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Final breakthrough for soft ground boring machines in San Francisco

The breakthrough of the second of two earth pressure balance machines (EPB) from Robbins marked the completion of twin tunnels for San Francisco’s central subway project in California, US.

The first specialised Robbins EPB machine completed its journey in early June and the both Robbins machines achieved advance rates of up to 40m in 24 hours and 513m in one month.

Two 2.5km long tunnels were excavated as part of the second phase of the San Francisco Municipal Transportation Agency’s (SFMTA) third street light rail transit project, which aims to be operational in 2018.

Low cover, nearby utilities and sensitive structures required analyses and design precautions in order to limit settlement impact, especially when crossing directly below live rail tunnels for the Bay Area rapid transit (BART). Compensation grout pipes were put into place as a contingency, but were not needed as the machines passed just 3.4m below the rail lines with minimal settlement.

The tunnel boring machines (TBMs) were operated by the Barnard, Impregilo and Healy joint venture (BIH JV) and excavated through ground ranging from soft soils to thinly bedded siltstone, shale and sandstone bedrock, as well as concrete diaphragm walls.

A mixed face cutterhead was selected and designed to excavate the anticipated wide variety of ground, while active articulation was integrated between the TBM shields to lessen the risks of segment damage, ring deformation, and settlement during boring through curves.

“BIH JV did a superb job driving the two Robbins EPB machines through the tight 137m radius curves and varying ground while keeping a close eye on face pressures, annulus grouting and steering producing settlements well below our estimates. In my book, the EPB technology applied here has set a new standard for what can be done in urban soft ground tunnelling,” said Matthew Fowler, project manager for the Parsons Brinckerhoff and Telamon joint venture design team, who were in charge of utility relocation and tunnel design.

Work on two open cut station sites and one mined site is underway and when the second phase of the third street light rail project is complete, the 2.7km central subway line – excavated at depths ranging from 15m to 45m below the city – will extend light rail service as well as provide direct connections to two of the Bay Area’s largest regional commuter rail services.

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