SEATTLE - Gov. Chris Gregoire and Seattle Mayor Greg Nickels signed an agreement Saturday to replace the aging Alaskan Way Viaduct with a deep-bored tunnel.

The agreement ends eight years of debate about how to replace the seismically vulnerable viaduct on Seattle's waterfront.

The Seattle City Council voted unanimously Monday to authorize the state and the city to sign the pact, which lays out a plan for the replacement and sets a goal of completing the new tunnel replacement by 2015.

The tunnel project is expected to cost more than $4 billion. Under the agreement, the city will be responsible for at least $927 million of the project's costs.

The agreement details investments in the Highway 99 corridor, city streets, a new Alaskan Way, Mercer and Spokane streets, the waterfront promenade, and central seawall. It includes accountability for projects, implementation roles and funding responsibilities.

"The time for moving forward is now," Gregoire said. "This agreement means we will work with the city of Seattle to take down the seismically vulnerable Alaskan Way Viaduct and replace it with a bored tunnel that provides capacity for the future."

Gov. Gregoire, Mayor Nickels and former King County Executive Ron Sims announced their decision in January to replace the Alaskan Way Viaduct with a bored tunnel. The state Legislature endorsed the bored tunnel during its last session.

"This is an historic day for Seattle," Nickels said. "We finally connect our city to Puget Sound and ensure that people and freight move swiftly and safely. Generations to come will reap the benefits of this agreement."

State Rep. Judy Clibborn, House Transportation Committee chairwoman, said, "This partnership will help to ensure our investment is made wisely and efficiently."

Mayoral candidate Mike McGinn previously vowed to stop the project if he's elected, but after Monday's council vote said, "Although I disagree with this decision, it will be my job to uphold and execute this agreement."

Joe Mallahan, who is running against McGinn, expressed support for the tunnel and applauded the council for moving the project forward. He also criticized his opponent for blatantly changing his mind about the plan.

Construction of the new south mile of the Alaskan Way Viaduct near the sports stadiums will begin in March. The design of this portion of the project, with an estimated cost of $200 million, is complete.

The next step for the central section of the Alaskan Way Viaduct is a continuation of the ongoing environmental process.

A second supplemental draft environmental impact statement, which analyzes the bored tunnel alternative and builds upon the previous review of the other alternatives, will be published for public review in early 2010.