Seattle’s ‘Bertha’ Pauses, Then Resumes Bore on Final Segment

After a final, Feb. 28-March 6 maintenance stop, the world’s largest-diameter tunnel-boring machine has moved less than two blocks away from the disassembly pit in Seattle. TBM “Bertha,” churning a 1.7-mile-long tunnel for a state Route 99 replacement of the Alaskan Way Viaduct, required a stop less than 1,000 ft from the pit to confirm that the 57.5-ft-dia machine was 6 in. off course alignment. This adjustment was the second to correct Bertha’s course, which is set by an onboard guidance system. The Washington State Dept. of Transportation expects Bertha to complete mining in May. Then, contractor Seattle Tunnel Partners will spend months taking apart the machine.

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