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Bolts to secure rock wall over highway

Posted: Monday, July 19, 2010

KETCHIKAN - More than 40 orange dots on a cliff face overlooking the Tongass Highway 10 miles north of Ketchikan mark the spots where rock bolts will be installed to prevent the kind of problems Ketchikan has experienced in the past.

"Ketchikan is full of rock bolts," Ralph Swedell, the Department of Transportation's geologist for the Southeast Region, said.

Older and more recent experiences tell what problems are to be avoided, he said. Rocks and cliff sides can fall apart suddenly and cover the highway, placing pedestrians and car riders in danger, stopping traffic for lengthy periods, possibly undermining houses, he said.

One example occurred on Feb. 10, 2009. Massive boulders tumbled down along a 190-foot stretch of North Tongass Highway, just north of Ridge Road near Peninsula Point. The slide stranded motorists and school buses on both sides of the rock pile. No one was injured, but part of a home dangled over a new cliff the rocks left behind. It and another nearby house were declared unlivable and were removed.

The North Tongass Highway widening project has included several blasting sites, including that now-exposed cliff face with the many orange dots.

Swedell said the rocks there have many surfaces, which have been studied for their potential to break apart. The geologist's project has been to measure the orientations of the various surfaces and project them into the cliff, using time-tested manual methods as well as computer programs.

He studied "discontinuities" in the rock and determined the potential "failure modes," attempting to predict the worst things that could occur, said Swedell.

With results from those procedures in hand, the department determined the amount and direction of force necessary to prevent the bad scenarios, he said.

On the rock wall that collapsed in February 2009, DOT Project Manager Jack Lee said that the state had installed bolts in that cliff, too, in 1991-92, but those had all been the same length, which allowed the rock to shear off at the same point. He said bolts for the new project are the latest technology, and will do a better job keeping the rock in place.

"They're designed to hold," he said, describing them as "30-foot, 1-inch honkers."

On the current project, preventive measures were designed to include a large factor of safety, to provide two or three times as much protective capability as considered necessary, said Swedell.

He said rock bolts apply a force to the face of the rock surface to hold it in place. The same are used in mines to prevent surfaces of tunnels from caving in, said Swedell.

Lee said Thursday that 32 rock bolts have been installed in two other slopes in the project.

Each bolt is "good for certainly over 100 years," Swedell said.



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