From the Seattle Times in spring 2010:
Q: Marvin Jahnke, of Burien, says he recently received an informational bulletin from Seattle's Department of Transportation (SDOT) outlining the $168 million Spokane Street Viaduct project.
He says he's puzzled. "How is it that this amount of refurbishment can be put into a viaduct that is older than the Alaskan Way Viaduct, and built on the same sort of ground as the Alaskan Way Viaduct, and yet the Alaskan Way Viaduct must be torn down?" he asked.
He says he's observed support piers under the Spokane Street Viaduct being rebuilt in recent years without much public fanfare, yet SDOT seems to go public with its periodic measurements of Alaskan Way Viaduct settling.
"Could the salvation of the Spokane Street Viaduct be that there are no expensive views to be gained by its demise?" he asked.
A: While the Alaskan Way Viaduct, which was built in the 1950s, and the Spokane Street Viaduct, built in the 1940s, may appear similar, they have some very distinct differences, says John Buswell, SDOT's roadway structures manager.
"For example, the two-level Alaskan Way Viaduct has a significant weakness at the point where the top roadway is connected to the lower roadway," said Buswell. That is not an issue for the single-level Spokane Street Viaduct.
He said the seismic retrofit solution for the Spokane Street Viaduct has, in part, been to build a new parallel bridge. The new bridge is being built to modern earthquake standards and is designed to hold up the older portion.
Yes, the city has invested over the years in the older portion, Buswell said, and SDOT believes that has preserved the Spokane Street Viaduct. But, "there will come the day when the old section can no longer be maintained and will need to be replaced."
On the other hand, the Alaskan Way Viaduct's age and vulnerability are showing, says Jugesh Kapur, state bridge and structures engineer for the Washington State Department of Transportation (WSDOT). It's pretty hard to ignore crumbling concrete, exposed rebar, weakening column connections and deteriorating railings.
When the Alaskan Way Viaduct was built, the structure's columns were not placed deep enough into stable soil, Kapur says. Instead, that viaduct stands on fill soil that Kapur says can become quicksandlike in an earthquake.
The state did consider retrofitting the old Alaskan Way Viaduct, Kapur said. "However, several studies have shown that a retrofit would be a poor investment."
By WSDOT's figures, the cost of retrofitting the Alaskan Way Viaduct would be about 80 percent of the cost of a new structure. Kapur says federal guidelines recommend replacing a structure when retrofitting exceeds 50 percent of new construction costs.