State commits to highway tunnel under Seattle

SEATTLE—Washington Governor Christine Gregoire signed a bill into law on May 12 that commits state funding to replace the Alaskan Way Viaduct with a bored tunnel beneath downtown Seattle. The legislation approves $2.4 billion to construct a bored tunnel and remove the current structure. The funding will also be used to replace the south end of the viaduct with a new surface roadway. The Legislature also directed Washington State Department of Transportation (WSDOT) to evaluate whether an additional $400 million can be raised through tolls.

In addition to a bored tunnel, the plan includes replacing the seawall between Colman Dock and Pine Street, building a new Alaskan Way surface street including a new connection to Elliott and Western avenues, constructing a waterfront promenade, and implementing transit and surface street improvements. The total cost for all the improvements is $4.24 billion. The Port of Seattle, King County, and the city of Seattle will fund the remaining $1.44 billion through local sources.

The bored tunnel will be located at depths of 60 to 200 feet, where WSDOT expects to encounter dense natural soils during construction, based on information from previous downtown Seattle tunnel construction projects. The agency is conducting geotechnical investigations to further its understanding of the types of soil along the tunnel alignment and says it will obtain soil samples about every 300 feet along the alignment.