What’s in a name? If you ask San Francisco’s Municipal Transportation Agency, when it comes to 350-foot long tunnel borers, it’s not only a tradition, it’s a safety precaution. “It’s bad juju to not name tunnel-boring machines. It’s done all over the world,” says Mike Sinon, safety manager for Barnard Impreglio Healy, who heads up the construction team that’s tunneling a new underground central subway for San Francisco that will stretch from the south of Market street Caltrain station to Chinatown.
San Francisco’s tunnel borers, Mom Chung and Big Alma, named respectively for the first American-born female Chinese physician and Alma de Bretteville Spreckels, a San Francisco socialite and philanthropist, each weigh 750 tons. The machines are made up of three parts: a rotating cutting wheel, which is attached to a steel cylinder, and 300 feet of tunneling equipment. “The machine spews a conditioning foam that softens the rock and dirt,” explains Michael Cabanatuan, writer for the San Francisco Chronicle, “Which is ground up and inhaled by the cutter head and transported by a giant screw onto conveyor belts that haul the diggings out of the tunnels. The machines excavate and build about 50 feet of tunnel a day. Big Alma is quicker at 54 feet a day compared to Mom Chung’s 44-foot average, but Mom Chung holds the performance record of 96 feet in a single day.”