Idaho Springs’ mining history made a reappearance on July 19 when a sinkhole caused by an abandoned mining tunnel under the road appeared in the left eastbound lane of Interstate 70 near the Hidden Valley exit.
What started as a foot-long hole in the asphalt grew to more than 12 feet deep and 10 feet wide. Colorado Department of Transportation crews closed part of I-70 as they worked through the night to fill it.
“Hopefully nothing else crops up,” said CDOT spokesman Bob Wilson.
Wilson said work crews finished the project at 3 a.m. and had to let the concrete dry for four hours. The road was fully open for traffic at 7:50 a.m. July 20.
“They worked on it all night and got it stabilized, filled it with concrete, let it dry, paved over it,” Wilson said. “So I think there’s just a slight bump there, but I’ve already heard from a couple of people who drove over it that it doesn’t even seem like anything is there.”
Sinkholes caused by subsidence from abandoned mines are not unusual for Clear Creek County.
CDOT project engineer Jim Van Dyne said sinkholes can potentially pop up at any time, so the roads always are being closely monitored.
“That particular area is well known for many mineshafts and access shafts that crisscross through the area,” he said.
Van Dyne said several years ago, a geotechnical drilling project helped map the underground area and the voids caused by mining. The void under the sinkhole that opened July 19 was among them.
“Frankly, there are just so many voids down below the ground in Colorado and that area,” Van Dyne said. “You don’t want to dig them up and start looking for them and trying to fill them all because it would be a budget buster. Some of the holes are incredibly deep.”
In one case about eight years ago, just west of Idaho Springs near mile marker 239, a sinkhole opened on I-70 that was 20 feet wide and went down more than 100 feet.
“That one was an actual mineshaft. … I actually had to build a bridge underground to go over that shaft because it was so large. We didn’t know how much it would have taken to fill it with concrete,” Van Dyne said.
Van Dyne said crews never did reach the bottom of that sinkhole.
“We decided in that case to build a structure over that hole and then to put the highway back on top of it,” Van Dyne said.
A quick fix
Van Dyne said that because of the number of subsurface voids in the area and their potential to become sinkholes, CDOT is always monitoring road surfaces for any deviation.
“When we find something that’s not right or looks like it is starting to fail in some way, we jump on it,” Van Dyne said.
Many factors could cause a void to open under a road, including the area receiving a lot of rain.
“The water in a big storm could come in. It can migrate stuff around underground,” Van Dyne said.
CDOT crews used a large backhoe on the July 19 sinkhole to verify there wasn’t another shaft underneath that would break open. Crews used 34 cubic yards of concrete to fill the hole, and covered it with 12 inches of asphalt.
“It’s unlikely that we’ll have many future events, although over time, those holes open up here and there, and then we just fix them as we come to them,” Van Dyne said.