SEATTLE — Seattle Tunnel Partners finished disassembling Bertha, the world’s largest tunnel-boring machine, on Monday and said the damage the machine sustained when it broke down in December 2013 was more extensive than previously thought, which means they have to revise their schedule for when they will resume drilling.
Also Monday, state transportation officials released the findings of two reports on the ground settlement near the Seattle Tunnel Project. Both said the removal of water near the repair pit that was dug to access Bertha was a factor, but so was natural settlement and the removal of water for other construction projects.
Building a two-mile tunnel under Seattle was the project of choice to replace the Alaskan Way Viaduct, which was damaged in a 2001 earthquake. Crews spent much of 2014 digging the 120-foot-deep access pit to reach the front of the machine so it could be pulled out and repaired.
Concerns about ground stability were sparked late last year when monitors detected some settlement near the pit. Officials measured 1 inch of settlement since November. They said they plan to expand the overall monitoring program.
Program administrator Todd Trepanier said having reliable information is essential to the decision-making process, and the studies will help inform future decisions as they work to replace the State Route 99 viaduct.
“We all agree that public safety and protecting infrastructure are our top priorities,” Trepanier said in a release. “This is an incredibly complex issue, but all of us – the state, our contractor, the city – have a shared interest in reaching consensus and acting in the interest of public safety.”
Transportation spokeswoman Laura Newborn said the department doesn’t have a set number for how much ground settlement would be considered too dangerous to continue. “It depends on where the settlement occurs, what type of settlement it is— differential vs. uniform,” she said in an email.
In addition to the damage to Bertha being more extensive than thought, tunnel officials said the machine suffered some minor damage when it was taken apart. The outer seals and steel retainers that hold them in place were destroyed, they said. The cutter drive motor also suffered damage, they said.
Seattle Tunnel Partners has also decided to replace the inner seals to make them more compatible with the new outer seals and easer to access if needed. The new seals were designed and made in Japan and are scheduled to arrive in Seattle in late May, they said.
The company will have a revised schedule for the tunnel-boring machine by June, Newborn said.
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