Shaky assumption: An Alaskan Way Viaduct myth is dispelled

More movement this week on the Alaskan Way Viaduct – literal, not political. State engineers said yesterday, Jan. 23, that the elevated waterfront freeway has sunk another one-eight of an inch since the last check six months ago. In all, it's settled more than 5 inches since they started measuring shortly after the 2001 Nisqually earthquake.

It's like an old man getting shorter. The Viaduct has osteoporosis.

I was always under the impression that the fundamental problem with the highway was its location. It was built on fill, in an area that used to be part of Elliott Bay. That was before the city erected a seawall, threw in some dirt, and moved the shoreline from Western Avenue out to where it is now at Alaskan Way. Atop all that is where the Viaduct sits.

Since loose soil can "liquefy" in an earthquake, the Viaduct will supposedly disappear into oblivion when a major shake hits, just like throwing a rock into quicksand. That's more or less how I've heard the warning over the years.

What I learned just recently, however, after talking with Tom Madden, the state's chief Viaduct engineer, is that the columns are actually quite deep and firmly connected to the "hardpan" soil 65 feet down, well below the "fill" dirt.

Did everyone else know that? Why am I just learning that the Viaduct is on fairly solid footing?

That doesn't mean that the Viaduct wouldn't shake in a major quake, or that a huge amount of "sloshing soil" around the upper part of the columns wouldn't be dangerous. But the relationship between the Viaduct and an earthquake is not as obvious as I always thought it was. I guess I feel slightly better about driving on it.

There are only four columns along the entire central waterfront that pose a problem (out of several hundred). They are the ones that have the osteoporosis. Indeed, when officials talk about the Viaduct sinking, it's those four they are talking about. They sit right near the foot of Columbia Street. The rest of the Viaduct, at least along the water, remains in its original position.

(The portion opposite and south of the stadiums apparently is connected to the fill dirt; plans are now under way to rebuild that portion, even before a Viaduct replacement option is agreed on.)

Right now crews are shoring up the four problem columns, hoping to prevent further sinking. The mystery is why exactly those are the only ones going down. But it doesn't seem to be related to the fill dirt, since they were firmly connected to the hardpan soil when the structure was built. The ones just 60-feet in either direction are doing fine. I would have thought most, if not all, of the columns were a problem until talking to Madden. If all the columns were drifting down together, Madden said, there wouldn't be any bend in the structure nor the resulting cracks.

A big part of the problem with the Viaduct isn't so much how it was constructed in 1953. Sure, the codes have become more stringent. But it's the relatively recent discovery of the so-called Seattle Fault in the 1990s that really shook things up for engineers. It runs along the Interstate 90 corridor, under the stadiums, out to Alki and beyond. In other words, right under the Viaduct. And it's a fault line that is much closer to the surface than the others in the area, which increases the damage it could do. The Viaduct wasn't planned with that problem in mind. It could do devastating damage to the roadway – though, of course, a big shake like that would take a lot of other structures down with it.

So it's not quite as easy as saying that the Viaduct was "built on fill." But we still have a big problem on our hands.