The forward-churning movement of Bertha underneath downtown Seattle has the Washington State Dept. of Transportation optimistic.
In the continued effort to bore over 1.7 miles under Seattle for a future double-deck roadway that will replace the aging Alaskan Way Viaduct, progress has continued rapidly since early 2016, a nearly nine-month stretch of positive news for a project that was virtually stalled for roughly two years.
Bertha, the world’s largest tunnel boring machine, hit the one-mile mark in early November, checking off 5,280 ft of bored tunnel, meaning less than 4,000 ft now separates the front end of the machine from the receiving pit near the Space Needle where Bertha will emerge at the end of tunneling. Bertha’s cutterhead has most recently moved under First Avenue, just north of Stewart Street, more than 200 feet below the surface.
The one-mile mark in the project also signified a change in the tunnel route. Bertha has entered Zone 7 of the bore—the project is broken into 10 zones, each representing a slightly different soil type or direction—and will see Bertha slide north of Stewart toward Lenora Street, also existing from under First Street and veering toward Second Street. At more than 200 ft below ground, Zone 7 marks the deepest zone in Bertha’s route.
Jerry Roberge, one of Bertha’s six operators and with 27 years of experience running tunnel-boring machines, says the machine moves akin to a drill bit, pushing forward with the push jacks using the last ring built as the foundation for the push. The articulation joint near the cutterhead allows for turning as needed, based on a laser-guided path.
“It is like a moving factory,” Roberge says. “You perform one operation and then the next and it is the same thing repeated over and over again.”
Once Bertha reaches Lenora Street, Zone 8 starts the gradual climb toward the surface, passing underneath Second and Third avenues. It will also include a greater variety of soils, as the surface above Zone 8 has seen its share of dirt moved over the years, including the regrade that leveled Denny Hill in the early 20th Century.
Follow Tim Newcomb on Twitter at @tdnewcomb.