The total feet mined by Bertha, the world’s largest diameter tunnel-boring machine currently moving under downtown Seattle, hasn’t been impressive in the last few weeks simply for distance, but the fact that Bertha continues adding to its mining total is a feat worth noting.
After a roughly two-year shutdown and fix, Bertha had barely moved forward again before Gov. Jay Inslee shut down the project on Jan. 14 after a sinkhole formed behind Bertha’s path.
The project to bore a new tunnel to replace the Alaskan Way Viaduct was given a provisional okay to start mining again on Feb. 23 and now, early in March, Bertha was given a second round of clearance, this time to keep moving about another 120 ft north toward a planned maintenance stop.
The conditional permission granted in February to resume tunneling lifted the “suspension for cause” placed by Inslee and allowed Seattle Tunnel Partners to move forward with a 25-ring demonstration period. During the time of building the 25 concrete tunnel rings, Seattle Tunnel Partners had to demonstrate new quality assurance protocols, new personnel at key positions within the tunneling operation, restructured daily tunneling meetings and updated tunnel work and quality plans, including calculations of the amount of soil removed during excavation of each tunnel ring.
The conditional lifting of the suspension came after Washington State Dept. of Transportation’s team of tunneling experts evaluated documentation submitted by Seattle Tunnel Partners.
Now that the 25-ring provisional mining portion has wrapped up, giving Bertha a total of 1,437 ft of bored section, WSDOT agreed to allow STP to move forward the additional 120 ft to the planned maintenance stop. In all, 15 percent of the tunnel is now complete.
The next big moment for Bertha will come during the maintenance inspection. And then, after that, every foot successfully mind will prove a big moment, all the way until the machine finishes the 1.7-mile tunnel.
Follow Tim Newcomb on Twitter at @tdnewcomb.