Tunneling 100 ft below a busy city with varying substructure is a delicate job, especially when the work comes within 8 ft of existing tunnels. Such is the case on San Francisco's new $1.5-billion Central Subway Project, which began major subterranean excavation last month.
"The tunnels pass through both soft ground and Franciscan formation, which is heterogeneous rock that is not predictable except in its unpredictability," says Sarah Wilson, a San Francisco Municipal Transportation Agency (SFMTA) resident engineer.