Tunneling

Tunneling

Bertha's 57-foot-wide cutterhead being lowered by crane into the launch pit

The SR 99 Alaskan Way Viaduct, an elevated concrete structure built in the 1950s and vulnerable to earthquakes, is being replaced with a 1.7-mile tunnel running beneath downtown Seattle. Replacing the viaduct with a tunnel allows the highway to remain open for much of construction, thus minimizing closures and impacts to traffic. Once the tunnel opens, the viaduct will be taken down to clear the way for new public space along Seattle's downtown waterfront.

The tunneling machine known as Bertha began tunneling beneath Seattle in July 2013 (photo at right: the machine's cutterhead being lowered into the launch pit near Seattle's stadiums). The machine completed its tunneling on April 4, 2017 and was fully dismantled and removed from the tunnel on Aug. 23, 2017.

+ Bertha's tunnel route

+ Tunneling breakthrough and disassembly

+ How a tunneling machine works

+ Performing routine maintenance

+ Repairing Bertha