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 after work by Seattle Tunnel Partners (STP), said Todd Trepanier, Highway 99 administrator for the state Department of Transportation (DOT).
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 said.
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It would take an earthquake or similar catastrophe to force a traffic shutdown, Trepanier said.
But the settling follows approximately 5½ inches of sag since the 2001 Nisqually Earthquake. The DOT previously sank thin steel piling 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 that controlling settlement is the contractors’ responsibility and that other methods are available.
On Monday, City Councilmember 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 of 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 Tuesday he would like a public discussion and a written DOT commitment saying exactly how much settlement is safe, before any crisis might 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 nearly
35 percent water. 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 seawall.
Trepanier said, “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 piling, 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 set for this weekend. The structure is continuously measured by electronic devices.
Mike Lindblom: 206-515-5631 or email@example.com. On Twitter @mikelindblom