Alaskan Way Viaduct Replacement-Corridor SR99

The project consists of a two-mile-long tunnel underneath downtown Seattle that will replace the SR 99 Alaskan Way Viaduct . Replacing the viaduct with a tunnel allows the highway to remain open for much of construction, thus minimizing closures and impacts to traffic. The project is carried out by the world’s largest tunnel boring machine nicknamed Bertha, 17.4m diameter, manufactured by Hitachi Zosen Corp.