Recent surveys found that the Highway 99 tunnel is a few inches off course, and Seattle Tunnel Partners has stopped digging until early next week to perform more measurements.
STP project manager Chris Dixon said late Wednesday afternoon that surveys found the concrete tunnel rings being installed behind tunnel machine Bertha varied “a couple inches” beyond the 6-inch tolerance limit of where a giant tunnel tube is supposed to be.
More surveys will be done, and a sensitive gyroscope is being trucked in from Ohio to increase the precision, he said.
“If we’re off alignment, we’ll make the necessary course corrections so we get back on alignment and enter the retrieval pit in the right place,” he said.
Bertha has gone 8,310 feet, or almost 90 percent of the 9,270-foot dig from Sodo to South Lake Union. Sometime this spring, the 57-foot, 4-inch-diameter cutter head is expected to break into daylight and be removed in pieces, just west of Aurora Avenue North.
The stoppage called Tuesday became evident when a muck barge at Terminal 46 wasn’t taking dirt Wednesday morning. The state Department of Transportation said maintenance work was planned but didn’t have details immediately available.
“Adjustments are common during tunneling, including on this project,” WSDOT said in an online update.
Bertha is now under Denny Way and has passed the last building along the route, the state said. The most recent STP schedule calls for digging to end in May.
If further measurements confirm Bertha is several inches east of the target, a day or two of engineering will be needed to chart the course ahead, Dixon said.
After restart, workers at the controls can steer Bertha by tweaking how much force is applied to each of the 56 hydraulic rams in the back to push the cutter head forward.
STP did similar steering in February 2015 to enter a massive repair-access shaft, which required Bertha to go straight rather than along the original curved route.
“It’s not a big deal,” Dixon said.
Operators who steer Bertha normally stay within a 3-inch-diameter bull’s-eye, using laser guidance, to give some cushion for the 6-inch limit.
However, the laser is shot from the rear of the machine, Dixon said, and the precise location of the cutter, some 330 feet or so forward, is hard to pinpoint.
“No TBM (tunnel boring machine) goes exactly in alignment,” he said.
Meanwhile, workers will replace some of the huge 600-pound cutting blades mounted inside Bertha’s cutter head, he said.
A work schedule filed in December showed four possible maintenance stops this year, but Dixon said Bertha might make it the last thousand feet without any need to replace the huge blades again, or to replace the hundreds of 75-pound scraping bits on the face of the cutter head.
Drilling on the $2.1 billion Highway 99 tunnel started in July 2013 and was interrupted by damage and difficult repairs, but last month the machine covered 930 feet. After Bertha reaches daylight, roadway installation, lighting and signal work will continue.
The rapid pace may slow this spring.
Sound Transit’s smaller tunnel drills have significantly slowed, sometimes to a couple inches per hour, when they grind through grout or concrete walls next to the underground station sites.
The four-lane, tolled tunnel is expected to open to traffic by early 2019.
STP has filed $480 million in claims for cost overruns based on nearly three years of project delays, including repairs to machine damage that happened in late 2013.