It will take 75 piles to enclose a 120-ft-deep circular pit in front of Bertha, North America’s largest tunnel-boring machine, which currently sits stalled under downtown Seattle.
The “repair pit,” as it is dubbed, will allow crews the chance to access the 57.5-ft-dia machine from the front, making it easier to replace the damaged main bearing and broken bearing seals, pieces that have forced the machine to sit essentially idle since December 2013. Bertha traveled just over 1,000 ft into a 1.7-mile dig under downtown Seattle for a new State Route 99 path that will eventually replace the aging Alaskan Way Viaduct before overheating and slowing.
Seattle Tunnel Partners, the contractor hustling to get Bertha running again, has started building the repair pit by injecting grout into the ground above the machine and relocating utilities. Next up is the construction of underground walls and then pit excavation, all part of a nearly one-year plan to have tunneling resume in March 2015. Below is the plan in pictures:
A below-ground view of the pit site.
Crews will inject grout between the existing piles and behind the machine.
The build of the pit’s walls comes after the relocation of utilities.
After lowering groundwater, crews will tunnel into the pit and excavate it before installing a crane atop the pit.
Bertha’s damaged pieces will get removed and fixed.
Crews can then resume tunneling, likely not until March 2015.
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.