Tunnel-boring machine Bertha will be felt under Seattle; Viaduct closures likely

by: John Knicely Updated:

SEATTLE - Crews are hard at work assembling Bertha, the world's largest tunnel-boring machine, which will make way for the new Highway 99 underneath Seattle.

When Bertha starts digging in the early summer, you will likely feel it in downtown Seattle.

The 300-foot-long machine is still in pieces near Seattle's waterfront. What can't be seen from the road is the 100-foot-deep launch pit, where a giant crane will lower the pieces for assembly.

"Then we have what we call a 'tunnel in a box,'" said Washington State Department of Transportation spokesman Travis Phelps. "It's a controlled environment this machine is going to move through for its firs 1,000 feet. And in there we can see, is the machine doing exactly what we need to do? Is there anything we need to adjust?"

Bertha is an electric powered machine that will drill 6½ feet at a time. Each dig will take 30 to 90 minutes. If you're in downtown Seattle you'll likely know when it's at work.

"You might feel it. It depends on what it's hitting underground," said Phelps. "You could actually hear it as well."

WSDOT anticipates having to close the Alaskan Way Viaduct while Bertha digs underneath it. At this point, that hasn't been determined, and it's unclear how long and how often the Viaduct will close.

The 99 Tunnel is unique, said Project Manager Chris Dixon, who has been doing digging projects for over 30 years. He said the toughest part will be building the underground highway as they continue to drill.

"When the machine gets out there 2,000 feet, we're going to start building the interior structure," said Dixon. "And that's going to follow along behind the tunnel drive."

The test drilling will take several months to work out the kinks and drilling on the dirt under Seattle is expected to start by late fall. The contractor has a deadline to open the new highway to traffic by the end of 2015.