SR99 Alaskan Way Viaduct and Seawall Replacement

Seattle, Washington


•    Design-build project produces world’s largest machine-bored tunnel
•    World’s largest TBM ever built will excavate the tunnel
•    Tunnel designed to increase safety 

HNTB is lead designer for the Washington State Department of Transportation’s SR 99 Alaskan Way Viaduct and Seawall Replacement project. The design-build project is constructing a tunnel with a two-level roadway to replace the aging, earthquake-damaged Alaskan Way Viaduct. 

At 57.5-feet in diameter and nearly 2 miles long, the tunnel will be the world’s largest soft-ground, machine-bored tunnel, designed to enhance safety and emergency-vehicle access. 

Despite coordination between multiple government agencies and numerous stakeholders, HNTB stayed on schedule by:
-    Creating a timeline that kept everyone on task
-    Locating all team members – owner, contractor, designer and subconsultants – under one roof, so issues could be addressed face-to-face
-    Leading weekly meetings to resolve conflicts and minimize costly changes
-    Being flexible to stakeholders needs 

The tunnel will free up precious downtown waterfront property where the old viaduct sat and minimize construction disruption.