Alaskan Way Viaduct

SR-99 "Bertha" Worlds Largest

Project: Alaskan Way Viaduct Replacement – SR 99 Tunnel

Customer: Seattle Tunnel Partners – Dragados


Ballard Marine Construction tunnel support division was brought in early by Seattle Tunnel Partners on this world record setting project (Largest diameter at time of start -The Tuen Mun – Chek Lap Kok TBM is now the world’s largest tunnel boring machine launched in June 2015. The Herrenknecht machine has a diameter of 17.6 m, 0.2 m more than Bertha). The J/V contractors were able to consult with our hyperbaric support managers to develop a first of its kind Saturation (SAT) Habitat specifically for tunnel work.  By being on board early we are able to insure that the hyperbaric systems fulfill the needs of both the owner and the prime contractor while adhering to all applicable codes and standards. The SAT system provides the capability of supporting up to 20 workers and their support crewing.  Ballard schematic reviews, air consumption calculations, site assessments, hyperbaric operations manual and aid in acquiring variances have supported this project from the offset.


“Bertha”, the massive TBM responsible for digging the SR 99 tunnel beneath Seattle, is the 2nd largest machine of its kind in the world. Tunneling beneath Seattle will allow the city to replace the SR 99 Alaskan Way Viaduct while minimizing closures of the highway during construction. When the tunnel opens, a two-mile stretch of SR 99 will move underground, allowing Seattle and the Washington State Department of Transportation to remove the viaduct and clear the way for new public space along Seattle’s downtown waterfront.

Bertha was manufactured by Hitachi Zosen Corp., a Japanese firm that has successfully built more than 1,300 tunneling machines, a number of them for large-diameter tunnel projects. As Bertha’s owner, Seattle Tunnel Partners, the contracting team that’s building the tunnel, is responsible for ensuring she functions properly.

More information on the Alaskan Way viaduct and seawall replacement program at