Recent surveys found that the Highway 99 tunnel is a few inches off course, and Seattle Tunnel Partners (STP) 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-in. 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.
Bertha has gone 8,310 ft, or almost 90% of the 9,270-ft dig from Sodo to South Lake Union. Sometime this spring, the 57-ft, 4-in.-diameter cutter head is expected to break into daylight and be removed in pieces, just west of Aurora Avenue North.
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.
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.
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.
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.