Bertha found the daylight North America’s largest tunnel-boring machine was searching for. It just isn’t exactly where crews hoped it would be when they started the 1.7-mile-long dig in 2013.
On Thursday, the machine that has sat stuck under downtown Seattle for more than a year popped through a 20-ft thick concrete access pit wall to daylight, pushing the repair process for the machine near a new phase.
Contractor Seattle Tunnel Partners has searched for a way to fix the Hitachi Zosen-made machine since it started overheating in December 2013. The fix includes a 120-ft-deep access pit in front of where the machine sits stuck just over 1,000 ft into the 1.7-mile dig under downtown Seattle. When the pit finished, 20-ft-thick concrete walls held everything in place. To bridge that 20-ft gap, STP planned to mine Bertha forward for the first time in over a year, through the concrete wall and into the open-air pit.
Bertha successfully mined through the wall, stopping every 6.5 ft to allow crews to build a tunnel ring behind it, hitting daylight for the first time on Thursday afternoon. STP wasn’t positive Bertha would be able to mine the concrete and had a backup plan that included chipping away the wall from inside the pit to open space for Bertha. But STP didn’t need that backup plan.
Due to the angle of the machine, the top portion of the cutterhead broke through the southern wall of the access pit first.
Friday saw a break from the digging, allowing STP time to clean out the bottom of the access pit and the cradle that will house Bertha during the fix. As expected, a mixture of dirt, concrete and water dropped to the bottom of the pit during the machine’s breakthrough. Crews used vacuum trucks and other tools to remove the material.
Bertha must travel an additional 35 ft before the disassembly process can start. So far, Bertha has moved nearly 22 ft.
The disassembly of Bertha’s front end will allow STP to use a crane to lift portions of Bertha to the surface and allow Hitachi Zosen to install a new main bearing and more robust seals to hopefully limit material from entering the main bearing.
There remains no hard and fast timetable on the length of time the fix will take.
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.