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.

Bertha's cutterhead was lowered into the launch pit near Seattle' stadiums in May 2013 (see photo at right).

+ Bertha's tunnel route

+ Tunneling breakthrough

+ How a tunneling machine works

+ Performing routine maintenance

+ Repairing Bertha