It took only 10 days for Seattle Tunnel Partners to successfully mine beneath the Alaskan Way Viaduct—State Route 99—with Bertha, a positive step in moving forward on the 1.7-mile bored tunnel that will eventually replace the raised roadway.
The viaduct was closed as a precaution during the world’s largest boring machine’s trip through ground under the seismically vulnerable structure, creating traffic unrest in downtown Seattle. The viaduct reopened for the Monday morning commute, earlier than scheduled.
During the process, construction crews installed 52 concrete tunnel rings and excavated 342 ft. Once the machine hits 385 ft. of tunneling under the viaduct, Seattle Tunnel Partners will allow for a short break to let crews rest. The 10 days of tunneling occurred around-the-clock.
Before reopening the viaduct, structural engineers with the Washington State Dept. of Transportation completed a thorough inspection of the viaduct, confirming the stability of both the structure and the ground. The mining was through “complex soils” at one point just 15 ft. below the viaduct’s foundation, the closest the machine will come to any structure at any point during its drive beneath Seattle.
Before Bertha took off from a planned maintenance stop to start the viaduct portion of the tunneling, WSDOT flew a video-equipped drone through the SR 99 tunnel to show the construction process. Done between shifts to avoid disrupting workers, the video captures the progress made so far on the project.
As Bertha continues to move under downtown Seattle, it will soon dip below buildings and streets—the next major street it will go under is Western Avenue—as it will remain under some of the busiest portions of downtown for the duration of the tunnel drive.
Follow Tim Newcomb on Twitter at @tdnewcomb.