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SR 99 Tunnel Passes 4,000-ft Mark

SR 99 tunnelSeattle Tunnel Partners crews have now mined more than 4,000 ft as they continue their push toward the future north portal of the SR 99 tunnel. As of Aug. 22, the 57.5-ft Hitachi TBM had traveled approximately 4,000 ft of the 9,270-ft long tunnel route, and installed 607 of the concrete rings that form the tunnel’s exterior walls.
The top of the cutterhead is approximately 165 ft below the centerline of First Avenue, approaching Union Street. Crews will continue mining beneath First Avenue for several blocks. Then, at Stewart Street, where First Avenue veers to the west, tunneling crews will continue mining north en route to the spot near Seattle Center where Bertha will emerge.
The Washington State DOT noted that the program has an extensive network of instruments in place to monitor ground movement. Crews continue to see little to no movement as they tunnel through a mixture of clay, sand and gravel beneath downtown.
The entire tunnel route, including descriptions of each of the 10 zones through which the TBM is mining, can be found here. Tunneling statistics are updated on that page on Mondays and Thursdays. You can also track tunneling progress on Twitter @BerthaDigsSR99.

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