The size of the Sound Transit tunnel-boring machine Pamela—at 21 ft in diameter—doesn’t compare to the world’s largest TBM, Bertha, at 57 ft in diameter, tunneling in the same city. The woes of Pamela don’t compare either, but the two machines do have one thing in common: issues.
While Bertha sat idle for about two years after a breakdown—the machine boring the tunnel under downtown Seattle to replace the Alaskan Way Viaduct is on hold again after a sinkhole issue—Pamela’s issues haven’t proven as catastrophic, lengthy or publicized. But the machine did have a six-week stoppage during its effort to bore a tunnel as part of a four-mile light rail tunnel from the soon-to-open Husky Stadium station at the University of Washington north to Northgate.
During boring, according to Ahmad Fazel, executive director for Sound Transit’s construction management, and reported by the Seattle Times, the six electric motors suffered hairline cracks on the gear that rotates the drive shaft and rotary cutterhead.
The machine was about 650 feet north of the planed University District Station—the new route has a station at Northgate, Roosevelt and the U District before tying into the soon-to-open University of Washington/Husky Stadium station—trying to mine through exceptionally hard soil. After limited repairs, Sound Transit plans to mine with Pamela at a reduced speed and torque and then perform more extensive repairs once the machine reaches the future station.
Fazel presented findings on the breakdown to elected officials earlier this month, with Sound Transit saying there was a delay in reporting the work stoppage until the damage was found more serious than originally believed. Sound Transit anticipates it can urge the machine through the next few weeks of mining and toward a more proper fix.
The entire $1.9 billion project to extend the current extension is planned for a 2021 opening, which has not changed.
Follow Tim Newcomb on Twitter at @tdnewcomb.