As lead designer and engineer-of-record, HNTB managed construction, testing and commissioning for the WSDOT Alaskan Way Viaduct Replacement Program in Seattle. HNTB designed the permanent structures and facilities, including the tunnel liner, interior stacked roadway structure, approach cut-and-cover tunnels, approach roadways and operations buildings located at each portal.

Designed to enhance safety and emergency vehicle access, the SR 99 Tunnel features a two-level roadway tunnel to replace the aging, earthquake-damaged Alaskan Way Viaduct. At 57.5 feet in diameter and nearly 2 miles long, the tunnel in downtown Seattle is the world’s second-largest soft-ground, machine-bored tunnel. It also features:

  • state-of-the-art ventilation, fire detection and suppression systems
  • an emergency egress system with its own ventilation system
  • a security system with closed-circuit TVs
  • an intelligent transportation system with variable message signs

From a mobility perspective, the tunnel creates a convenient and efficient north-south passage, and from a real estate perspective, it will be instrumental in transforming the downtown waterfront, making it a much more residential, commercial and tourist-friendly environment. The tunnel also frees up precious downtown waterfront property where the old viaduct sat and minimizes construction disruption.

Location: Seattle, WA

Client: Washington State Department of Transportation