Tunnel machine Bertha is expected to reach daylight midday Tuesday.

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Tunnel machine Bertha is expected to reach daylight midday Tuesday.

The drill was 25 feet away as of 2 p.m. Monday, said Joe Hedges, Highway 99 administrator for the Washington State Department of Transportation.

It’s currently moving at a reduced speed of a half-inch per minute through grout-filled soil. Bertha should reach the head wall of the cubelike retrieval vault by Monday night, then stop.

At around 8 a.m. Tuesday, the digging will resume at even lower speeds, through a 5-foot-wide wall of concrete pillars.

Sooner or later, the round cutter head will bite through, moving at a 4 percent upward slope. Concrete chunks will slowly be dislodged, tumble to the vault floor and break apart if all goes according to plan. Seven sprinklers have been installed over the site to reduce dust.

WSDOT has installed an online vault cam and will tweet using the hashtag #Berthabreakthrough. Seattle television stations will be using a shared camera that also posts live on a WSDOT project page. The Seattle Times will have a photographer on scene.

Bertha breaks through

Three more weeks are needed to push the entire front end into the vault, according to contractor Seattle Tunnel Partners.

Then, Bertha’s steel shield, cutters and drive parts will be carved or dismantled into segments of 20 tons or less. They’ll be removed by crawler cranes, rather than a red lift tower that was used for repairs two years ago. Pieces will be trucked away for recycling or reuse.

The tunnel is currently scheduled to open for traffic in early 2019. It’s the biggest part of a $3.1 billion replacement for the Alaskan Way Viaduct, which was built in 1953 and damaged in a 2001 earthquake.