The cutterhead of Bertha, the world’s largest tunnel-boring machine at a diameter of 57.5 ft, doesn’t sit where Seattle Tunnel Partners and owner Washington State Dept. of Transportation were hoping, but Bertha’s cutterhead rests exactly where needed right now: above ground in downtown Seattle.
After Mammoet lifted the cutter drive unit from 120 ft below Seattle with a custom-made skid crane in April—the crane is 116 ft wide, 90 ft long and 105 ft tall with a distance between the towers of 75 ft—crews have worked to disassemble the machine designed to replace Seattle’s aging 60-plus-year-old Alaskan Way Viaduct with a 1.7-mile bored tunnel.
With disassembly complete, damage assessment is ongoing.
Seattle Tunnel Partners plans to replace the main bearing and outer seals of the machine as expected, but the contractor also decided to replace the inner seals to make them more compatible with new outer seals and make them easier to access should the need arise, according to information provided by the state.
The new inner seals were designed and manufactured in Japan and should arrive to Seattle by the end of May.
Damage to the machine was more extensive in some areas than anticipated and some minor damage also occurred during disassembly, according to the state. For example, the outer seals and the steel retainers that hold them in place were destroyed. There was also damage to the cutter drive motor pinions and the main bearing bull gear.
Original estimates provided by Seattle Tunnel Partners called for the Hitachi Zosen-built machine to start tunneling again in August, but the state says the contractor “will not provide a revised schedule for resuming mining until they fully understand the scope of repairs.”
Bertha has sat idle since December 2013 when overheating brought the $80 million machine to a stop after tunneling just over 1,000 ft—about 11 percent—into the project. After months of uncertainty as to the reason for the overheating, Seattle Tunnel Partners determined the seals protecting the main bearing were compromised by dirt and debris.
Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He also writes for Popular Mechanics, Sports Illustrated and more. You can follow him on Twitter at @tdnewcomb or visit his website here.