The tip-tip-tap of small hammers against the granite Hood Tunnel on the road to Sylvan Lake on Monday morning might have sounded like the music of mountain miners looking for copper in the rock wall — but it was only troubleshooters from the state, inspecting the first of seven Black Hills tunnels this week for structural integrity.
"Hear the different sounds?" asked Steve Kamarainen, region bridge engineer with the South Dakota Department of Transportation. "This sounds hollow, and this sounds harder."
An SUV with Wisconsin license plates approached the one-lane tunnel, and the crew cleared.
"We try to go in the fall so we minimize impact to Custer State Park," said Todd Thompson, bridge maintenance engineer. "We also try to do it when the weather's nice."
A slurry of ice and snow comes crashing to the pavement from one of the rocky spires above the tunnel. Over the weekend, reports say between 6 inches to a foot of snow fell in the park.
"Once the sun came up over the hill, things warmed up," Thompson said.
The seven tunnels on Needles Highway and Iron Mountain Road, blown out of the granite with dynamite in the 1920s to retrofit the scenic highways, have for years been monitored by state geologists for any weathering, cracking or loose rock. But recently, the Federal Highway Administration under the U.S. Department of Transportation required more routine checking every two years. This week's visit by engineers and a geologist to the tunnels is only the second. They'll look for soft spots in the rock and come back again to check for erosion.
"With a rock tunnel, it's pretty straightforward," said Thompson, holding a bound copy of the National Tunnel Specifications in his hand. "You either have good rock, or you have unsound rock."
You have free articles remaining.
In 2015, the state Department of Transportation gave official names to these seven tunnels that aesthetically frame the pine-clad hills. The jagged maws are wide enough for two motorcycles or a (snug fit) camper and provide cover for many of the millions of annual summertime roadsters to the Black Hills. There's no concern the tunnels will collapse. But they will, gradually, erode just like any rock shelf.
"Hopefully, a long time (they'll stay up)," said Thompson.
Jay Tople, a state geologist, used the pick-end of his hammer to dust off rock shavings from the hoary granite surface as Kamarainen took readings.
"Zero-plus-two, and we've got a one-by-one, and a one-by-two," Kamarainen said.
Today, the crew will close both Highway 87 Needles Highway and Highway 16A Iron Mountain Road as they inspect Needles Eye Tunnel and C.C. Gideon Tunnel. Iron Mountain Road will remain closed Wednesday for the inspection of Doane Robinson Tunnel. Both roads will be open on Thursday and Friday, but travelers should expect delays. On Friday, motorists will be diverted around the four-lane Miner's Gateway Tunnel in Keystone on Highway 16A.
By the weekend, the crew will be finished with the tunnels, just in time for the snow to melt.
"It's part of the deal of living in the Hills," said one of the men flagging traffic while warming his hands in the cab of one of the three vehicles the team drove up in. There weren't the sounds of many vehicles Monday morning, only the high-altitude wind, the droplets from the icicles, and the sounds of those hammers, gently tapping the rock.