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Seattle tunnel mining resumes, Bertha headed for city center

The southbound upper roadway inside the new SR 99 tunnel was coming along in December. (WSDOT)

Crews with Seattle Tunnel Partners resumed mining under Seattle, following 10 days of maintenance on Bertha the boring machine.

The work resumed after crews replaced about 300 of nearly 500 scrapers on the machine’s cutterhead.

“Completing routine maintenance is an important part of ensuring that the machine continues to operate properly. STP will continue to perform inspections and maintenance as needed over the remainder of the tunnel drive,” according to the Washington State Department of Transportation.

Bertha is less than 2,500 feet from the pit that will signify the end of its journey. It will emerge near Seattle Center.

Crews excavated 691 feet in December. According to the Bertha blog, it has progressed more than 30 feet in January.

The top of Bertha’s cutterhead is about 160 feet below Third Avenue in Belltown. It remains in Zone 8.

The soil in Zone 9, which runs under Battery Street, is mostly sand and gravel and “increasingly dry,” according to WSDOT.

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