Bertha, the world’s largest-diameter tunnel-boring machine, sits at a low point in its 9,000-ft bore under downtown Seattle. But, this time, the much-maligned machine finds itself at a low point it wants.
Located about 120 ft beneath Spring Street, the 57.5-ft-dia machine has reached a planned maintenance stop 3,088 ft into the project that, ultimately, will give Seattle a bored tunnel to replace the aging Alaskan Way Viaduct. After a two-year-plus delay that required raising Bertha’s 4,000-ton face to the surface for repairs, getting to this point under Spring Street—the lowest grade the machine will reach in moving north to its exit portal, near the Space Needle—has elevated the mood of the project team, says Chris Dixon, Seattle Tunnel Partners project manager.