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February 25, 2014 at 12:35 PM

WSDOT says sinking viaduct isn’t a threat

State officials are trying to assuage public worry about the safety of the Alaskan Way Viaduct in the wake of reports that sections of the elevated highway are sagging slightly.

The section around Yesler Way has settled up to four-tenths of an inch following work by Seattle Tunnel Partners, said Todd Trepanier, Highway 99 administrator for the state Department of Transportation.

He said the causes were construction of a nearby buried concrete block, called a “safe haven,” where tunnel boring machine Bertha can stop for maintenance; and the loss of groundwater in December, when STP was pumping water away to allow for safe inspections of Bertha’s clogged cutter head.

“That settlement does not endanger the Viaduct or bring it closer to closure,” he told reporters at noon today.

It would take an earthquake or similar catastrophe to force a traffic shutdown, Trepanier said.

But the settling follows approximately 5 1/2 inches of sag since the 2001 Nisqually Earthquake. The DOT previously sunk thin steel pilings to reinforce some columns, and STP already inserted dozens of narrow concrete cylinders near Yesler — where Bertha is supposed to pass beneath the Viaduct later this year. Trepanier said controlling settlement is the contractors’ responsibility and other methods are available.

On Monday, City Councilman Mike O’Brien asked for a traffic control plan in case the Viaduct has to be closed due to settling.

He pointed out that the project to date hasn’t exactly unfolded as DOT predicted. The tunnel drilling is about four months behind schedule, with months more delay likely as STP digs a giant vault to remove the cutter head and fix damaged seals on the main bearing this spring.

O’Brien said in an interview Tuesday that he’d like a public discussion and a written DOT commitment saying exactly how much settlement is safe, before any crisis may occur. Lacking that, he said, “the politics of what we think about infrastructure come into play and cloud a safety discussion.”

“However complex this settling issue is, we need to know something more than, ‘We’ll know it when we see it,” O’Brien said.

Excavation of a rescue shaft for Bertha requires a 120-foot deep pit, below sea level in soil that’s close to 35 percent water content. One option for STP is to design a “watertight bottom” to prevent groundwater loss, Trepanier said. In Sarnia, Ontario, workers needed a thick concrete pit floor just to support the tunnel machine and equipment against sinking.

Interest in the settlement question resurfaced two weeks ago, when Jared Smith, Mayor Ed Murray’s chief of waterfront projects, mentioned the four-tenths figure, and nearby work on the city’s new sea wall.

Trepanier mentioned, “There’s nothing new here, for some reason there was some revisiting of the issue.” Seawall work this month, including the construction of deep concrete pilings, didn’t cause settlement on the Viaduct, he said.

He was asked repeatedly by reporters how close DOT is to exceeding safe limits to keep Viaduct traffic moving. He didn’t offer a number, saying it depends on the exact location of settling, and whether some deck sections sag more than others.

“If the whole Viaduct was settling at the same rate, it could probably settle clear to the ground,” he said.

A routine inspection is planned this weekend, and the structure is continuously measured by electronic devices.

Comments | More in | Topics: alaskan way viaduct, highway 99

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