Sources: Single Bore Tunnel Remains the Best Option for BART Silicon Valley Phase II Project | VTA;

Can Single-Bore Tunneling Transform Urban Subway Construction? — TBM: Tunnel Business Magazine (tunnelingonline.com)

Single-bore tunneling involves constructing a single, large-diameter tunnel that can accommodate both trackways, and allows construction of the passenger platforms within the tunnel itself, significantly reducing surface-level disruption compared to typical cut-and-cover station construction. This method uses one large diameter tunnel boring machine (TBM) and enables construction of station entrances and ticketing facilities to occur off-street, minimizing impacts on businesses, vehicle traffic, and pedestrians compared to traditional twin-bore construction methods. Twin-bore tunneling, in contrast, uses two smaller tunnel boring machines and often requires a cut-and-cover method (excavation from the street downward) for station construction, leading to extensive utility relocations and lengthy disruptions at the surface.

The single-bore tunneling method 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. While the Phase II Project will be the first transit project in the United States to use the single-bore tunneling method, it is not the first in the world – due to its well-known benefit of minimizing surface-level disruptions, this method has been used for tunneling beneath densely populated urban areas. The Barcelona Metro Line 9 is being constructed as a single-bore tunnel. In addition, Toronto’s Scarborough Subway Extension is being constructed using a single-bore tunnel.

VTA has conducted multiple comprehensive evaluations, including an Independent Comparative Analysis Risk Assessment. These efforts recognized that the single-bore construction method would result in deeper stations, and has the potential for some higher operation and maintenance costs. However, VTA concluded that the benefits of using a single-bore tunnel outweigh the differences identified. Single-bore construction will significantly reduce surface level disruptions and the long-term economic impacts typically associated with twin-bore and cut-and-cover construction.  Overall, the single-bore construction method was deemed cost-effective and less disruptive than the twin-bore cut and cover method, leading to its adoption in 2018 after receiving key approvals, including the FTA’s Record of Decision for the project upon their approval of the Final Supplemental Environmental Impact Statement.

There’s a misconception that VTA could simply revive old twin-bore designs from 2008 for the BART extension, but these designs are outdated and would require significant updates to comply with current standards and environmental regulations, adding years and costs to the project. Additionally, VTA has already committed to the less disruptive single-bore tunneling method by ordering a highly specialized, large diameter TBM, which allows for building station platforms and entrances underground Switching now to a twin-bore construction method would cause major project delays and increase costs. Thus, any potential savings that twin-bore construction methodology might provide would be negated by escalated costs and schedule delays.

Early Works Construction began in mid-2024, with Tunneling and other Heavy Construction activities starting in late 2025. For more details on upcoming construction activities, visit VTA’s BSVII Extension Construction page. 

VTA is able to limit construction impacts through innovative tunneling technology, mainly the single-bore tunnel method Construction of a single-bore tunnel uses a tunnel boring machine (TBM), which will remove soil through the tunnel itself, keeping the street above intact. While utility relocations and tunnel connections may require some street disruptions, construction of the main project structures (stations and ventilation and emergency egress facilities) will largely occur off-street, similar to the construction of buildings with underground parking. Construction will be similar to other low-rise building developments as seen around San José and Santa Clara.  Intermittent driveway, access changes and lane closures will occur throughout the construction process. Advanced notices of detours will be provided for all transportation modes and communicated to affected stakeholders and the public. 

VTA’s BART Silicon Valley Extension Program will bring Bay Area Rapid Transit (BART) into Santa Clara County and ring the San Francisco Bay with frequent rail service. The program is a 16-mile, six-station extension of the existing BART system.   

VTA’s BART Silicon Valley Program is managed by VTA in cooperation with BART. The complete extension is being built in two phases. Phase I, the Berryessa Extension, was a 10-mile, two-station Project that opened for service in June 2020. VTA’s BSVII Project is a six-mile, four-station extension of the regional BART system from Berryessa/North San José Station and is projected to serve 55,000 weekday riders by 2040. The extension will traverse through downtown San José to the City of Santa Clara, providing riders with a fast and convenient transit alternative, while enhancing connections to other regional transit services.  

