The inside of the 120-ft-deep access pit has never looked so welcoming to a tunnel-boring machine.
As the fix nears for Bertha, North America’s largest tunnel-boring machine, the same machine that hasn’t tunneled since December 2013, new milestones have us looking forward to Bertha reaching the light—and surface—again, just not the way owners Seattle Tunnel Partners had hoped.
With Bertha stalled under downtown Seattle about 1,000 feet into the 1.7-mile dig to create a new State Route 99 tunnel, crews spent months digging an access pit, lined with 20-ft-thick concrete to keep it secure. The most recent milestone came with the installation of a concrete cradle at the bottom of the pit.
Over the last week, crews placed several base layers, formed the cradle with rebar and poured nearly 1,000 cu yards of concrete to finish up the cradle, which will support the machine after it moves through the pit’s southern wall.
Of course, it remains to be seen if the machine can actually get through the concrete.
Todd Trepanier, program administrator of the Alaskan Way Viaduct Replacement Program, says the next step is to tunnel through the concrete wall.
“The length of time it takes Bertha to reach the pit will depend largely on her ability to mine through and digest the concrete,” he says. “If she’s unable to mine through the wall, STP will create an opening from within the pit to give her an unobstructed path forward.”
Once Bertha makes it free and clear into the pit, the red gantry cane poised atop the pit’s opening will hoist the front end of the machine to the surface for repairs.
Next step: crack open 20 ft of concrete. Then? Repair the $80 million machine.
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.