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Troubled Viaduct Develops New Cracks

Friday, March 14, 2014

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New cracks discovered on a seismically unstable double-deck highway in Seattle warrant a closer look, transportation officials say.

A recent inspection of the Alaskan Way Viaduct revealed new cracks in the 61-year-old elevated road's girders and supports, the Washington State Department of Transportation announced Monday (March 10).

WSDOT says the road is still safe, but the new cracks, along with the movement and widening of existing ones, have prompted bridge engineers to schedule a follow-up, "in-depth" inspection on March 22. 

Alaskan Way Viaduct
Photos: Flickr / WSDOT

Since the 6.8-magnitude 2001 Nisqually earthquake, WSDOT has been working to replace the Alaskan Way Viaduct. The structure is inspected four times a year.

"While the viaduct remains safe for travel, engineers need a second inspection to gather more information about the cracks before they can make repairs," WSDOT stated. 

13 Years of Cracking

Built in 1953, the Alaskan Way Viaduct carries SR 99 along Seattle's waterfront and sees about 110,000 vehicles per day. 

The viaduct, as well as its supporting Alaskan Way Seawall, was damaged in the 6.8-magnitude Nisqually earthquake in 2001.

After the earthquake, sections of the viaduct sunk several inches, and WSDOT quickly stabilized the structure. But if the earthquake had lasted a few moments longer, engineers said it would have collapsed. 

The cracks were most likely caused by additional settlement from the 2001 Nisqually earthquake, reported

WSDOT viaduct cracks

The discovery of new cracks, as well as the widening of existing ones, warrants an extra inspection, WSDOT says.

Ever since the earthquake, the viaduct "requires constant maintenance and attention to stay in service," WSDOT says, which is why the agency inspects it four times per year.

During the March 22 inspection, WSDOT says it will conduct an in-depth evaluation of the area, perform tests to determine how the cracks respond to heavy loads on the viaduct, and look for other issues. 

Engineers will also install monitoring devices on the columns to keep an eye on how the cracks grow and move over time. The data will be used to identify potential repairs, which may include injecting epoxy into the cracks, according to WSDOT. 

Moving Underground

The cracked section of the viaduct is located a little over a half-mile north of the massive SR 99 tunneling machine, Bertha. The tunnel is being built in order to take the viaduct out of service "before the next significant earthquake," according to WSDOT. 

Although the agency has not officially determined the cause of the cracks, it says they are not related to the tunneling work. 

Bertha, the world's largest tunneling machine, started digging July 30, 2013. However, work has been stalled since December after an issue with the machine's seal system. 

Bertha tunneling machine

Bertha, the world's largest tunneling machine, is being used to dig a tunnel underneath Seattle to replace the seismically unstable viaduct.

The new, two-mile-long tunnel is expected to open by the end of 2015. 

Half of the viaduct has already been demolished and replaced.

The other half has an automated closure system that WSDOT installed in 2011. If the system's earthquake monitoring device detects significant ground movement, it will simultaneously lower all nine traffic gates and close the viaduct within two minutes. 

The viaduct replacement projects are estimated to cost $3.1 billion. 


Tagged categories: Bridges; Cracking; DOT; Inspection; Roads/Highways; Tunnel

Comment from Dennis Justice, (3/14/2014, 5:07 PM)

Should have built the suspension bridge...

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