ALAMEDA, Calif. — The Seattle City Council Monday ratified agreements designed to smooth the path toward construction of a tunnel to replace the elevated Alaskan Way viaduct along the city’s waterfront.

Monday’s 8-to-1 council vote is enough to override the promised veto from Mayor Mike McGinn, an opponent of the tunnel who says he fears that the city could be stuck with the bill for cost overruns on the project, even though the state has promised to foot most of the bill using bonds backed with fuel taxes.

The damage done to the viaduct by the 2001 Nisqually earthquake made it clear that some sort of replacement was needed for the double-deck elevated highway, now 58 years old.

There has been a lack of consensus in Seattle over how to replace it.

That resulted in conflict between those who want to get rid of the viaduct in order to reconnect the waterfront piers with the city street grid — as happened in San Francisco after the 1989 earthquake led to the demolition of the Embarcadero Freeway — and those who want to preserve the traffic throughput of the viaduct, which carries 110,000 vehicles a day on State Route 99.

The compromise solution, brokered by Washington Gov. Christine Gregoire, is to build a deep-bore tunnel that will carry State Route 99 under downtown, while removing the northern half of the viaduct. The southern half, farther from downtown, will be replaced by a surface road.

In 2009, state lawmakers agreed to support the governor’s plan by passing a bill allocating $2.4 billion in funding toward the $3.1 billion project, most of which will come through general obligation bonds backed by state fuel taxes.

But that legislation also requires Seattle to pay for any cost overruns, helping fuel a third, anti-tunnel faction in city politics — McGinn won the mayor’s office in 2009 on an anti-tunnel platform.

Even if the council overrides his veto, a citizen’s group has already submitted signatures for a November ballot initiative to bar the use of any city right-of-way for a tunnel to replace the viaduct.

The Port of Seattle pledged $300 million toward the project, which will result in improved connections to its shipping terminals.

The state’s plan also presumes $400 million will be raised by imposing a toll on the new tunnel, which would be used to back revenue bonds.

The Washington treasurer’s office has reported that the tolling plan is feasible, though lawmakers have not yet adopted any tolling legislation.

City politicians have continued to squabble even as the state Department of Transportation moved ahead with the project. In January, the DOT signed a $1.35 billion design-build contract with Seattle Tunnel Partners, a consortium led by Dragados USA and Tutor Perini Corp.

“With this contract, we are confident that the tunnel will be built within budget and delivered on time,” Washington Transportation Secretary Paula Hammond said when the deal was announced. “More than 90% of the design-build work will be performed for a fixed price.”

Monday’s vote approved agreements between three city agencies and the state government to cooperate on the tunnel.

With the state moving forward on tunnel design, City Council member Sally Clark on Monday said it was imperative for the city to have a seat at the table while decisions are made.

“As they make plans to move stuff, to dig up stuff, to destroy stuff, and rebuild stuff, we’re guaranteeing that it’s of the highest quality,” she said.

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