At Risk. Alaska Way viaduct was damaged in 2001 earthquake. (Photo courtesy of Washington State DOT)

The city of Seattle and the Washington Dept. of Transportation are moving on plans to replace the 51-year-old Alaska Way Viaduct with a 5,300-ft-long tunnel estimated to cost up to $4 billion.

Parsons Brinckerhoff, New York City, is beginning environmental impact studies for the six-lane tunnel that would replace the double-deck concrete artery carrying 110,000 cars per day, plus the city’s sea wall. Both were critically damaged in 2001 when a 6.8 earthquake hit the Puget Sound region, forcing WDOT to temporarily shut down the roadway (ENR 4/9/01 p. 16). "We’re answering the wake-up call of the Nisqually earthquake," says Richard Conlin, chairman of the City Council’s transportation committee.

For two years, WSDOT held more than 80 community briefings and nine public hearings, gathering feedback on six different proposals for repairing the viaduct. In September, the city and WSDOT eliminated proposals to build a new bridge, construct a bypass tunnel, widen the Alaska Way arterial or simply demolish the viaduct. That left two options: construct a tunnel or rebuild the viaduct. Now, rebuilding is only being considered as a contingency plan in case the agencies fall short of raising the estimated $3.4 billion to $4 billion for the tunnel. Click here to view graphics

Rebuilding the viaduct would cost $1.3 billion less than constructing the tunnel and would be completed a year quicker, WSDOT officials say. But the city council and local communities are throwing their weight behind the tunnel option as a way to beautify the city and boost its economy.

Since the earthquake, WSDOT has implemented an extensive monitoring system on the viaduct, restricted lane-use for larger vehicles and lowered the speed limit. If any portion of the viaduct shows signs of additional stress, the agency will take additional precautions including further limits on vehicle speeds or closures, says David Dye, WSDOT urban planner.

Preliminary designs revealed at the Dec. 6 press conference announcing the plan show that the tunnel roof would be depressed about 10 to 15 ft below street level and its floor 31 ft below that. The walls would extend nearly 15 ft below the bottom of the tunnel. Dye says cut-and-cover tunneling with secant piles or cylinder piles is likely.

The final environmental study is expected to be completed by the end of 2006. Depending on available funding, construction will begin in 2009 with a projected completion date of 2016. As a joint project between the city, WSDOT and the Federal Highway Administration, the viaduct and seawall project would be paid for by a combination of state, local and federal dollars.

The planned scope of the project is comparable to that of Boston’s Central Artery/Tunnel project. Dye says agency planners studied CA/T and similar projects to capitalize on lessons learned. He says that although the agencies are working to present accurate cost estimates and completion dates, there are still unknown conditions, such as potential hazardous materials below ground, that could affect these figures.

The city also announced it will seek $1 billion in FHWA funds plus $100 million from the Army Corps of Engineers for a 1,700-ft stretch of stand-alone seawall that is not part of the tunnel.