After sitting virtually stalled in her tracks for more than a year, Bertha, North America's largest-diameter tunnel boring machine (TBM), has finally been rescued. Before she can begin excavating again, contractor Seattle Tunnel Partners (STP) must open up the 57.5-ft-dia cutter head to see how extensive repairs need to be.
Bertha had sat under downtown Seattle since December 2013 after tunneling 1,083 ft of its 9,270-ft mission to construct a new Highway 99 replacing the aging and seismically vulnerable Alaskan Way Viaduct.
But this month, crews finally reached Bertha after digging a 120-ft access shaft in front of where she sat and plan to haul a portion of the 326-ft-long machine to Seattle's surface for repairs.
"We don't know what the full extent of the repairs will be until we get all of the components to the surface," says Chris Dixon, STP project manager. "Assuming that it's the main bearing and seals that we've been talking about and things go according to plan, we'd be resuming tunneling in August."
After Bertha shut down in 2013 due to overheating during early tunneling, the STP joint venture of New York's Dragados USA, ACS Group of Spain and California's Tutor Perini Corp. started planning to replace seals in the main bearing that allowed debris to infiltrate the machine. It was soon determined that the machine needed a new main bearing, which manufacturer Hitachi Zosen of Japan had already built as part of the original contract.
Crews mined through 20 ft of concrete that made up the temporary shaft wall. For the rest of March, crews expect to weld pad eyes onto Bertha's steel shield and then cut them into three pieces in order to lift the shield out of the pit.
Removing the shield exposes the cutter-head drive unit and cutter head at the front of the machine, which is the main component that will be removed. That task involves a 2,200-ton pick that requires a 1,300-ton custom-engineered gantry system from Dutch-based Mammoet, sitting ready above the access shaft.
"We're making preparations to lift those three front-shield parts out and then we're also in the process of disconnecting everything within the TBM so that the cutter drive unit and cutter head can be lifted out," Dixon says.
In April, STP and Hitachi Zosen plan to disassemble the parts at the surface and then reassemble the new components in May, "hopefully lowering those components back into the shaft" in the reverse order they were removed," says Dixon.
Inside the machine, bigger openings in the cutter head, longer mixing arms to stir dirt, a heavier bearing and steel rods and plates to reinforce the front end will give Bertha more robustness.