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

The Alaskan Way Viaduct was an elevated concrete structure built in the 1950s to carry SR 99 through downtown Seattle. By the early 2000's, the structure was vulnerable to earthquakes and wearing out. The viaduct was replaced by a 1.7-mile tunnel that opened in February 2019. The tunnel's design and approach to construction allowed SR 99 to remain open for much of construction, thus minimizing closures and disruptions to vehicle and freight traffic. The viaduct was demolished in 2019 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