RAPID CITY, S.D. (AP) - South Dakota officials are inspecting seven scenic tunnels in the Black Hills this week for structural integrity.
Engineers and a geologist are monitoring the tunnels on Needles Highway and Iron Mountain Road to determine unsound rock, weathering and erosion, the Rapid City Journal reported. They do so by gently tapping hammers onto the tunnels, which were blown out of granite with dynamite in the 1920s.
“Hear the different sounds?” said Steve Kamarainen, region bridge engineer with the South Dakota Department of Transportation. “This sounds hollow, and this sounds harder.”
The inspection comes as the Federal Highway Administration requires more routine checkups every two years.
South Dakota officials aren’t concerned the tunnels will collapse, but the tunnels will gradually erode, just like any rock shelf.
“With a rock tunnel, it’s pretty straightforward,” said Todd Thompson, a bridge maintenance engineer. “You either have good rock, or you have unsound rock.”
Inspection crews try to monitor the tunnels in the fall to minimize the road closures’ impact on Custer State Park, Thompson said. Millions of travelers take road trips to the Black Hills in the summer.
“We also try to do it when the weather’s nice,” he said.
But that doesn’t always work out. Snow fell in the park over the weekend, as much as 12 inches (30.5 centimeters) in some areas. Inspection crews expect to finish their work by this weekend - just in time for the snow to melt.
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Information from: Rapid City Journal, http://www.rapidcityjournal.com
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