SR99 Alaskan Way Viaduct and Seawall Replacement
World’s largest bored tunnel on budget and ahead of schedule
• 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.