Case Study: Alaskan Way Viaduct Replacement Tunnel


Alaskan Way Viaduct Replacement Tunnel Overview. Image obtained from,,

The Alaskan Way Viaduct Replacement Tunnel, completed in 2017 and opened to traffic in 2019, was one of the most ambitious urban tunneling projects in US history. However, it was not without issue – the project suffered over 2 years of delays.

The project used a TBM named Big Bertha which was, at the time, the largest in the world (57 feet). Drilling under the water table through the glacial soils and tills of downtown Seattle’s coastline led to subsidence hazards affecting surface infrastructure. In some places, Bertha passed within 20 feet of existing infrastructure.

Following extended stoppages due to an unresolved issue requiring emergency rescue and repair of the TBM cutterhead which led to 2.5 inches of subsidence in downtown Seattle, the 1.76-mile tunnel was completed over 2 years late. Today, the tunnel is fully operational, though litigation regarding the cause of the extended stoppages continues.