At What Point Will The Viaduct Be Unsafe? It's Complicated, State Says

The Alaskan Way Viaduct is safe to drive on, state officials told the Seattle City Council on Monday afternoon. That follows the weekend’s news that the viaduct may be sinking more than expected.

Just west of the viaduct, a giant wood and metal wall shielded public viewing of a giant pit where Bertha, a boring drill, has been stuck for nearly a year. Bertha is supposed to bore through Seattle, creating a tunnel to replace the Viaduct.

Construction crews had been pumping water out of that pit – necessary to create a dry area so that construction crews can work on Bertha once the excavation is complete. But the water pumping may have caused the viaduct to settle an additional 1.25 inches in November.

State officials were careful to not explicitly blame the water pumping for the soil settlement, but the correlation was strong enough that they are considering halting the pumping.

Stopping the pumps is a big decision, said Robert Robinson, a consulting engineer for the Washington Department of Transportation. Robinsons said the pumps aren't like a light you can switch on and off at will.

"Soil, like everything else, has a buoyancy," Robinson said. "When we take the water out, it settles."

Thus, a tricky cycle: If you stop pumping after removing water, the soil would expand. Restart pumping, and the soil compacts even more than the first time. That could increase how much the viaduct and surrounding buildings sink.

Todd Trepanier of WSDOT had hoped to reassure the Seattle City Council yesterday. But Councilmember Jean Godden suggested the state transportation department scrap its planned presentation and answer the council's questions. Councilmember Mike O'Brien grilled Trepanier, asking how much the viaduct would have to sink before being deemed unsafe.

Trepanier said the engineering was too complex to answer simply.

O’Brien wasn’t satisfied with that response: "I don’t know how worried to be, because there’s a complete lack of transparency about what’s going on down there," he said.

WSDOT officials apologized for the department’s failure to quickly inform the city council when it had concerns about the viaduct’s settlement. State officials promised to come back before the council in a week, Dec. 15, to discuss backup plans – and what they'll do if the sinking gets worse.