ construction strategy. Geotechnical investigation showed that the site mostly consists of soft ground with a high-water table. The tunnel boring machine will need to be specified to cope with these conditions.

Preliminary design of the large-diameter single-bore tunnel – including interdisciplinary building information modelling (BIM) to establish the internal diameter of the single-bore tunnel, precast concrete lining, adit (horizontal passageways) configuration, tunnel portals, and internal structures. Preliminary design has taken account of the trains’ range of motion and the spatial requirements for ventilation/smoke extraction, access and emergency egress, power supply, utilities, and roadside facilities.

Preliminary design of the four stations – including deep shafts and mined adits to connect the station headhouses to the tunnel. At the Downtown San José and Diridon stations, the primary construction activities will be off the street, allowing traffic to continue and limiting disruption to businesses and the travelling public.

Track and systems development –including preliminary design of direct fixation trackwork and crossovers.

Ventilation system analysis and design – based on the large-diameter single-bore tunnel and unique station configurations. Computational fluid dynamics (CFD) analysis demonstrated that the system will be able to handle smoke and fire design requirements effectively, keeping passengers safe and aiding emergency responders. Coupling CFD with our custom-made STEPS software, we modelled passenger egress to demonstrate that our design meets or exceeds emergency evacuation requirements.

Development of a digital delivery system – the digital delivery system complies with ISO 19650, the international standard for digital delivery, and uses the latest technologies for efficient production and collaboration between our team, our client VTA, the progressive design-build contractor, and 40 subconsultants. BIM enhanced multidisciplinary co-ordination during preliminary engineering and will continue to do so going into advanced design. The unified digital model of the project will be handed on to the VTA as an asset management tool for future operation and maintenance of the extension.

BART Silicon Valley phase II is expected to open in the early to mid-2030s. The extension will bring economic vitality through community connections to thousands of jobs around the bay, mixed-use development around the route, and cleaner air through a reduction in road traffic.

Single-bore tunnel

14.6m

wide

8km

long

We began in 2019 by examining the concept designs to assure VTA that a single-bore tunnel was possible. The smaller, 13.1m (43ft) option, would have been less expensive to build than the larger bore option from a pure tunnelling perspective, but the changing internal configuration, from side-by-side guideways switching to stacked stations, brought complexity.”

Michael Lehnen, project manager, Mott MacDonald