The BSVII Project will support mobility for transit dependent populations, increase access to employment centers and opportunities, and stimulate economic development in Silicon Valley. Around stations, the Project will help foster vibrant, pedestrian-friendly, transit-oriented communities that strengthen neighborhood identities and support local businesses and microenterprises. 

The BSVII Project will support Silicon Valley’s economy, connecting its residents to millions of jobs and generating 75,000 new diverse job opportunities – including 43,000 jobs directly related to the project, and another 32,000 jobs tied indirectly or through the impacts of spending from the project. Direct employment supported by the project would also result in over $3.5 billion in new labor income, with a total effect on labor income exceeding $5.6 billion. VTA’s vision is one of empowerment, inclusion, and progress, leaving a lasting legacy for Santa Clara County and the Bay Area. Please visit the Project Benefits page for more information on the BSVII Project’s benefits. 

When the BART District was formed in the 1960s, Santa Clara County voters opted not to join the District. As population and job growth rose in Santa Clara County, congestion along the I-880 and I-680 corridors grew as well. To prepare for this growth, the Santa Clara County voters approved a 30-year half-cent sales tax increase to fund BART system extension in the Santa Clara County, known as the 2000 Measure A.  Since then, Santa Clara County residents have returned to the polls in 2008 and 2016 to reinforce their support for VTA’s BART Silicon Valley Program through multiple ballot that would continue to fund construction and operations of the extension. As the recipient and manager of these funds, VTA is responsible for designing and constructing VTA’s BART Silicon Valley extensions to BART’s standards and will pay for operations and maintenance of the extension. 

As Santa Clara County’s Congestion Management Agency, VTA is responsible for designing and constructing the BART Silicon Valley Extension Program (Phases I and II). While VTA will have ownership of all the property, facilities, and equipment associated with the extension, BART will be responsible for operating and maintaining the system in a way that ensures a safe and seamless experience for passengers. Through the 2008 Measure B that was passed by Santa Clara County voters, VTA will pay BART for operations and maintenance of the Extension Program.

Approximately five of the 6-mile BSVII Project will be in a subway tunnel, starting at Las Plumas Avenue, just east of US-101, and coming back to grade just north of I-880 and Newhall Street. The last mile of the alignment will be at grade, parallel to the existing Caltrain Corridor, with an end of line station at the end of Brokaw Road. The 28th Street/Little Portugal, Downtown San José, and Diridon stations will have underground passenger platforms while the Santa Clara Station will have ground level platforms. 

The passenger platforms for the three underground BSVII stations will be approximately 75 feet below ground, which is comparable to other stations throughout the United States. Locally, it is less deep than Muni’s Union Square and Chinatown Central Subway Stations in San Francisco, with the Chinatown Station approximately 100 feet below ground. Washington Metro, which is the second busiest heavy rail system in the United States, has multiple stations that are much deeper, including stations that are 117 feet, 145 feet and 196 feet deep.  

The BSVII tunnel will be approximately 54 feet in diameter and needs approximately that much ground cover above it. 

The Phase I and Phase II alignments and station locations were determined through a rigorous process. Initially, a Major Investment Study conducted by VTA in 2001 assessed 11 transit alternatives and alignments for the I-880 and I-680 corridors. Among these, BART emerged as the preferred transit mode due to its projected ridership. The VTA Board of Directors finalized the alignment and station locations in the 2004 Environmental Impact Report, further supported by voter approval of subsequent sales tax measures, including 2008 Measure B, 2016 Measure B, and the 2018 Regional Measure 3.  

With the completion and adoption of the Project’s environmental clearance and issuance of a Record of Decision by the Federal Transit Administration in 2018, the Project’s alignment and station locations became fixed and permanent. Altering the alignment or station locations would require the Project to be re-defined, requiring new environmental clearance and additional design work. As a result, such changes would significantly extend the schedule for providing revenue service along the BSVII alignment. 

