As part of WSDOT’s Alaskan Way Viaduct Replacement Program, the completion of the State Route 99 Tunnel was a transformative event in Seattle’s storied transportation history and a breakthrough for the worldwide tunneling industry.

Creating a fast, convenient north-south option for motorists who want to bypass traffic congestion in the city’s central business district and providing enhanced 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 57.3 feet in diameter, the colossal tunnel winds for 2 miles under downtown Seattle, carrying two southbound lanes stacked atop two northbound lanes. It is one of the world’s safest and longest continuous machine-based tunnels in soft ground.

As lead designer and engineer-of-record for the design-build project, HNTB managed construction, testing and commissioning. The firm 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.

Additional key features of the tunnel include:

  • 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

The completed tunnel has set a new standard for innovative, creative and safe tunneling solutions under densely populated cities. And while the project has transformed the tunneling industry, it is also helping shape downtown Seattle as it continues to provide a convenient mobility solution and frees up precious downtown waterfront property to transform into a more residential, commercial and tourist-friendly environment.

Location: Seattle, WA

Client: Washington State Department of Transportation