How did VTA determine a single-bore tunnel was the best option for Phase II? Has the single-bore tunnel been used elsewhere?

During the environmental clearance phase of the VTA’s BART Silicon Valley Phase II Extension Project, both the twin-bore and single-bore tunneling methodologies were evaluated and assessed regarding constructability, system operations, passenger experience, economic development, costs, schedule, and risk impacts. This included an Independent Comparative Analysis Risk Assessment conducted in 2017 with findings presented to the VTA Board of Directors in September 2017.The conclusion of this analysis showed both tunnel configurations are generally cost neutral when comparing all factors including construction, schedule, and the known and unknown risks. The overall operation and maintenance costs associated with single-bore systems are comparable to twin-bore systems, as the total linear feet of track remains the same. The deeper single-bore stations are only expected to nominally increase the operations and maintenance costs related to vertical circulation elements and those may be further optimized during the design development process to reduce these costs. 

Since then, as part of the project development phase, VTA benchmarked the Phase II Project (using the single-bore method) with other major federally funded rail transit projects that used the twin-bore construction methodology. Focus was on two California projects: SFTMA’s Central Subway and LA Metro’s Purple Line. For both projects the original average cost per mile constructed was comparable to the Phase II Project. However, actual costs for these projects trended higher than originally projected due to utility relocations and construction impacts, which included business disruptions. These cost increases were associated with the cut-and-cover construction required for twin-bore systems. Cut-and-cover construction necessitates significant utility relocation work and causes traffic and pedestrian access and circulation problems, which is the primary cause of disruption to typical downtown businesses. Since the single-bore method can be constructed with minimal surface disruption, it would reduce and avoid multiple years of business and economic disruption. Additionally, in September 2020, VTA, BART, and FTA held a peer review with members from LA Metro, Sound Transit, SFMTA, Silicon Valley Clean Water, Virginia DOT, and Design Build Institute of America in attendance. The members provided their feedback and recommendations on project scope, delivery methods, and overall approach and provided beneficial lessons learned from their similar large capital projects.

Single-bore tunnel methodology is not new technology. There have been larger tunnels constructed in North America, including some in similar ground conditions. A tunnel of similar size was completed in Miami for a highway in 2014. The main difference from other single-bore tunnels is how the train tracks would be configured within the tunnel. While this project will be the first transit project in the United States to use a single-bore tunnel, it is not the first in the world. The Barcelona Metro Line 9 uses a single-bore tunnel. In addition, Toronto’s Scarborough Subway Extension is constructing a single-bore tunnel.