Twin earth-pressure-balance tunnel-boring machines Mom Chung and Big Alma emerged into a water-filled retrieval pit in the heart of San Francisco in June, having raised the bar for cutting through variable soils without affecting existing infrastructure. The 350-ft-long, 750-ton TBMs, part of the $1.5-billion Central Subway extension of the Third Street Light Rail transit project, charted a course through the dense subgrade and emerged within an inch of the target, 1.7 miles away from the launch pit.
"We demonstrated new assumptions about how much ground loss can be expected," says Matthew Fowler, project manager for the Parsons Brinckerhoff/Telamon Joint Venture design team. Careful measurement and monitoring boosted accuracy, requiring less fill to overcome soil displacement. The strategy will make future projects less costly, Fowler says.