VTA is using innovative construction methods to minimize disruptions to small businesses along the Project alignment. The single-bore tunneling construction method, jointly chosen by VTA and BART, significantly minimizes street-level impacts compared to traditional twin-bore methods that are typical in major transit projects. Unlike twin-bore construction, which often requires street closures for multiple years during station construction, the single-bore approach largely confines construction to off-street areas, minimizing travel disruptions. As a result, the BSVII Project will resemble traditional building construction, with most work occurring off-street with temporary lane and sidewalk closures. 

Additionally, to meet the environmental commitments of the BSVII Project, VTA developed and is implementing a Construction Outreach Management Program (COMP). The purpose of the COMP is to inform and educate the public and other stakeholders about the construction methods, construction schedules, and associated activities. The COMP includes three main components:  

• The Construction Education and Outreach Plan (CEOP) outlines how and when VTA communicates with the public before and during construction. 
• The Construction Transportation Management Plan (CTMP) outlines how VTA coordinates access and circulation for all modes of transportation and mobility during construction. 
• The Emergency Services Coordination Plan (ESCP) outlines how VTA coordinates with local emergency services to minimize impact to their response times due to the project’s construction. 

Included in the CEOP is the Thriving Business Program (TBP). VTA will be implementing the TBP for businesses near work areas once major construction begins. The TBP is a proactive effort to support the local small business community and provide targeted resources during Project construction. The purpose of the TBP is to enhance small business resiliency, encourage foot traffic to businesses that rely on “walk-in” clientele, support businesses through technical guidance and marketing assistance, provide grant funding to assist businesses during construction-related disruptions, and create positive community benefits beyond the Project itself. Visit the Thriving Business Program page for more information on the TBP.  

VTA has a long-standing partnership with both cities. This includes coordination with city staff within departments such as Planning, Department of Transportation, Engineering, Public Works, Cultural Affairs, and Economic Development. This relationship and partnership will continue throughout engineering and construction. 

VTA is committed to keeping station area communities informed on current and future construction activities. For up-to-date information on the Project’s construction progress, as well as street and lane closures, detour alerts and construction impact alerts, please visit the BSVII Construction Page. 

Excavated material removed from the tunnel will be temporarily stored within VTA’s construction staging area at the West Portal and Newhall Yard and Maintenance Facility and may be reused as fill material or loaded onto trucks for reuse or disposal. Excavated material removed during tunnel construction will require several dump trucks to move the material off-site. As such, VTA will coordinate with the City of Santa Clara and City of San José to determine the most direct and least disruptive routes to transport the excavated material. 

Once the tunnel construction is complete, construction will begin at the Newhall Yard and Maintenance Facility and the Santa Clara BART Station. The Newhall Yard and Maintenance Facility is a key component of the BSVII extension, as it allows for end-of-line maintenance and storage of BART vehicles. Without this facility, BART trains would need to return to the East Bay at the end of service each day. 

Within the Newhall Yard and Maintenance Facility there will be a BART train maintenance shop, train car wash, maintenance and engineering offices, a yard and control tower, and other support facilities. Most maintenance activities will occur inside enclosed buildings, resulting in minimal noise from trains. The operations at the Newhall Yard and Maintenance Facility will commence with passenger service of the Project. 

In preparation for construction, VTA proactively developed a plan to outline the modifications to access and circulation in and within the station areas during construction. This plan is referred to as the Construction Transportation Management Plan (CTMP). The goal is to maintain site conditions that allow for efficient construction while minimizing impacts to the community to the extent feasible. To develop the CTMP, VTA and the Contractor engaged in regular coordination with key stakeholders including the Cities of San José and Santa Clara, and adjacent businesses and developments within the construction area. Watch the video of the March 2023 Public Meeting to learn more about the CTMP and early construction at the West Portal. Future CTMPs will be drafted for the other constructions areas as more details about the construction means and methods are developed.