Revamp may close viaduct

The Alaskan Way Viaduct could be closed for almost four years if a tunnel is built to replace the roadway, according to the state Department of Transportation (DOT).

For years the state has said the viaduct would remain open during construction, but new cost estimates indicate that it's poised to make a major reversal on that position.

In releasing the new estimates for replacing the aging roadway with a tunnel, the DOT said closing the road could shave years off construction time.

The viaduct-replacement scenario that might lead to the longest closures involves a section north of Pike Street where the tunnel would end. The tunnel would have to be connected to a bridge leading to the Battery Street Tunnel, said David Dye, viaduct-project manager with the DOT.

Dye said that section alone could cause closures of three years or more.

In 2004, the state studied what would happen if it were to close the viaduct during construction of its replacement. The study was prompted by a group of Seattle waterfront-business owners that complained closing the viaduct during construction wasn't considered in the environmental-impact statement written for the viaduct options.

The state study found that by closing the viaduct, it could save $300 million to $500 million, but it would add 25,000 cars to Interstate 5 downtown each day by 2010. It also would increase downtown street traffic 36 percent, to 33,000 cars a day. Dye said those numbers haven't been updated but will be in the environmental-impact statement.

"The scheme to maintain traffic on the viaduct is very complicated, and there's a lot of risk associated with it that needs to be more thoroughly evaluated," Dye said.

Under the new estimates, rebuilding the viaduct, rather than replacing it with a tunnel, would be much cheaper, would cause less traffic disruption, but would take up to two years longer to build.

Dye cautioned that the new estimates are preliminary.

Officially, the state and city of Seattle want a tunnel to replace the viaduct, damaged in the 2001 Nisqually earthquake. But while there is enough money to rebuild the structure, $2.45 billion, the state is several million dollars short of paying for a tunnel. The city is confident it can find the extra money.

These are the four options the state is considering. All envision construction starting in 2008:

• "Tunnel complete": Build a tunnel along the central waterfront, replace the sea wall along the north waterfront, add a walkway over Highway 99 from Victor Steinbrueck Park to the waterfront, widen curves in the Battery Street Tunnel and lower Aurora Avenue North north of the tunnel. Cost would be $3.7 billion to $4.5 billion and would take seven to 10 years to build.

• "Tunnel core": Build a tunnel along the central waterfront and a bridge up to the Battery Street Tunnel, with no lowering of Aurora or changes to the tunnel other than safety improvements. The sea wall would be partially replaced. The cost would be $3 billion to $3.6 billion, and construction would take seven to 10 years.

Both of these tunnel options assume a complete closure of Highway 99 for 18 to 42 months. The longer the closure, the shorter the construction time.

• "Rebuild complete": Rebuild the viaduct along the central waterfront. This option includes complete sea-wall replacement and a lowered Aurora Avenue. Cost would be $2.6 billion to $3.1 billion, and construction would take 11 to 12 years.

• "Rebuild core": Rebuild the viaduct and sea wall along the central waterfront. No lowering of Aurora. The cost would be $2 billion to $2.4 billion and construction would be 11 to 12 years.

Rebuilding would take longer than a tunnel because the roadway would remain open, at least during peak hours, during construction, Dye said.

"Much more work needs to be done," he said. "But this updates the information and frames the issue for us better. We bracketed the best and worst case."

Susan Gilmore: 206-464-2054 or sgilmore@seattletimes.com