The 1950s structure, which was closed last weekend for its semi-annual inspection, started sinking into Seattle’s shoreline fill after the Nisqually earthquake of 2001. On the lower deck, Paananen, the viaduct’s project director for the Washington State Department of Transportation, pointed out the damage that drivers can’t see from behind the wheel.

Just south of the First Ave. off-ramp, for instance, the railing curves downward with the sinking structure. At another spot between Columbia Street and Yesler Way, the white fill of cracks streaks the underside of the upper deck. Unfilled cracks are measured with little, stick-on “crack monitors,” which engineers showed off at another point. They also gave away souvenir pieces of rubble.

The $915 million in immediate work on the Alaskan Way Viaduct involves (1) buttressing the columns in the highway’s midsection, the weakest part; (2) relocating utility lines; (3 and 4) upgrading the Battery Street Tunnel; and (5) replacing the highway from King Street to Royal Brougham. Map courtesy WSDOT.

It’s not quite as bad as saying that the viaduct is held up with duct tape and bailing wire, but it’s getting there, which is one reason that Gov. Chris Gregoire ordered WSDOT to renovate and replace parts of the structure after Seattle voters said no March 13 to both a new viaduct and a tunnel.

Paananen said that the work, which is slated to cost $915 million, includes shoring up the central, weakest part of the viaduct along the waterfront, leaving it to the city and state to arrive at a plan for the central piece by 2012, the governor’s deadline for tearing it down.

Starting in late 2008, WSDOT will move all the viaduct’s utility lines, renovate its north end and the Battery Street Tunnel, and replace its south end largely with a surface highway that will have new exits and on-ramps for the stadiums at Atlantic St. and Royal Brougham.

“We’re replacing about 40 percent [of the viaduct] and retrofitting about 15 percent, leaving 45 percent in the middle to be decided,” Paananen told the group. That decision, he said, should take about two years.

Columns that support the central viaduct, particularly two that stand between Columbia St. and Yesler Way, have already sunk four and three-quarter inches into the fill that makes up Seattle’s waterfront. Results of the new inspection won’t be out until March 30, but the limit, said state bridge engineer Jugesh Kapur, is six inches. “Beyond that point,” he said, “the structure would be too stressed for traffic.”

Starting this fall, after tourist season ends, WSDOT plans to spend about six months drilling 10-inch-round “micropiles” beside the column footings to a much lower depth of 30 to 40 feet, where they’ll be in solid ground. Much of what’s above that, Kapur said, is uncompacted fill that could liquefy in an earthquake.

“We are quite sure that will stop the settlement,” Kapur said. But “we are not beefing it up for earthquake conditions. That would be too much at this point.”

From Lenora Street north to the entrance of the Battery Street Tunnel, soil conditions are much better, Paananen said, allowing for a seismic upgrade and tunnel renovation pegged at $125 million. The plan is to lower the tunnel’s four lanes about two feet to provide a 16-foot clearance for large trucks. New lighting, ventilation, emergency exits and fire systems will also be installed.

WSDOT plans to keep half the tunnel open during construction, which will be completed in 2010 or 2011.

From King Street southward, the viaduct will be torn down and replaced with a four-lane surface road. At Massachusetts Street, it will turn into an overpass to cross the railroad tracks, then return to grade as a six-lane highway. Including the new on- and off-ramps at the stadiums, Paananen said the project will cost $600 million and start sometime in 2009, with completion in 2012.

WSDOT spokesperson Emily Fishkin said there has been discussion of rerouting SR99 traffic from West Seattle onto a looped ramp from the West Seattle Bridge to the Spokane St. Viaduct, where drivers could use the Fourth Ave. exit.

“But that’s not part of the $915 million,” Fishkin said.