SJ BART extension insight: Double-bore tunnels cheaper/simpler than single-bore

Image by Wikimedia Commons

Transit analysis blog Systemic Failure sheds light on why many Bay Area transportation advocates, and even BART staff, are questioning VTA's insistence on developing a single-bore tunnel through DTSJ to Santa Clara: single-bore (two tracks stacked deeper underground) incurs extra operational costs/hurdles, while impairing passenger accessibility.

For the past year, the VTA has been selling the idea of doing its BART tunnel in a single bore instead of twin bores. BART staff has been very opposed. If you don’t know what this all means, here is a diagram to show the difference:

The twin bore on the right is the usual BART configuration. The single bore on the left is what VTA is proposing to build.

The VTA describes the single-bore concept as a new and revolutionary approach to reduce costs and construction impacts. In fact, the idea has been around forever, and is really only useful in situations of limited ROW or for other technical issues. The single-bore design has a number of downsides, which are obvious just by studying the above diagram.

The first problem is that platforms are deeper underground. Passengers would have to descend several additional levels to reach the trains. The station would also have fewer entry points on the surface. This layout would be especially bad if it were used at Diridon station and the CHSRA persists in building its HSR tracks on an aerial. Transferring from BART to HSR would entail a trip from 85′ underground to some 60-80′ up in the air. If you enjoyed playing Chutes-and-Ladders as a kid that might be fun, but not so enjoyable for people with luggage or wheelchairs.

The second problem with the single-bore is that the stacked design does not allow for track crossovers. Crossovers give BART the operational flexibility to move around a disabled train. The stacked design eliminates a planned crossover near the downtown station.

The single bore station also brings higher operational costs. Running all those additional elevators and escalators to the lower depths adds $1.5 million in annual costs. The VTA staff report concedes that the construction costs are comparable for single vs. twin bore. Thus taking into account the higher operational expenses, the single bore has higher life-cycle cost.

This article originally appeared in Systemic Failure. Read the whole thing here.

Related:

• Perspective (part III of III): What it'll take to grease the wheels of local/State transit

• Transportation professionals on BART extension: Let's actually examine costs of single vs. twin-bore tunnels

• Perspective: Who's reviewing BART's (highly questionable) fiscal choices?

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