In 2001, The Santa Clara Valley Transportation Authority (VTA) completed a study that identified an extension of the Bay Area Rapid Transit (BART) regional rail system would provide the best transit option to two highly congested San Francisco Bay Area commute corridors. The planned 25.6km (16 mile) extension ultimately would include six stations and an 8km tunnel under Downtown San Jose. In 2008, a phased delivery approach was pursued. The first phase, a two-station, 16km extension broke ground in 2012 with passenger service commencing in June 2020.

As the first phase neared completion, design progressed on the four-station, 9.6km second phase. Early in design, twin-bore tunnels were planned for the 8km that would traverse under Downtown San Jose. To minimise street level and business impacts in the heart of Silicon Valley, VTA explored innovative single-bore tunnelling methodology and received environmental clearance for both twin- and single-bore options. VTA is now proceeding with a US$6.8bn plan to construct an approximate 14.6m diameter single-bore tunnel that accommodates two trainways with portions in stacked and side-by-side track configurations. This allows two downtown stations to be built off street and connect to the tunnel by short mined adits. This minimises surface disruptions to residents and businesses while massive construction occurs under the street.

VTA’s large diameter single-bore project will be a first for the US rail transit industry. Other large diameter tunnelling projects have primarily been for roadways. These have led to increased confidence in the construction method. Conventional subway construction typically entails cut-and-cover for stations. However, this would have required the station excavation to be around 457m long and 18m deep from curb to curb on one of the city’s busiest streets. Construction would be highly disruptive and likely to have severe consequences for businesses recovering from Covid-19 impacts.

The large-diameter, single-bore tunnelling method avoids most in-street impacts because the majority of tunnel and trainway construction occurs within the tunnel, and the stations are constructed off-street. Advancement of tunnelling and underground excavation methods allows operators to precisely monitor and control ground movement behaviour, as demonstrated in the SR-99 tunnel construction in Seattle, to achieve minimal surface settlement and associated building/ utility impacts. Hundreds of soil borings and cone penetrometer soundings performed for the project have revealed the geologic conditions in the area are alluvial soils that include clays, silts, sands and fine gravels. Water pressures in the groundwater basin are generally high, with the water table near the ground surface, and in some places above the ground surface where artesian pressures may exist.

With the promise of a regional BART service and reduced construction impacts, many commercial, residential and mixed-use developments have been planned around the future stations, including the future Google Campus near BART’s future Diridon Station, residential units near the downtown San Jose BART station, and a bloom of economic activity and vitality near the future Santa Clara and 28th Street/Little Portugal stations. Creating access to a one-seat ride throughout the Bay Area has spurred a flurry of residential, retail and office development that will substantially improve access to jobs, housing and recreation for residents of Santa Clara County.

A solid contracting strategy is key to delivering an infrastructure project of this magnitude. VTA reached out to other agencies that have undertaken similarly sized and complex tunnelling and transit projects, as well as reaching out to the contracting community to generate a comprehensive contracting plan to deliver this project.

The project will be delivered in four contracts:

  • Systems – includes all rail transit and communication infrastructure.
  • Newhall Yard and Santa Clara Station – includes the yard, maintenance facility and above-ground station.
  • Stations – includes the three underground stations, streetscapes and final sitework.
  • Tunnel and Trackwork – the largest of the four contracts, includes demolition, site preparation and station excavation support, the main tunnel and connecting adits, tunnel ventilation, egress facilities, guideway and trackwork. To be delivered as a progressive design-build; the remaining contracts will be delivered as traditional design-build.

Early procurement activities are underway. VTA anticipates releasing the Request for Proposal (RFP) to the shortlisted proposers in summer 2021, with RFPs for the other contracts released in 2021 and early 2022. Construction is anticipated to begin 2022, with major construction complete in 2028, followed by two years for testing and certification.