On January 6, 2011, the Washington State Department of Transportation (WSDOT) signed a contract with Seattle Tunnel Partners for the biggest piece of the SR 99 Viaduct replacement project, the 1.7 mile long tunnel carrying traffic from the south end of the Seattle waterfront to near the Seattle Center. This is a design-build contract with an estimated value of about $1.35 billion.
The SR 99 viaduct was built in the 1950s and concerns have been expressed about its ability to survive a major earthquake, in part because its foundations rest in part on fill materials that may liquefy in such an event. After studying various options, WSDOT has decided to replace the viaduct with a bored tunnel, which will require the use of one of the largest tunnel boring machines ever built. The SR 99 replacement project has many other parts, some of which are already in the preliminary phases of construction.
The WSDOT website has lots of interesting information about the project, including video simulations of how the existing viaduct and nearby seawall might collapse during a major earthquake and how it will look to drive through the completed tunnel.