Tunnel-boring machines Big Alma and Mom Chung might be finished with Muni's Central Subway project, but they have thousands of feet left in their arsenals. Upon refurbishment, they would be suitable for more local projects, such as extending the subway farther or building tunnels for BART's extension to San Jose.
When Big Alma broke through the ground last week, and at an event Monday with city officials celebrating the end of tunneling, its cutter head appeared more worn out than that of Mom Chung, which surfaced June 2. Each machine carved through 8,500 feet of rock and soil, but both finished in “beautiful shape,” said Sarah Wilson, resident engineer representing the San Francisco Municipal Transportation Agency.
Big Alma likely encountered more abrasive ground, or the tunnel-boring crew elected not to change its cutters and scrapers because the last stretch was primarily soil.
“I don't think they will have any trouble reusing the machines,” Wilson said. “They just will need to put a new set of cutters on.”
As typically happens in the construction world, the contractor — in the Central Subway's case, it is Barnard Impregilo Healy — resells the tunnel-boring machine to the manufacturer, The Robbins Co., to refurbish and make available for other projects.
Already, interest has been expressed for refurbished Big Alma and Mom Chung from around the world, according to Central Subway head tunnel design engineer Matt Fowler.
On another note, the machines, with approximately 20-foot diameters, are very well-suited size-wise for BART's extension to San Jose, Fowler, 57, said.
“It would be wonderful,” he said. “It's not unheard of that they stay close to home.”
BART is currently building more track south from Fremont and into northern San Jose, with the goal of one day going into the downtown area. For a future project, 5 miles of the BART extension in San Jose would require underground tunneling, although designs are still being refined. If the refurbished Central Subway tunnel-boring machines meet the criteria for BART, they “certainly would be worth considering,” said Bernice Alaniz, a spokeswoman for the Santa Clara Valley Transportation Authority, which is building the extension because it is out of BART's district.
An extension of the Central Subway to Fisherman's Wharf is being studied. But even if that moves forward, Big Alma and Mom Chung are not designed to make a turn that would be necessary for the project.
For now, the machines need to be extracted from the retrieval shaft at the old Pagoda Theater in North Beach.
Big Alma will be the first to go. A tunnel-boring machine cradle will be installed at the bottom of the retrieval shaft and Big Alma will be pushed 16 feet out. Its cutter head will be sliced in half and removed with a crane. The remainder of the machine will also be taken apart for removal.
Mom Chung will follow, and both machines — named after historic San Francisco women — should be fully disassembled within 16 weeks.