SEATTLE -- The contractors drilling a tunnel under downtown Seattle say so much groundwater is flowing into the tunnel, it will likely be January before they can look at what's blocking their boring machine.
The state Department of Transportation wants to see what mystery object stopped the machine on Dec. 6. The object is thought to be an unstable boulder, or a giant rock brought along by a glacier sometime in the distant past.
Chris Dixon of Seattle Tunnel Partners said a sealed-off chamber is nearly full of water, even though six wells have been dug to try to pump water away from the boring machine known as “Bertha.”
“Imagine being in a swimming pool , the deeper you dive down in a swimming pool the more pressure you feel,” said Matt Preedy, WSDOT Alaskan Way Viaduct Replacement Program Deputy Administrator. “The ground has water in it and because the ground has water in it, anything in front of the machine is going to feel the exact same pressure as if it was in a swimming pool.”
Preedy says for workers to get a look at Bertha, they have to reduce the pressure by pumping out some of the ground water.
“That’s what’s going on now,” said Preedy. “We have de-watering wells that are operating; we have some more to install.”
Ten wells will be drilled to suck out the water and relieve pressure, allowing workers to safely access the boring machine. Crews will create a bubble of compressed air to keep all the ground and water in front of the machine from pouring through the holes in the machine.
“If the water pressure can’t be reduced, then the workers have to be specially trained,” said Preedy. “They have to be divers and they wouldn't necessarily be diving in water. They'd be diving in compressed air.”
Eventually, once tunneling teams get down there, the hope is they can use hand-drills to break up what’s blocking Bertha and finally free her path.
“I’m very confident we're going to identify what it is that's preventing us from moving forward,” said Preedy.
The machine is about 60 feet under Seattle streets and about one-tenth of the way toward completing a 1.7-mile tunnel.
It’s too early to tell if the project will take longer or cost more. Project managers are trying to soften the impact by getting a jump start on non-tunneling work like roadways, excavations and operations buildings.
KING 5's Zahid Arab and Liza Javier contributed to this report.