One of two major north-south corridors through Seattle, the Alaskan Way Viaduct went into service in 1953. This double-decked elevated section of State Route 99 runs along Elliott Bay in Seattle’s downtown and Industrial District.
Fifty years later, the viaduct was carrying more than 100,000 vehicles per day — but its vulnerabilities had become obvious. A similar viaduct in Oakland was destroyed by the 1989 Loma Prieta earthquake, and in 2001 the Nisqually earthquake damaged the viaduct and its supporting seawall. Engineers said the viaduct was within a few seconds of complete collapse.
Repairs to the viaduct, well into its 75-year design life, cost more than $14 million. The seawall was in disrepair, and the soil it supported around the viaduct’s foundations was subject to liquefaction.
In 2007, officials decided to replace the southern end of the viaduct while continuing to consider options for replacing the waterfront section. Foundations for the new side-by-side roadway were sunk up to 260 feet (79 meters) deep for added stability, and wider lanes and shoulders were provided.
In January 2009 officials from the state, county, city, and port recommended a bored tunnel to replace the waterfront section of the viaduct — the only option that would allow the state highway to remain open during construction. The following year, it was decided to bore a tunnel 2 miles (3.2 kilometers) long and 57 feet (17 meters) in diameter, requiring the construction of the world’s largest tunnel boring machine (TBM).
Working with the Washington Department of Transportation, Mott MacDonald served as Project Management Assistant Consultant. Our first major tasks included these:
- Developing a project management plan mandated by the Federal Highway Administration
- Developing a project management template to be used by WSDOT on this and potentially other megaprojects
- Design review of deliverables
- Development of the program management process using best-in-breed software tools
Mott MacDonald played a key role in guiding the alignment of the tunnel. Avoiding the original narrow corridor through the historic Pioneer District, the new alignment reduces the risk and cost of beginning the tunnel in an area of poor fill soils and debris, where areas of Puget Sound have been filled for waterfront development. We also helped evaluate contractor proposals before the execution of a design-build contract in January 2011.
The grand opening of the SR 99 tunnel was celebrated in February 2019, with a public walk-through, STEM activities, an art festival on the viaduct, an 8-kilometer fun run, and a 12-mile bike ride.
Seattle now enjoys an improved transportation corridor. Drivers use the tunnel to bypass downtown traffic. New and restored surface streets connect SR99 with the downtown area, and a new overpass allows drivers to avoid train blockages near Seattle’s busiest port terminal.
For our work on this project, Mott MacDonald received the Outstanding Civil Engineering Achievement Award from the American Society of Civil Engineers and an Award of Merit from Engineering News-Record.