To get just over a quarter of a mile from the finish of the 1.7-mile-long bored tunnel under downtown Seattle, Bertha had to dig under another State Route 99 tunnel. And the machine accomplished that task, passing about 90-feet below the bottom of the 62-year-old Battery Street Tunnel.
Now Bertha, the world’s largest tunnel-boring machine, is well into its ninth of 10 stages and nearing a finished mining date later this year.
Seattle Tunnel Partners has now tunneled through over 7,500 ft of its 9,270 ft and installed over 1,150 of the 1,426 concrete tunnel rings needed to complete the project. Zone 9, Bell Street to Denny Way, sent Bertha beneath the Battery Street Tunnel, constructed in the 1950s and without an upgrade since. Its electrical and mechanical systems have proven difficult to maintain and the Washington State Dept. of Transportation plans to close and fill in the tunnel when the new SR 99 opens to traffic.
WSDOT reports that the ground along the current tunnel route remains stable as Bertha moves toward the finish line near Seattle Center, home of the Space Needle. Crews have moved less than 1,600 ft from the end of the tunnel drive, just over a quarter of a mile, and continue climbing in elevation at a steady rate. Currently the distance between the top of the 57.5-ft-dia machine and the ground surface is about 115 ft. The tunnel, at its deepest point, is about 215 ft deep.
As Bertha inches toward daylight near the Space Needle, the machine will continue beneath Fifth Avenue and the Seattle Center Monorail, with the top of the machine passing approximately 90 ft below the Monorail’s supports.
Once Bertha mines a total of 8,365 ft, the machine will enter the 10th and final stage of the process.
While Bertha continues forward, crews inside the tunnel construct the double-deck highway, with the new roadway—the stacked system places the northbound lanes on the bottom and the southbound lanes on the top—now stretching more than 3,800 ft., which places it near the southern edge of Pike Place Market. The southbound roadway is now over 42 percent complete.
With so much of the tunnel complete, work started two weeks ago on the mechanical, electrical and plumbing systems. Crews began installing the systems near the southern entrance to the tunnel around the stadium district and will continue north.
As every crew works its way north, the light at the end of the tunnel grows ever brighter for Bertha.
Follow Tim Newcomb on Twitter at @tdnewcomb.