According to the Washington State DOT, Seattle Tunnel Partners has temporarily stopped mining to prepare Bertha, the SR 99 tunneling machine, for the final 1,000 feet of the 9,270-foot tunnel drive.
Crews stopped mining yesterday to verify Bertha’s position along the tunnel route before making their final push to the disassembly pit. Recent surveys show that Bertha may be several inches off of the tunnel alignment.
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.
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. “It’s not a big deal,” Dixon said.
Adjustments are common during tunneling, including on this project. STP made a similar adjustment to correct Bertha’s course after crews mined out of the pit that was used to access and repair the machine. 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.
According to STP’s most recent schedule, Bertha will arrive at the disassembly pit in May. The pit is located approximately 960 feet north of the cutterhead’s current location beneath Denny Way and Sixth Avenue North. Mining rates will continue to vary based on maintenance needs and soil conditions. . Bertha has traveled a total of 8,310 feet and built 1,270 of 1,426 concrete tunnel rings.
In the meantime, crews will continue to perform routine maintenance on the machine and will replace some of the huge 600-pound cutting blades mounted inside Bertha’s cutter head for the next several days. They plan to resume mining next week.