Single bore option for BART to San Jose 01 Jun 2017

Extension of the Bay Area Rapid Transit system, BART, to the city of San Jose on the south end of San Francisco Bay in northern California, is into perhaps its most positive stage of development and realisation. After years of stalled progress, funding sources are now identified and with a procurement programme in place, the project developers are exploring signal bore options for the underground stations and track alignment ahead of plan start of construction in 2019.

Fig 1. Details of the BART extension under San Jose

The initial plan for the 6-mile extension of BART services beneath downtown San Jose (Fig 1) was to link three underground stations with conventional twin-tube TBM-bored running tunnels. Planners of the underground alignment of the BART to San Jose metro rail extension are now exploring the single-tube, twin-track running-tunnel concept with the tunnel divided by a horizontal deck for trains to run in the upper and lower sections of the tunnel in a stacked configuration and allow the platforms at stations to be accommodated within the same single-tube, TBM-bored tunnel (Figs 2 and 3).

Known also as the Barcelona method, the concept was developed in Spain and used on the city’s Line 9 project and to positive effect. The advantages of the system are several fold. In particular, the disruption and cost of extensive open-cut excavation of stations into the streets of the city is avoided. Excavation at stations is confined or restricted to smaller off-line ingress and egress access structures to cross passage adits to the underground station platforms, stacked and aligned in the single-tube double-track tunnel. In addition, a single tube double track tunnel of approximately 12m diameter excavates less material than twin parallel single tube tunnels of 6-7m diameter each, and uses less concrete material in its precast segmental lining. The cost of one larger diameter TBM is also potentially less than the cost of two smaller diameter machines. The pros and cons of the two different methods are being considered in the current schedule of studies.

Current estimates of the (9.5km) underground Phase II Silicon Valley Extension beneath San Jose is costed by the Santa Clara Valley Transportation Authority (VTA) at US$4.7 billion. Of this, US$1.5 billion (nearly 30%) is being applied for allocation from the New Starts Program of the Federal Transit Administration (FTA). $2.5 billion will be raised via a local sales tax increase approved by Santa Clara County voters in the November 2016 ballot.

In December 2016, a VTA spokesperson told TunnelTalk: “Based on the current schedule, construction procurement activities are planned for late 2018 with heavy construction scheduled to start by mid-2019. The format and specific procurement process is likely to be finalized in early 2018, with the evaluation of contracting strategy and packaging likely to begin in mid-2017.”

The Phase II project has provisional New Starts Project Development (NSPD) status with the FTA and design is at an advanced stage ahead of confirmation of environmental permitting and expected FFGA (full funding grant agreement) from the FTA by December 2017.

Fig 4. The extension to Silicon Valley will add another 4.5 miles of underground infrastructure to the 112-mile BART network

The BART to San Jose project has had a checkered history.

In 2001, VTA and the BART District entered into a comprehensive agreement to extend BART services into Santa Clara County. VTA’s BART Silicon Valley Program, is responsible for ensuring the intent of the comprehensive agreement is fulfilled. Beginning in 2002, VTA initiated planning, environmental, and conceptual engineering activities for the full 16-mile extension from the Warms Springs Station end of the East Bay alignment in Alameda County all the way to downtown San Jose and Santa Clara city (Fig 4).

In 2005, VTA initiated preliminary engineering activities followed by 65% engineering activities in 2007. By the end of 2008, VTA made the decision to deliver the program in phases. One of the major funding sources for the BART Silicon Valley Program is the sales tax measure passed by Santa Clara voters in 2000. As a result of the economic downturn in 2007/ 2008, the amount of expected revenue from the tax measure was substantially less than originally projected. As such, in 2008, the decision was taken to deliver the program in two phases.

Subsequent to this decision, design activities for Phase II were put on hold, while Phase I for the 10-mile at-grade extension from the BART Warm Springs Station in Alameda County to Berryessa Station in San Jose, continued and into construction. Phase I construction is currently ahead of schedule and the extension is expected to be open for ridership in late 2017.

Planning and environmental clearance efforts for Phase II were restarted in 2013. Current focus for the 6-mile Phase II project to extend services from the Berryessa Station underground through downtown San Jose and to surface again for an at-grade station at Santa Clara, includes the reevaluation of tunneling methodology and the tunnel alignment and stations configurations.

Operating station tunnel on the Barcelona Metro Line 9

The tunnel segment of Phase II is approximately 4.5 miles long beginning at the east portal in the Alum Rock area of San Jose and concluding at the west portal adjacent to I-880, south of Avaya Stadium in San Jose. Based on the current design, tunnel excavation would be by using a pressurized closed-face TBM. VTA anticipates a decision to be made on the tunneling methodology in mid-2017 and if the single-bore tunneling methodology is adopted, preliminary studies indicate the off-street station and entrances would be cut-and-cover construction with the platform in the bored tunnel.

The Phase II project is located in the Santa Clara Valley, which extends southeastward from San Francisco Bay, and is a northwest/southeast trending valley within the Coast Ranges Geomorphic Province of Northern California. The Valley is an alluvium-filled basin located between the Santa Cruz Mountains to the southwest and the Diablo Range to the northeast and is covered by alluvial fan, levee, and active stream channel deposits with marine estuary deposits located along the bay margins. These unconsolidated deposits cover Tertiary through Cretaceous age bedrock.

The water table in the area is approximately 10ft below ground surface with the tunnel being constructed below the water table at an average depth of 40ft below ground surface to the tunnel crown. TBM operation and hyperbaric interventions are anticipated and under hydrostatic pressures of typically between 1 and 2 bar. Beneath the rivers and retaining walls, where the tunnels are at their deepest, hydrostatic pressures of between 3 and 4 bar are anticipated.

As design progresses into final stages for the project, the only threat to its construction is suggestion from the Trump administration that it will cut severely the Federal budget allocations to mass transit projects.