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Sinking Structure Safe for Traffic

Thursday, November 19, 2015

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A portion of the Alaskan Way Viaduct—an elevated highway in Seattle—has continued to sink since its last inspection just six months ago, according to Washington State Department of Transportation officials.

The affected portion runs between University Street and south of Seneca Street, KOMO News noted, where the viaduct has settled anywhere between a quarter inch and a half inch since its spring inspection.

During the Oct. 31 inspection, engineers also discovered new cracks in columns and girders, and that some known cracks had widened by a half millimeter.

By Adbar / CC BY-SA 3.0 via Wikimedia Commons
By Adbar / CC BY-SA 3.0 via Wikimedia Commons

A portion of the Alaskan Way Viaduct is reported to have settled anywhere between a quarter inch and a half inch since its spring inspection.

Although WSDOT shared its findings in broad strokes, Laura Newborn, a state transportation spokeswoman, said the inspection report is preliminary and not available yet for public disclosure.

Ongoing Inspections

The Alaskan Way Viaduct replacement project—with the help of the tunnel-boring machine nicknamed Bertha that is tunneling underneath the city—will ultimately take the place of the 2-mile-long elevated double-deck road originally built in the 1950s.

Damage from a 6.8 magnitude earthquake in 2001 led to the need to stabilize and repair the structure and restrict heavy vehicle traffic.

Since the 2001 Nisqually earthquake, WSDOT has done a significant amount of work to reinforce and maintain the viaduct to keep it safe for traffic until the replacement tunnel, also known as the SR 99 Tunnel Project, is complete, the agency’s website says.

Twice a year, in the spring and fall, engineers thoroughly inspect the structure. Two of these inspections are done visually by crews in bucket trucks, and don’t require a closure of the viaduct, according to WSDOT.

The other two inspections require a weekend closure of the structure that allows crews to:

  • Measure existing cracks and look for new ones;
  • Check for structural movement; and
  • Evaluate the integrity of the viaduct’s foundations.

Inspection Findings

In the announcement of its findings, WSDOT clarified, “It's important to note that not all settlement is significant.”

WSDOT went on to say that no single number represents an acceptable level of settlement and that limits vary along the length of the viaduct according to ground conditions and the condition of the structure in each area.

“How the ground settles is also important,” the agency added. “A structure that settles uniformly is less likely to be damaged than a structure that settles unevenly.”

In the case of this section of the viaduct, the crew found the settlement to be uniform in nature.

Although the total settlement since 2001 measures 3.5 inches, the inspectors say there’s no immediate risk; it’s just a sign that the structure is reaching the end of its lifespan.

By Cliff CC BY 2.0 via Wikimedia Commons
By Cliff / CC By 2.0 via Wikimedia Commons

Although the total settlement since 2001 measures 3.5 inches, the inspectors say there’s no immediate risk; settlement would have to reach 6 inches to prompt emergency stabilization work.

"These small, subtle changes [are] concerning and that's why we are replacing the viaduct with a tunnel eventually," WSDOT spokesperson Travis Phelps told KOMO. "But it's not something that's going to cause us to go 'hey, this structure is dangerous, we need to shut it down right now.'"

Newborn told the Seattle Times that if the viaduct were unsafe, they would take action.

“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.

According to WSDOT standards, established last decade, a target measurement of 6 inches of settlement would prompt emergency stabilization work. This might include concrete injections into the soil, braces around the columns or long steel rods that are planted around the underground foundations, the Times reported.

What Caused the Cracking?

When asked about the cracking that was found in the columns and girders, Newborn said the cause was unclear, but engineers suspect it may have resulted from repair work (widening an expansion joint) performed early in 2014 where two bridge decks were pushing against each other in that area. Epoxy was used to fill in several cracks at that time.

Another contributing factor, according to senior bridge engineers, is that “slow, residual soil motion” connected to the earthquake event may still be affecting the structure.

The next inspection is scheduled for March 2016.

   

Tagged categories: Bridge Piles; Bridges; Cracks; Department of Transportation (DOT); Engineers; Environmental Controls; Inspection; North America; Roads/Highways; Safety; Traffic control

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