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WSDOT Looks To Tamp Down Viaduct Concerns, But Has No Easy Answer On When To Start Worrying

Seattle Tunnel Partners
Portions of the viaduct dropped more than an inch last month, but officials say the structure is still safe.

Officials overseeing the replacement of Seattle’s Alaskan Way viaduct are trying to tamp down safety concerns. But under questioning Monday from Seattle City Council members, they had a hard time coming up with an answer for when people should start to worry.

The exchange came as engineers continue to investigate what’s causing the viaduct to sink. It dipped about 1.25 inches in mid-November.

Settlement itself is nothing very new, having been a known problem since the Nisqually quake in 2001. Engineers and planners have kept a closer eye on it during the Highway 99 tunnel project, and it’s thought that work connected to rescuing the tunnel boring machine known as Bertha may have contributed to the latest instance.

The question is how much settlement poses a safety hazard.

No Easy Answers

Washington transportation officials used to say 6 inches was the threshold, but now they say it’s much more complicated. In fact, parts of the viaduct have already sunk that much since 2001. But officials say retrofits have shore up portions of the structure, so that number no longer makes sense.

City council member Mike O’Brien asked several times for clarity from WSDOT’s Todd Trepanier.

“[WSDOT] has failed to actually articulate exactly what is the threshold, other than to say, ‘Trust us, we will let you known when we’ve crossed it,’” O’Brien said.

“The reason it’s not just the 6 inches as you described, it really is a case-by-case,” Trepanier answered.

Trepanier and his staff said there are many variables, including how fast the settlement happens and which part of the viaduct is affected.

Even Or Uneven?

A big concern would be to find the viaduct is settling unevenly, which can bend the structure out of shape and damage it. Officials and engineers said they have seen only tiny amounts of that deformation and no cracks or damage – nothing to worry about yet. But they had only general answers for how much of that so-called “differential settlement” would force them to shut the viaduct down.

They’ll talk more about that next week: the city council has asked them to come back and discuss how an emergency closure would work.

Meanwhile, the company operating Bertha is considering whether to stop its groundwater-pumping.

That dewatering, part of the effort to tunnel down to Bertha and hoist it up to the surface for repairs, can cause deep soils to compress. Trepanier said the company is still pumping for now, but is reviewing whether it should taper off or stop.

Gabriel Spitzer is a former KNKX reporter, producer and host who covered science and health and worked on the show Sound Effect.