Tunnel machine Bertha is back on a slightly shifted course this week, after measurements confirmed it was 6 inches east of the correct position. It should break into daylight near Aurora Avenue North sometime this spring.

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Tunnel machine Bertha is back on a slightly shifted course this week, after measurements confirmed it was 6 inches east of the correct position.

The giant drill is under Denny Way at Sixth Avenue North. It resumed grinding north Monday afternoon, after a stoppage last week that included three rounds of surveys. Operators will gradually steer the machine back on course.

Seattle Tunnel Partners (STP) took advantage of the time to replace some of the 600-pound disc-shaped cutter blades that are mounted in Bertha’s front spokes.

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An onboard guidance system that provides a bull’s-eye target for operators within a control room, was “set to the new tunnel alignment,” said an update from the Washington State Department of Transportation (WSDOT). Similar adjustments occurred in February 2015 when Bertha had to be steered into a giant open-air vault for repairs, near the state ferry terminal.

WSDOT says tunnel contractors are considering whether to perform additional measurements this week using a gyrotheodelite, which establishes position based on finding true north, despite being underground.

Bertha moves forward by pushing its rear end off the freshly installed concrete rings of the future Highway 99 tunnel, using hydraulic jacks. Each ring is built from a standard set of 10 pieces that vary slightly in width — so engineers can order one or more rings to be rotated slightly to tweak the curvature of the tunnel. The goal is for Bertha to be in exact alignment when it breaks into daylight near Aurora Avenue North, sometime this spring. It has traveled 8,310 feet, with less than 1,000 feet remaining.

The $2.1 billion downtown-bypass tunnel from Sodo to South Lake Union is expected to open to traffic in early 2019, followed by demolition of the Alaskan Way Viaduct. Court disputes are underway about who will pay which share of some $480 million in STP claims, based on project delays and repair costs.