New Direction for Subway Tunneling?

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subway illustration

While most new subway construction in North America over the last couple of decades has used the twin-tunnel approach, two agencies are planning new lines using a single, large-bore tunnel.

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In Northern California, the Santa Clara Valley Transportation Authority (VTA) is planning to construct a large-bore tunnel as part of the 6-mile BART Silicon Valley Phase II Project in San Jose, which includes a 5-mile, 43-ft diameter tunnel that will house bi-directional traffic. Meanwhile in the Greater Toronto Area, Metrolinx is also planning a large-bore tunnel for its Scarborough Subway Extension, a 4.8-mile extension using a 35-ft diameter tunnel.

These would be the first single-bore transit tunnels constructed in North America, according to VTA. Successful implementation of these projects could pave the way for similar approaches going forward, according to Hatch’s Gary Kramer, who wrote in our February 2021 issue: “The industry is seeing a need for ever larger tunnel diameters primarily for transit systems. … Transit stations with platforms built into such tunnels can eliminate the impacts associated with in-street cut-and-cover structures. We at Hatch have been involved and advocating this technology strongly for some time and are optimistic that large bore TBMs will become more state-of-the-practice (and not just state-of-the-art). The ‘mega-bore’ fears will subside as the feasibility and effectiveness of large bores are confirmed.”

Mike Wongkaew of HNTB wrote in our August 2019 issue that the large-bore option would help to “reduce the need for cut-and-cover station construction including impacts on streets, traffic and utilities, which can be very disruptive to communities and businesses situated within dense urban areas. The continuous space within the large bore also increases flexibility in locating stations and helps identifying station locations and track crossovers readily along the tunnel alignment, which enhances user experience and operational reliability.”

While the construction costs of large-bore tunnels are higher than constructing smaller tunnels, the large-bore approach can lead to a reduction in station and crossover construction, right of way acquisition, and surface disruption mitigation costs, Wongkaew wrote, adding “The large bore can be cost competitive to twin bore when all cost aspects of the project are thoroughly analyzed.”

VTA is currently evaluating proposals from three short-listed contracting teams for the BART Phase II Extension Project. Construction is anticipated to begin in 2022, with completion in 2030.

Metrolinx has contracted a Strabag-led consortium, which includes Arup Canada Inc. and Brian Isherwood & Associates Ltd., to build the Scarborough Subway Extension. The $757.1 million (CAD) contract was finalized in May 2021, and crews are currently working to build the launch shaft. Metrolinx recently posted photos of the fully assembled TBM at the Herrencknecht factory in Schwanau, Germany, which is expected to arrive in Toronto in early 2022. Project completion is expected by 2029-30.

Will these projects set the stage for future subway projects? Stay tuned!

Jim Rush, Editor/Publisher, TBM

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