Another Seattle tunnel machine has been damaged, and spent six weeks going nowhere.
This time, a Sound Transit drill nicknamed Pamela halted some 650 feet north of the future University District Station on Dec. 28 and was out of service until Thursday, according to Sound Transit.
New light-rail service to Capitol Hill and Husky Stadium is farther south so it will not be affected, and it will likely open for passenger service as planned March 19.
Elected officials on Sound Transit’s capital committee were briefed Thursday by Ahmad Fazel, executive director for construction management. He said the 4-mile extension from Northgate to Husky Stadium is still likely to open in 2021, as scheduled, on the $1.9 billion budget.
The six electric motors that turn tunnel-boring machine Pamela were damaged, and Fazel said hairline cracks formed on a huge circular ring, called the bull gear, that spins the drive shaft and rotary cutterhead. After limited repairs, the machine restarted around noon Thursday, he told the committee.
The damage apparently is related to difficulty spinning through “very hard ground,” Fazel said.
Pamela will run at reduced torque and speed, needing two months to finish the short distance to the station, Fazel said.
At that point, more extensive repairs are planned. In essence, Sound Transit contractors will make use of its unfinished underground station in the U District in much the same way Hitachi-Zosen and Seattle Tunnel Partners built an open-air repair vault to reach the damaged Highway 99 tunnel machine Bertha last year.
There are many differences. Bertha has struggled with soft, wet soil, as opposed to hard glacial till in the U District. And each transit tunnel, at 21-feet diameter, has a cutting face only one-seventh the size of 57-foot Bertha.
Pamela is one of two light-rail boring machines. Brenda last year needed cutter-teeth replacement, but it continued and has already passed the U District Station, heading south toward Husky Stadium.
If repairs to Pamela fail, contractors could use the other tunnel machine, Fazel said. Or they could switch to “sequential excavation mining,” which entails using conventional digging machines to scrape out dirt a few feet at a time, then spraying concrete to support the walls, then more scraping.
Repairs or a change in methods could consume three to eight months, but the excavation schedule still has five months of contingency time, known as float, Fazel said.
The agency didn’t announce the breakdown or report it to elected officials for six weeks. In the highway project, state highway officials were roundly criticized after their monthlong lag in disclosing details about the Dec. 6, 2013 stall of their machine, in particular the presence of an abandoned steel pipe in Bertha’s path.
Asked why the agency sat on bad news, Sound Transit spokeswoman Kimberly Reason said contractors initially thought the damage was less extensive, involving just a single motor, until later when they disassembled more components.
“After four or five weeks, it was finally determined the bull gear had hairline fractures and cracks,” she said.
Meanwhile across town, Bertha has now been idle for nearly a month, since Gov. Jay Inslee suspended drilling Jan. 14, in response to a sinkhole near Elliott Bay.
The Washington State Department of Transportation and Highway 99 contractors haven’t settled on a restart plan for a project that’s trending more than two years beyond its December 2015 completion goal.
But crews Thursday morning did clear one hurdle, by replacing 22 pilings at Terminal 46, which were dislodged when a barge full of tunnel muck tilted in January.
