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SR 99 tunnel
SR 99 tunnel project workers are shown placing the rings that make up the Seattle tunnel’s outer wall. (Via WSDOT / YouTube)

Spinning and pushing its way beneath Seattle, Bertha the SR 99 tunneling machine moved past the 5,280-foot mark on Thursday morning, creeping ever closer to an end point it’s expected to reach next spring.

Like a “moving factory,” Bertha continues to bore the tube and construct the rings that make up the outer wall of the nearly 2-mile tunnel. The journey began south of downtown, ran beneath the waterfront, and will conclude near the intersection of Sixth Avenue North and Harrison Street. GeekWire got an inside look at that receiving pit last month.

According to the Washington State Department of Transportation, the machine’s cutterhead is now located more than 200 feet below First Avenue, just north of Stewart Street. As it enters the seventh of 10 tunneling zones, Bertha will be at the deepest point of the operation.

Bertha stages
Bertha’s progress, and where she still has to go. (Via WSDOT)

So how do engineers with the SR 99 tunnel project get Bertha from point A to point B, curving and descending and rising along the way? A new video offers footage from inside the tunnel and a machine operator explains the high-tech process.

“You’ll probably never see anything like this on any other TBM (tunnel boring machine),” says Jerry Roberge of Seattle Tunnel Partners as he shows off Bertha’s command center. “This has got all the bells and whistles.”

The video points out that an elaborate computer system measures 6,000 different points on the machine 24 hours a day, 7 days a week. There is no GPS underground, the narrator says, noting that the entire tunnel route is mapped with a laser-guidance system that moves forward as the TBM moves forward, creating a roadmap for workers.

“This machine actually responds very well,” Roberge says. “It doesn’t take a lot of pressure or power to get it to go where you want it to go.”

Roberge further explains how the machine is steered much like a car and how push jacks apply pressure similar to the way you would push a drill bit into a surface.

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