On April 23, a ship carrying the first of two Robbins mixed ground EPBs docked at San Francisco’s pier 80. Robbins is providing the two 6.3-m (20.7-ft) diameter EPBs for the city’s newest rail route – the Central Subway. The machines for the Barnard/Impregilo/Healy JV were nicknamed “Mom Chung” and “Big Alma,” after local historical figures.
The completed Central Subway line will provide service between the Chinatown and Mission Bay areas. “The unique challenges of this project include the dense urban environment and curved tunnel alignment. Use of the TBM will help us to minimize impact to the community, and the design of the TBMs will help us to negotiate curves using active articulation,” said Alessandro Tricamo, Chief Engineer for the Barnard/Impregilo/Healy JV. Both machines were designed to enable smooth excavation around tight turns down to 140 m (450 ft) in radius.
The two EPBs were additionally engineered with mixed ground cutterheads using knife edge bits and carbide tools to excavate parallel 2.5-km (1.5-mile) tunnels. Shaft-type screw conveyors will aid in excavation through poorly consolidated silt and sand deposits of the Colma formation below the ground water table.
Robbins horizontal belt conveyors will travel behind both machines for efficient muck removal during tunneling. “We estimated that a continuous conveyor would help us improve the production rate of the TBMs on steep slopes (up to 7 percent), where a traditional mucking system using a locomotive would have been a challenge,” said Tricamo.
Project owner San Francisco Municipal Transportation Authority (SFMTA) has set a planned launch date for the first machine in mid-June, and for the second machine by the end of August. The Central Subway is part of the SFMTA’s Third Street Light Rail Transit Project. Phase 1 of the 10.9-km (6.8-mile), two-phase project began revenue service along the Third Street corridor in April 2007, restoring light rail service to a high transit ridership area of San Francisco for the first time in 50 years.
When Phase 2 is complete, the Central Subway line will extend light rail service to the Financial District and Chinatown, as well as provide direct connections to BART and Caltrain, two of the Bay Area’s largest regional commuter rail services. The Central Subway is scheduled to open to the public in 2018. It is estimated that the new subway will serve about 65,000 passengers daily by 2030.