OWATONNA — Though there haven’t been any sinkholes that have swallowed homes in southeast Minnesota recently, that doesn’t mean they aren’t present across the landscape.
Jeff Green, a hydrogeologist for the Minnesota Department of Natural Resources, said there are currently about 10,000 sinkholes that have been inventoried in this section of the state, but it’s difficult to get an exact figure.
“There’s no way of telling how many sinkholes there are,” Green said. “We don’t go out and actively go across the entire landscape searching for them. We don’t have the manpower or the resources for that.”
Sinkholes in Minnesota, known as cover-collapse sinkholes, are common because of the presence of soluble rocks like limestone and dolomite, that can wear away as water runs through the soil and reacts with gases in the soil produced by plant roots and rotting organic matter. The water mixed with carbon dioxide forms a carbonic acid that erodes the stone.
“People think it’s because the limestone collapses, and that is not how they form,” Green said. “Those rocks go from the Mississippi river all the way to Mankato … Limestone naturally has joints and fractures in it, and the water will get into those joints … and the soil sitting on top starts to get flushed into those openings in the rock and they get carried away into the groundwater system.”
Once the soil is gone, there is only a chasm of air where the soil once was. Eventually, the ground on top becomes too heavy and collapses, causing a sinkhole — an event that can be dangerous, even deadly.
Earlier this month, 37-year-old Jeff Bush was killed in Florida after a sinkhole opened up under his home. Sinkholes pop up across Florida every year, said state geologist Jonathan Arthur, who noted that the sinkholes could be occurring due to drought conditions.
“An extensive drought can cause soil and sediment over a cavity to be extremely dry and collapse,” Arthur said earlier this month.
But it’s not just Florida that is at risk.
Sinkholes are nothing new for residents of Steele County. In March 2004, a portion of County Road 45 — near the interchange with Highway 14 and what is now Federated Insurance Companies’ A.T. Annexstad Building — collapsed, leaving a gap in the road that was 15 feet deep and 20 feet wide. The collapse happened just moments after a sports utility vehicle and a gravel truck had driven over that exact spot in the road.
Steele County Public Works Director and Engineer Anita Benson said there are two different types of holes that form — sinkholes, like the ones described by Green, or holes that occur due to infrastructure under the streets, called “suck holes.”
Steele County Highway Maintenance Manager Beth Brady said suck holes occur most often in culverts that run under roads. Gaps occur in some of the piping that allow moisture to get into the fissure, bringing the soil with it and leaving a gap of air below the surface of the ground.
Brady said in most cases, suck holes begin in the ditches and work their way toward the middle of the road, allowing them to be caught before they become a problem. The county now ties pipes together to keep the gaps from forming.
“The center-line culverts that go underneath the road are typically concrete,” Brady said. “MnDOT now has a spec where you tie all the joints (of the pipe). They drill holes in them and put these ties in and tighten them, and they keep the pipe together.”
The county has begun taking what local officials call a “proactive approach” toward suck holes to stop them before they become a problem. When a project is being done, the county inspects the culverts to make sure they are structurally sound. If the piping needs to be tied, it is done during a road project to prevent digging up a new street later. Brady said if a problem were to occur in the roadway, the county would address it right away.
“Now is a good time of year, because everything is draining and frost wants to work its way out,” Brady said.
Owatonna Public Works Director Kyle Skov also noted that there are two types of sinkholes.
“There’s the naturally occurring ones, where you have limestone bedrock which is in southeast Minnesota, and we don’t have a lot of issues with that,” Skov said, noting that a portion of School Street collapsed last year. “When School Street collapsed last year, there was a sinkhole associated with that. That’s more of a man-made one due to our infrastructure.
“We’ve got storm sewers and sanitary sewers that can create a sinkhole. We keep track. After (the School Street collapse), we found another sinkhole, so we do inspect. But they are also hard to find.”
Skov said that he doesn’t have concerns about the sinkholes described by Green. Green said while the types of sinkholes that occur in Minnesota are similar to the ones that happen in Florida, they aren’t of the same magnitude.
“We have big sinkholes here, and we have sinkholes that pop up every year — we have a very dynamic landscape,” Green said. “It’s the same process but it’s a different magnitude.
“Put it this way, I don’t lay in bed at night worrying that a sinkhole is going to get me.”