Highway 99 tunnel machine Bertha, which is currently near Spring Street, restarted Monday after maintenance crews found its cutting teeth in better shape than expected.

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Tunnel-boring machine Bertha restarted Monday after being stopped for about four weeks so some of its steel cutting teeth could be replaced.

The giant drill is currently 120 feet below Spring Street near First Avenue, according to a Tuesday update from the Washington State Department of Transportation (WSDOT).

Contractors are building a $2 billion, double-deck Highway 99 tunnel to replace the old Alaskan Way Viaduct. It’s currently expected to open in 2018.

Related video: Riding up to Bertha through the Hwy 99 tunnel

A shuttle moves through the Highway 99 tunnel construction site under Seattle, up to the tail end of tunnel-boring machine Bertha. Read more. (Ken Lambert / The Seattle Times)

The stoppage began June 23. Crews worked underground in short shifts, at 2½ times atmospheric pressure, and found they needed to replace 33 of Bertha’s roughly 750 steel cutting bits.

That so few needed replacing is a favorable sign that clay soils aren’t eroding the rotary cutting face and tools. Project engineers have estimated that 9 tons of the 838-ton disc would flake off along Bertha’s 1.7-mile route.

Previously, one of the smaller machines that dug the Brightwater sewer tunnel north of Seattle was ruined by erosion, and one of two Sound Transit machines cracked in hard soil near the University District.

Bertha’s restart came a few weeks earlier than expected, said WSDOT spokeswoman Laura Newborn. The state hasn’t released a schedule update from Seattle Tunnel Partners, but STP manager Chris Dixon has said two more maintenance stops are likely.

The machine has gone about one-third of the way from Sodo to South Lake Union. Drilling should finish early next year, if there are no problems and Bertha sustains a consistent pace of about 40 feet per day.