A key adviser to Gov. Christine Gregoire said Tuesday that replacing the Alaskan Way Viaduct along the waterfront with a deep bored tunnel "is probably the most viable option" if the numbers pencil out.

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OLYMPIA — A proposed tunnel to replace the Alaskan Way Viaduct is making a comeback.

A key adviser to Gov. Christine Gregoire said Tuesday that replacing the viaduct with a deep-bored tunnel “is probably the most viable option” — if the numbers pencil out.

The comments came after Gregoire, King County Executive Ron Sims and Seattle Mayor Greg Nickels issued a joint statement saying they would miss today’s deadline for deciding how to replace the viaduct and needed more time to study the options.

They now expect to make a decision sometime in January, according to the Governor’s Office.

The reason for the delay?

“The interest in the tunnel has led us to take some additional time to study it right,” said Ron Judd, a senior adviser to the governor and her lead negotiator on the viaduct.

Earlier this month, the state, city and county transportation planners announced two finalists to replace the viaduct: another elevated structure or a surface route in which southbound traffic would run on Alaskan Way and northbound traffic on Western Avenue.

At that time, the state transportation planners had dropped the idea of a bored tunnel but said it could be built later as a “stand alone” project along with the surface option.

However on Tuesday, Judd said transportation planners will be crunching numbers and talking to international tunnel experts to see if a tunnel is feasible now.

“I think the governor would say that if we could make the numbers work, that is probably the most viable option,” Judd said. “But that option is going to mean that there has to be a real meaningful partnership with the city and county and Port [of Seattle] to make it happen.”

Sims could not be reached for comment, but he has supported a surface-street option and his spokeswoman said Tuesday that he hasn’t changed his mind.

Seattle Deputy Mayor Tim Ceis said Nickels, who once supported a cut-and-cover tunnel, is taking no position on the deep-bore tunnel.

“We’re waiting to see where this information takes us. We’re not supporting or opposing it, but listening to the outcome of the process we all set in motion.”

A cut-and-cover tunnel is essentially a trench that is then covered. A deep-bore tunnel is drilled under the surface.

House Transportation Chairwoman Judy Clibborn, D-Mercer Island, said she doesn’t care if the governor misses the Dec. 31 deadline set by the Legislature for making a decision.

Clibborn said she met with Gregoire recently and advised her, “Don’t make it so you have to make a decision by a certain day if you need more information. Get it right. It’s more important that we get the right decision.”

In a statement announcing the decision was being delayed, Gregoire, Sims and Nickels wrote, “We only have one chance to do this right. It is imperative that we find the best option that addresses not only financing, but also solves safety, economic, capacity, and environmental concerns.”

State, city and county officials have spent years debating how to replace the 55-year-old viaduct, which carries state Highway 99. Engineers say it could collapse in an earthquake.

In 2007, Seattle voters rejected proposals to replace the structure with another viaduct or a tunnel.

That led the state and city to work on less controversial parts of the project, at its north and south ends, while figuring out how to replace the one-mile waterfront section.

$2.8 billion earmarked

The state Legislature has earmarked $2.8 billion for a replacement project, funded by state gasoline taxes and federal bridge grants.

So far, about $1.1 billion has been budgeted for a Sodo interchange, Battery Street Tunnel safety improvements and other work to be finished by 2012, when the waterfront section is due to be torn down.

That leaves around $1.7 billion for the rest of the project.

The surface option is estimated to cost $3.3 billion, and the elevated version $3.5 billion. Those costs include the $1.1 billion already set aside for the Sodo and Battery Street Tunnel projects, and a boost in transit service during construction.

Since the surface and elevated options were announced, however, tunnel advocates, including residents, business interests and environmentalists, have urged the governor to reconsider a deep-bore tunnel.

Supporters say a tunnel would carry more traffic than the surface-street proposal while eliminating the elevated highway they say walls off the city’s waterfront.

Even environmentalists, who pushed for the surface plan, say the tunnel should be studied in case surface streets can’t handle the traffic volumes.

Cost was chief concern

The cost of a tunnel, however, has been the state’s chief concern.

In November, the state said a deep-bore tunnel could cost as much as $3.5 billion, not including money needed for the work north and south of the existing viaduct or the temporary transit improvements.

Tunnel supporters have said that estimate is too high.

Judd said planners will be looking at whether a single bored tunnel could be done cheaper and faster than a twin-bored tunnel, which had been considered. It’s too soon to say whether the finances can be made to work, he said.

“There’s definitely been a consensus coming around the idea of a combination of surface and deep-bore-tunnel options that meet most of the people’s interest in the viaduct issue,” Ceis said. “We’re taking that input seriously and doing the work necessary to see if it can work.”

He would not discuss financing for a tunnel or how much money the city could contribute.

“I don’t want to talk about contributions. We’ll see how the finances work out,” he said. “This obviously costs more money.”

The Discovery Institute, a Seattle think tank, has been pushing for a deep-bore tunnel to replace the viaduct.

“This is definitely good news,” said Bruce Agnew, who heads the Discovery Institute’s Cascadia Center, which focuses on transportation issues. “The more that they look at the numbers, the life-cycle costs, the tunnel wins out over the other options. There’s momentum for the tunnel.”

Andrew Garber: 360-236-8266 or agarber@seattletimes.com