The continued settlement isn’t severe enough to require that the Alaskan Way Viaduct be closed to traffic or have an emergency repair, a watchful state says.
A new inspection shows the Alaskan Way Viaduct has sunk another quarter- to a half-inch near the Seneca Street exit.
State bridge engineers say there’s no immediate risk.
However, the total sagging at this location has reached about 3½ inches since the 2001 Nisqually earthquake — raising the question of how long the 62-year-old structure along the Seattle waterfront can endure before a deep four-lane tunnel replaces it.
Former Gov. Chris Gregoire declared in 2008 that the viaduct would be demolished by 2012, and the south half was removed on time. The remainder, which runs along the Seattle central waterfront, is planned to continue carrying traffic until the new Highway 99 tunnel is completed, which currently is projected for April 2018.
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The damage isn’t severe enough to require emergency repairs, said Laura Newborn, spokeswoman for the Washington State Department of Transportation (WSDOT), which announced the inspection results Friday morning. The routine viaduct inspection was conducted Oct. 31.
“If the viaduct was unsafe, we would close the road. If bridge experts find something in need of repair, WSDOT repairs it. Our bridge experts concluded no repairs are needed as a result of the last inspection,” Newborn said.
Crews also found new cracks in columns and girders, and that some previous cracks have widened by a half-millimeter, the state said.
It’s unclear what caused the damage. One possibility, Newborn said, was that repair work last year, to widen an expansion joint, released pressure. In early 2014, two bridge decks were pushing against each other in that area.
Senior bridge engineers said last year they suspected that slow, residual soil motion related to the 2001 quake may still be affecting the viaduct.
They applied epoxy to fill several cracks that emerged in 2014.
The Seneca Street ramp is six blocks away from the giant 120-foot-deep vault where tunnel-boring machine Bertha is being reassembled, following a two-year stall and repairs. In addition, city seawall construction is under way, as well as building construction and underground utility repairs between Bertha and Seneca Street.
Based on standards WSDOT established last decade, a total 6 inches of settlement would trigger urgent stabilization work. This can include concrete injections into the soil, braces around the columns, or long steel rods that are planted around the underground foundations. The state did such work in the Yesler Way area in 2008, when total settlement reached about 5½ inches.
Newborn said the Oct. 31 inspection report is preliminary and not available yet for public disclosure. Tom Baker, WSDOT’s chief bridge engineer, is unavailable until Monday, she said.