By Ronak Naik and Anthony Bauer, P.E.

When the Santa Clara Valley Transportation Authority (VTA) began considering a tunnel through downtown San Jose, California, in the early 2000s, twin-bore construction was the likely method.

Today, advancements in tunneling technology are allowing VTA to move forward with a $6.5 billion plan to construct a single-bore tunnel large enough to accommodate two trainways and station platforms. The relatively new method promises to minimize surface disruptions to downtown residents and businesses that will be virtually unaware of the massive construction project occurring below them.

Represents an Industry First

Project rendering.

The VTA’s proposed project would be a first for the U.S. transit industry. There is only one other large-diameter, single-bore transit tunnel in the world, in Barcelona, Spain. Other large diameter tunneling projects have been successfully completed around the world, leading to increased confidence in the construction method, which allows owners to pursue these minimally invasive concepts.

Conventional subway construction entails the use of cut-and-cover techniques that involve removing the street surface, relocating and protecting existing utilities, and excavating from the street level. For example, using cut-and-cover techniques in San Jose would require the station excavation to be approximately 1,500 feet long, 60 feet deep, and extend from curb to curb on one of the city’s busiest streets and downtown centers. Construction would be highly disruptive to daily activities and likely have devastating consequences for existing businesses and potential developments.

The large-diameter, single-bore tunneling method avoids most in-street impacts because the tunnel, trainways, and boarding platforms are constructed below ground within the tunnel. Modern tunnel boring machines (TBM) use “smart” technology that allow operators to precisely monitor and control ground behavior through pressurization, as demonstrated in the SR 99 tunnel construction in Seattle, to achieve nearly zero surface settlement and associated building/utility impacts.

Facilitates Economic Development

With the promise of the Bay Area Rapid Transit (BART) expansion service and minimal surface disruption during tunnel construction, development has begun to flourish in downtown San Jose. As it currently stands, many commercial, residential, and mixed-use developments have been planned around the future BART stations.

The large-diameter, single-bore methodology minimizes surface disruptions, thus reducing environmental concerns and simplifying construction. With a single-bore tunnel’s off street station entrances, the majority of station construction can occur independently from tunnel construction. Decoupling these activities allows the owner to issue separate tunneling and station construction contracts resulting in greater procurement and schedule flexibility.

Further, large-diameter, single-bore tunnels eliminate mined cross passages which accompany twin-bore tunnels—often one of the highest risk activities in an underground transit project. Excavation of cross passages sometimes cause severe surface settlements, sinkholes or inflows into the tunnel. A single-bore tunnel provides the ability to locate cross passages within the tunnel itself to connect adjacent trainways.

Some considerations on the single-bore concept include the limited number of contractors with large-diameter tunnel construction experience and higher capital costs for the TBM. However, depending on the project characteristics, a single-bore tunnel configuration can be cost competitive with a traditional twin-bore arrangement. With this developing technology and increasing advances in TBM controls, it is critical to pay attention to contractor’s, designer’s, and machine manufacturer’s pre-qualification process as part of the tunnel contract procurements.

Positions the U.S. as a Leader

Single-bore tunnel construction can offer a number of environmental, community, and construction impact benefits and should be considered by any agency exploring an underground transit project. Demonstrating the single-bore tunnel’s viability in San Jose will serve as a model for other organizations exploring how to better construct infrastructure projects in cities.

Ronak Naik is a transportation engineer for the Santa Clara Valley Transportation Authority. Anthony Bauer, P.E., is West Region tunnel practice lead for HNTB Corp. See more stories about transportation design and construction industry technology and innovation in ARTBA’s “Smart Design & Construction” supplement.