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December 20, 2013 at 3:49 PM

What’s blocking Bertha? Groundwater delays inspection

So much groundwater is flowing into the front of the Highway 99 tunnel machine that it will be another two weeks before workers can get a look at what’s blocking “Bertha,” the tunnel-boring machine, Seattle Tunnel Partners’ Chris Dixon said this afternoon.

Water is spilling through the 57-foot diameter cutter head, into a sealed-off 5-foot mixing chamber for mud and rocks, which is nearly full to the top, officials said. Six wells recently have been sunk to try to pump water away from the cutting face, but Bertha is still filling up. The state Department of Transportation (DOT) is hoping to remove enough water that workers can see the mystery object that has stopped Bertha in her tracks since Dec. 6.  Four more wells will be added, which may or may not lower the water level inside Bertha.

A couple of days ago, crew members opened one hatch and took a very quick look at the cutting face, said Matt Preedy, DOT deputy project director. But they didn’t see much, he told reporters today at the Milepost 31 interpretive center in Pioneer Square.

The object is thought to be an unstable boulder, or possibly a giant “glacial erratic” rock that migrated to Seattle on a sheet of spreading ice in the distant past. If “de-watering” fails, the cutting-face area would need to be placed under extreme air pressure to fend off flows of water and mud, and then specially trained divers would venture into the sealed area.

The slow pace increases the possibility of cost overruns in the $1.4 billion contract or delays to the December 2015 completion date. The contract includes a shared $40 million risk fund to cover such surprises. When asked, “Is this a $40 million blockage, or more?” Preedy and Dixon, who is the project manager, said they didn’t know.

“It’s way to early to make any schedule or cost predictions at this point,” Preedy said.

Dixon said he’s confident the team will be able to remove the blockage, using hand-held tools and built-in drills on the machine, and restart the huge rotary cutters. It’s just that identifying the object is taking time because of the water flows.

 

 

Comments | More in Traffic & Transit | Topics: Bertha; Highway 99 tunnel

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