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​VTA to Further Analyze Tunnel Design for BART Silicon Valley Phase II

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Phase II of VTA’s BART Silicon Valley is reaching a critical phase in the environmental clearance process. On Thursday, December 7, VTA staff will present to the VTA Board of Directors next steps to advance the Phase II project into the final environmental phase and to prepare for the federal engineering phase.

The planned six-mile extension would extend BART from the Berryessa/North San José Station into downtown San Jose and then into the City of Santa Clara. Fast approaching, and necessary for state and federal environmental clearance and VTA to advance the project, is a defined project description which will include the locations for all four stations and the tunnel construction methodology to be used on the five-mile subway portion in downtown San Jose. 

As Santa Clara County’s Transit Agency and Congestion Management Agency, and as the owner and builder of the BART Silicon Valley Extension, VTA has a responsibility to fully explore options that will result in the safest and most efficient system being built with the least amount of impact on its customers, businesses and the community.  

In 2014, planning efforts were renewed for Phase II and VTA staff identified a viable option in delivering the five-mile subway portion using a single-bore tunnel, a configuration made practical by recent advancements in the use of large bore tunnel boring machinery. During the past two years, VTA engaged BART staff in exploring and undergoing a thorough analysis of the single-bore configuration. This single tunnel design would contain two tracks, each running in separate compartments in opposite directions. The team also studied the twin-bore configuration. Operating in many subway systems including the existing BART system, this includes a single track running in each tunnel. 

The draft environmental documents released for public review earlier this year included analysis of impacts related to both single-bore and twin-bore construction. VTA also engaged a number of third-party consultants to conduct a comprehensive analysis of the single-bore tunneling configuration. During that analysis, evaluation criteria were established, with BART, to provide a qualitative and quantitative assessment of the single-bore option, as well as review the previous work for the twin-bore configuration.

Once those studies were complete, VTA and BART agreed to engage a panel of peers from public transit agencies currently operating heavy rail subway systems with deep stations to review the single-bore concept with a focus on operations and safety. After three days of meetings where VTA and BART staff presented relevant aspects of the twin-bore and single-bore options, the panel opined that with some adjustments to address BART’s operational safety concerns: the single-bore tunnel can be operated safely as an extension of the BART system.  However, due to timing constraints related to the current federal funding schedule combined with BART’s strong preference for operating what it is was familiar with, the panel advised that at this time, twin-bore would be a preferred option for Phase II of VTA’s BART Silicon Valley Extension. 

VTA has embraced the input by the panel of its peers while noting the panel’s conclusions were based on the time constraints imposed by the timeline for the project to make its way into the federal funding process. Therefore, VTA will extend the project development timeline to demonstrate more thoroughly how the single-bore configuration can satisfy BART’s operational safety concerns in addition to further exploring construction impact mitigation strategies for the Downtown San José stations.   

The estimated time to refine the single-bore design is approximately three months. The request for an extension of time has been made to the Federal Transit Administration, the federal oversight and funding agency, which assures exhaustive due diligence before recommending a tunneling methodology. 

As with all activities related to VTA’s BART extension, VTA will continue to engage BART engineers and operations representatives in this process. Then, VTA staff will make a recommendation regarding tunneling methodology to the VTA Board of Directors and complete the environmental and funding processes, identified in the timeline below, for this critical and highly anticipated infrastructure project for Santa Clara County.

Phase II Project Timeline

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