The Alaskan Way Viaduct is on track to close for two weeks in March if tunnel-boring machine Bertha keeps up its progress.

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Tunnel-boring machine Bertha has broken through its concrete access vault and is tunneling beneath Seattle, the Washington State Department of Transportation (WSDOT) said in a Thursday update.

The Highway 99 tunnel machine, now approaching South Main Street, has moved 73 feet since it was repaired last month for restart, the agency said. It took more than two years to get the dysfunctional tunneler chugging along again.

Chris Dixon of Seattle Tunnel Partners, the contractors in charge of the project, said Bertha has left the “loose, more problematic soils we were into during the start of the drive.”

Still, Bertha is considered to be in a “testing phase,” Dixon said. Two crews are working 12-hour shifts, six days a week, until Bertha reaches a “safe haven” near Yesler Way for three weeks of planned maintenance, he said.

Dixon said STP will be pushing workers “about as much as we like someone to work” through the end of January. He said some have long commutes.

“By the time they get home to sleep, they’re getting 5-6 hours of sleep a night … That can take a toll over time,” Dixon said.

Once Bertha reaches the “safe haven,” the machine will stop for about three weeks as workers go into the chamber behind Bertha and check cutter tools and soil conditions.

“It’s our last opportunity to stop in what we consider to be a controlled environment that allows us to do quite a bit of work,” Dixon said.

After the pit stop, Dixon expects tunneling to resume in March beneath the Alaskan Way Viaduct (the elevated part of Highway 99 in Seattle) for 24 hours a day, seven days a week, Dixon said. “We don’t want to stop at any point in time underneath the viaduct,” he said.

As Bertha moves beneath the viaduct, WSDOT will close Highway 99 in downtown Seattle for two weeks as a precaution.