Clearing obstacle in tunnel machine’s path may take weeks
It could take several weeks to figure out what’s blocking the Highway 99 tunnel machine and to clear the path.
Seattle Times transportation reporter
It will take about two weeks to determine what is blocking the Highway 99 tunnel machine and plan a strategy to remove the object, Chris Dixon, project director for Seattle Tunnel Partners, said Wednesday.
The difficulty of diagnosing the problem suggests that it will take weeks more to clear the path for tunnel machine Bertha to resume digging toward downtown and South Lake Union.
Specially trained workers could be sent to the site next week, to peek outside the tunnel machine’s cutter head to check out what’s in Bertha’s way. The machine can retreat about 18 inches, and compressed air will be forced into the small space in front of the cutter head to fend off loose dirt and allow workers to view the problem.
The leading theory is that Bertha hit a boulder, but the soil around it is too soft to hold it snugly and allow the rotary cutter head to crack the rock apart. Drilling stopped after the cutting head spun without any forward progress Friday night, Dixon said.
The top of the tunneling machine is almost 60 feet below ground, while the bottom is 110 feet down, so the upcoming inspection — by professional divers accustomed to working deep below the surface — must occur at greater than atmospheric pressure.
Dixon said the quickest way to remove the object would seem to involve divers breaking it up using power drills and hammers, along with the drills built into Bertha. Seattle-based Ballard Marine Construction does tunnel work internationally and is on call for the Highway 99 project.
The other option is to work from above, under normal air pressure, to drill down and break up the object or to lift it out. That likely would require a protective wall or pit to be installed, holding back sand and groundwater — like the launch pit in Sodo.
“It would take several weeks to build that,” Dixon said Wednesday at a news conference on Pier 48, near where the machine is stuck.
An update is expected Friday.
It’s way too early to know whether the obstruction will cause cost overruns, or delay the December 2015 completion date of the tunnel that is set to replace the Alaskan Way Viaduct, said Matt Preedy, deputy project administrator for the state Department of Transportation.
He said planning must be done carefully, to ensure worker safety and avoid damage to the 57-foot diameter, $80 million machine — which still has 1½ miles to go toward South Lake Union.
Mike Lindblom: 206-515-5631 or firstname.lastname@example.org. On Twitter @mikelindblom