Conceptual rendering of a cross-section of a single-bore tunnel with stacked platform
Single Bore Tunnel Remains the Best Option for BART Silicon Valley Phase II Project
12/16/2021

Since 2016, the 5-mile tunnel portion of the 6-mile, 4-station planned BART extension through downtown San Jose to Santa Clara has been the most studied and talked about aspect of the project. After extensive due diligence, public review, industry feedback and discussions with cities and stakeholders on both the twin and single bore tunneling options, the single bore tunnel was selected and approved by both VTA and BART in 2018.

While VTA analyzed both tunneling options during the environmental phase, the single bore tunnel is the only option that was selected and received the required federal environmental clearance through issuance of the Record of Decision (ROD) from the Federal Transit Administration (FTA). With the ROD, VTA officially had the green light to move forward with the 4-station, single-bore project. Over the last 18-months VTA, in partnership with BART, the future operator and maintainer of the system, has worked diligently to advance a single bore tunnel configuration with associated underground stations taking into account operational, safety and passenger experience considerations.

Selection of the single bore configuration represented a recognition that the traditional method of building underground rail, through the use of twin-bores and cut and cover stations, is not always appropriate, especially in dense urban areas. This type of construction involves large excavations to build the underground stations, which disturbs the vast network of existing utilities that lies beneath the street’s surface and causes extensive utility relocations. These construction activities are very disruptive at the street level, and the resulting traffic impacts overall mobility in the community and can leave the local businesses inaccessible or difficult to access as well.

As the project delivery lead and transportation provider for Santa Clara County, VTA sought a solution that built on lessons learned from other major tunneling projects, incorporated feedback from the rail transit industry, and also protected the public from major construction impacts and risks. This includes discovery and impact of unknown utilities that are historically one of the major risks to capital projects leading to higher relocation costs and schedule delays, sometimes for years. Drawing on best practices from around the world and advancements in underground construction, VTA determined that constructing this extension with a single bore tunnel and off-street stations could alleviate many of the construction, traffic, utility, business and economic impacts typically seen with traditional twin bore construction, while still meeting the operational needs and providing a high level of passenger regional rail service.

Much has been said regarding station depth of the single bore but VTA is in good company with many other underground stations in California and nationwide. This includes the soon to be opened SFMTA Union Square/Market Street and Chinatown stations, which have comparable or greater depths. Passengers will be able to traverse to boarding platforms with high-speed elevators.

There is also a misunderstanding that VTA can simply ‘dust off’ twin bore designs that were previously developed and proceed with building this extension. In 2008, before the BART Extension Program was broken into two phases, this twin bore configuration with cut-and-cover stations was relatively advanced. However, those plans are now nearly 15 years old and would require major updates and redesign to meet the current codes and standards in addition to a new environmental clearance process. This would add years to the schedule and further delay revenue service. With only a few months to go before VTA anticipates awarding the first major construction contract for the project, such a change would add significant schedule and cost impacts and negate all the efforts and benefits of participating in the FTA Expedited Project Delivery funding program. Any potential cost savings, if determined, would also be exponentially negated with schedule delays and escalation.

Recently, inaccurate statements have been made and concerns have been raised regarding project costs and its contributing factors. At the onset of the environmental phase, initial cost projections were shared based on early concepts. These cost projections did not include financing charges, contingencies, or costs associated with risks, as these were not estimated at the time. Since then, as the project has continued to progress, time has been spent advancing the design through the preliminary engineering and procurement stages. While this has resulted in additional time added to the project schedule and associated cost increases, it has also allowed VTA to further establish a more comprehensive cost estimate inclusive of required financing costs and risks contingencies. It is important to note that the single bore tunnel configuration has not been identified as contributing to the cost increases associated with the project. FTA’s recent risk assessment also did not provide any indication that any potential additional risks or costs are associated with the selected tunnel configuration.

Currently, VTA is on-track to deliver a project that brings rapid and regional rail service to San José and Santa Clara, avoids major disruption, reduces difficult utility relocations, while still providing a reliable transit option for patrons. With environmental clearance and continued support and commitment from FTA, VTA looks forward to continuing to work together with stakeholders and the public to make this project the best it can be over the next year as it moves into final design and construction.

FAQ

How did VTA determine a single-bore tunnel was the best option for BSV Phase II?

During the environmental clearance phase of the VTA’s BART Silicon Valley Phase II Extension (BSVII) Project, both the twin-bore and single-bore tunneling methodologies were evaluated and assessed with regard to 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 BSVII 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 BART Phase II extension. 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, SoundTransit, SFMTA, Silicon Valley Clean Water, Virginia DOT, and DBIA 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.

How deep are the single-bore tunnel station platforms?

At approximately 90 feet below ground, the boarding station platforms are comparable to other underground stations in California and around the United States. Within the Bay Area, it is about the same depth as the soon to be opened SFMTA Union Square/Market Street and Chinatown Central Subway Stations. Washington Metro, which is the second busiest heavy rail system in the United States, has multiple stations that are even deeper, including ones that are over 100 feet deep.

Was the twin-bore tunnel environmentally cleared?

In early April 2018, the VTA Board of Directors approved VTA’s BART Silicon Valley Phase II Extension (BSVII) Project, certified the Final Subsequent Environmental Impact Report (SEIR) as CEQA lead agency, and selected the Single-Bore Option in the approved Recommended Project Description. In late April 2018, the BART Board of Directors, as a Responsible Agency under CEQA, accepted the SEIR and, pursuant to the Comprehensive Agreement, approved the BSVII Project as approved by the VTA Board, which included the selection of the Single-Bore Option. In June 2018, FTA, as the NEPA lead agency, issued the Record of Decision for the BSVII Project (approving the Final Supplemental Environmental Impact Statement) and selected the Single-Bore Option. Although the Draft 2016 and Final 2018 SEIS/SEIR included an evaluation of the Twin-Bore Option, it was not selected as the preferred option. Only the Single-Bore Option was selected as part of the approved project description. Also, although the VTA Board has approved the twin-bore design configuration in the past (in 2004 and 2007), only the single-bore design configuration has NEPA/CEQA clearance as of the 2018 approvals. Further information and documentation can be found in our Planning and Environmental page.

What would happen if we switched to twin bore now?

In 2008, before the BART Extension Program was broken into two phases, a twin bore configuration with cut-and-cover stations was relatively advanced. However, those plans are now nearly 15 years old and would require major updates and redesign to meet the current codes and standards in addition to a new environmental clearance process. This would add years to the schedule and further delay revenue service.

With only a few months to go before VTA anticipates awarding the first major construction contract for the project, such a change would add significant schedule and cost impacts and negate all the efforts and benefits of participating in the FTA Expedited Project Delivery funding program. Any potential cost savings, if determined, would also be exponentially negated with schedule delays and escalation.

 

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