Tunnel-boring machine Bertha has advanced 300 feet in the past three weeks, reaching a maintenance stop next to the Alaskan Way Viaduct.

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Tunnel-boring machine Bertha has reached the maintenance stop known as “Safe Haven 3,” just before its spring dive underneath the Alaskan Way Viaduct.

The giant drill traveled about 300 feet in the past three weeks, the Washington State Department of Transportation said in an update Monday afternoon.

This haven is a concrete box that was built in 2013, so that crews could reach the giant cutting face, to perform repairs and replace drill teeth.

The 57-foot, 4-inch diameter drill arrived Saturday morning, said Laura Newborn, a state project spokeswoman.

Maintenance could take about a month, and no restart date has been set, WSDOT said. Seattle Tunnel Partners (STP) intends to drive Bertha out of the box in April, and drill below the foundations of the viaduct, where Yesler Way meets the waterfront.

This is generally considered the riskiest part of the route from Sodo to South Lake Union, because of the potential to destabilize the viaduct or nearby brick buildings in Pioneer Square.

While the machine digs under the viaduct, WSDOT intends to close the viaduct to traffic for a couple of weeks as a precaution. Such a shutdown would also make it easier for STP to perform emergency grout injections to fill voids in soil or other urgent work if things go awry.

Already, the viaduct sank just more than an inch in late 2014, while groundwater was being removed at the job site — to enable repairs to Bertha during a two-year delay. And even before the project, both the state and tunnel contractors reinforced the viaduct columns and foundations at Yesler. They sank nearly six inches in the decade following the 2001 Nisqually earthquake.

Soft soils are near the surface, though Bertha is now descending toward firmer clay as it heads to downtown. The top of the tunnel machine is currently about 60 feet underground.

Just before this recent progress, Gov. Jay Inslee and WSDOT had suspended drilling work, in response to a Jan. 12 sinkhole.

Bertha has now gone 1,560 feet of the 9,270-foot route, since drilling began July 30, 2013. Barring any more crises, the four-lane, tolled tunnel could open in spring 2018.