A quick peek last week by a diver inside Bertha didn’t provide much help to officials trying to figure out what is blocking North America’s largest tunnel-boring machine (57.5 ft in diameter) under downtown Seattle. And now powerful water is making the investigation even more difficult.
Already crews have dug six wells from the surface down to the Bertha-made Highway 99 tunnel to help draw underground water away from the route. That helps clear water, but not enough. The Washington State Dept. of Transportation says they’ll get to 10 wells—up from the expected maximum of eight—to help empty Bertha’s five-ft chamber directly behind the cutter head and let divers safely spend time figuring out what is obstructing the machine, which hasn’t churned dirt since Dec. 6 when the machine stopped 1,019.5 ft into the project.
If extracting water through the well system doesn’t prove effective enough, pushing water out of the way using compressed air at high pressure represents the only other solution, a proposal that could also prove dicey in dislodging loose or fill soil within the tunnel. Too much pressure could disrupt the 55 ft of soil above Bertha, a situation that joint contractor Seattle Tunnel Partners and WSDOT hope to avoid.
When Bertha hit an unknown obstruction on Friday, Dec. 6, officials knew they’d have to get inside for the machine’s first “intervention” on the 1.7-mile path that will eventually become the new Highway 99, replacing the aging Alaskan Way Viaduct. The groundwater extraction started nearly immediately, meant to clear out pressure for the divers, giving them space and time to maneuver within Bertha, to see if the obstruction was in fact a boulder spinning against the cutter head in loose soil or possibly a clog in Bertha’s extraction process.
The fleeting look late last week didn’t provide much information.
While officials hoped to know the problem—and, therefore, the solution—last week, it may not be until sometime next week, in a best-case scenario, that any new information is available. Then, figure in more time for divers to potentially carve away at a boulder, if that is what sits in the path of Bertha, with handheld power tools and the delay in the boring will easily stretch past a month’s time.
As part of a nearly $2 billion contract of a larger $3.1 billion project to replace the viaduct, WSDOT hopes to have the new highway open December 2015. Bertha will need to get moving again to make that happen.
Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He also writes for TIME, Popular Mechanics, Sports Illustrated and more. You can follow him on Twitter at @tdnewcomb or visit his website here.