Viaduct inspection reveals new and widening cracks
During a recent inspection of the Alaskan Way Viaduct, Washington State Department of Transportation engineers found that new cracks had formed and existing cracks had widened on the 61-year-old overpass.
The cracks, near Seneca Street and Spring Street, were discovered on March 1. WSDOT will close the double-decked bridge for one day on Saturday, March 22 to conduct further inspections and tests and to install equipment to monitor the size of the cracks.
“They're really small hairline cracks,” said WSDOT spokesperson Travis Phelps. “When I say widen I don't mean big gaping holes you can put your hand in. The viaduct is safe or we wouldn't have reopened it after inspection.”
Since the 2001 Nisqually earthquake, WSDOT has observed similar cracks on other segments of the viaduct. During the next inspection, Phelps said, engineers will try to determine whether a spot repair will keep the bridge sound, or if more “robust improvements” are required. The department could inject an epoxy into the cracks to prevent them from spreading or widening.
One of the tests that engineers will conduct during the closure will involve driving a truck loaded with gravel over the cracked areas to see how the structure reacts. WSDOT has said that it is unlikely the cracks are related to construction work on the Highway 99 Tunnel. The machine digging the tunnel is currently stopped, awaiting repairs, a little less than a half mile from Spring Street.
The structure has hundreds of cracks that are millimeters in width, Matt Preedy, deputy program administrator for the Alaskan Way Viaduct Replacement Project, said during a recent press conference. The cracks are mapped, and monitored with gauges and photographs. To strengthen other damaged sections of the viaduct, WSDOT has wrapped portions of the structure with carbon fiber.
If too much cracking takes place on the viaduct’s concrete, weight could be transferred to the structure’s steel, causing it to lose strength. The most sensitive section of the viaduct, said Preedy, is between S. Main Street and Columbia Street. “It’s safe for everyday use,” he said. “If we have a big earthquake, that’s the one thing that can take the bridge out of service.”
The Highway 99 Tunnel is designed to replace the viaduct. The new stretch of underground roadway was scheduled to open around December 2015. But an expert panel recently estimated that repairs to the boring machine digging the tunnel would delay the project's completion until summer or fall of 2016.