WSDOT answers to Seattle City Council on Alaskan Way Viaduct settlement

The Seattle City Council regularly gets updates on tunneling progress and the status of the Alaskan Way Viaduct, but Monday’s meeting carried far more weight.

A weekend assessment of the roadway reveals it has sunk about an inch in the past three weeks, which Todd Trepanier, Washington State Department of Transportation’s Viaduct replacement project manager, told the council coincides with the time frame when Seattle Tunnel Partners started pumping water out of deep wells for a new access pit in front of the Seattle tunnel machine.

Trepanier said it’s important STP is able to control the sub-surface water to be able to build the 120-foot access pit that will allow crews to reach Bertha, the drilling machine stalled under the waterfront for a year.

In the contentious briefing with angry councilmembers who complained they learned of the sinking through media reports, Trepanier said the sinking, centered around King Street and Alaskan Way, appears to have affected nearby buildings.

“We do know we have this influence zone where there’s 30 some plus buildings that appear to have a uniform settlement in that inch range. Roughly about a third of those were in our preconditioned survey zone,” Trapanier said.

While this uniform settlement is acceptable for the roadway, Trapanier said the WSDOT has implemented protocols to immediately go back out and check those buildings.

“The other 20 plus, two-thirds of the buildings that did not have those preconditioned surveys, because they’re further out and weren’t expected to be in the influence zone of just the tunnel, we’re in the process of putting together how we’re going to contact those property owners and/or the people who are responsible for those buildings and going in and doing a survey to see in fact if this settlement … did cause a problem.”

As for simply shutting down the de-watering process, Trapanier said it’s not that easy.

“If you just shut (the pumps) off in a very non-engineering style method, you could damage buildings or you could damage the pit itself. And then you have to decide if you’re going to turn those back on – what is going to happen there as well.”

Council president Jean Godden made it clear the council is most concerned about public safety.

“In order to prioritize public safety, we must have transparency of information regarding both settlement around the viaduct and the stability of the structure.”

Councilmember Mike O’Brien was clearly frustrated with the communication process and the acceptable settlement threshold.

“WSDOT has failed to articulate what is the threshold other than to say trust us, we will let you know when we’ve crossed it. This has to be able to be articulated in whatever complexity you want to articulate it in so we, the general public, the council, and everybody understands at what point it’s gotten so unsafe that we need to do something else.”

Trepanier said it’s a case by case situation, but O’Brien wasn’t satisfied and requested a much lengthier, detailed explanation.

The WSDOT maintains the viaduct is safe for drivers and construction.

“We’re not taking any measures to close the viaduct based on one inch of movement,” said David Sowers, Operations at Washington State Department of Transportation.