San Francisco’s Central Subway rail tunnels, snaking through downtown areas, were required to be driven below the existing Bay Area Rapid Transit (BART) line. Robbins provided two 6.3 m (20.7 ft) diameter EPBs to bore twin tunnels for the city’s newest rail route. The machines, operated by contractor Barnard/Impregilo/Healy JV, were nicknamed “Mom Chung” and “Big Alma”, after local historical figures.
Geological testing revealed layers of mixed ground. Two 2.5 km (1.5 mi) long tunnels were excavated through ground ranging from soft soils to thinly bedded siltstone, shale and sandstone bedrock, as well as concrete diaphragm walls. The TBMs were designed with a number of special features to efficiently manage the varied geology, navigate the steep and turning alignment, and bore in what has been rated as “Potentially Gassy with Special Conditions” by Cal/OSHA.
Steering the TBMs accurately through tight curves was one of the key challenges of the project. 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. Both machines were designed to enable smooth excavation around tight turns with active articulation to excavate curves as small as 137 m (450 ft) in radius. Robbins continuous conveyors offered efficient muck removal throughout tunneling.
Low cover, nearby utilities, and sensitive structures required analyses and design precautions in order to limit settlement impact. This was especially true of a 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 3.4 m (11 ft) below the rail lines with minimal settlement. Careful monitoring of the key TBM parameters ensured that boring did not impact the critical structures over the tunnel that ran through the heart of downtown San Francisco.
The first of the two machines holed through on June 2, 2014, the second followed close behind and broke through on June 11, marking the completion of the twin tunnels. Both Robbins machines achieved swift advance rates of up to 40 m (131 ft) in 24 hours and 513 m (1,683 ft) in one month.