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Five things everyone should know about… sinkholes

1. They exist in Wisconsin. Parts of the Badger State have bedrock consisting of dolomite, an easily fractured rock that can be dissolved by water seeping down beneath the surface soil. That erosion can create an underground cavity that becomes a sinkhole if the surface soil above it collapses.

2. But they are relatively small. The past year has been full of hellacious reports involving sinkholes: the man who survived an 18-foot fall into a sinkhole on an Illinois golf course, the Florida man who died after falling into a 60-foot-deep sinkhole that had formed beneath his home. In Wisconsin sinkholes tend to be much more tame—smaller than 10 feet across. (And, while their depth varies, most sinkholes are about as deep as they are wide.) Wisconsin sinkholes are smaller due to the bedrock found here. Dolomite is less easily dissolved than limestone and other types of rock that allow for bigger sinkholes in other parts of the world.

HM-Five-Things-1-sinkhole

3. Some parts of Wisconsin are more prone to sinkholes than others. And to find them, follow the dolomite. It appears in a large V-shaped formation from Green Bay (including Door County) down to Dane County and then back up to St. Croix Falls. The map (right) shows Wisconsin’s karst, a landscape created when water dissolves rock—thus making it susceptible to such things as fissures, caverns and sinkholes.

4. Some sinkholes are not due to natural causes. A water main break can create a large underground cavity with sinkhole potential. Another cause: a ruptured tile drain, a system of perforated pipes installed beneath cropland to remove excess water from the soil. If a section of pipe ruptures (in what is called a “tile blowout”) it may draw in large amounts of soil, thus creating an underground cavity above it.

5. There’s a sinkhole on my property! First decide if the sinkhole is hazardous—and if it is, prevent access to it. Sinkholes should be filled to prevent falls and stop potentially contaminated water from flowing into the groundwater. The best way to fill a sinkhole is to use what is called reverse grading. Use large rocks at the bottom, switch to cobbles and gravel, and end with sand. Then place a seal over it using either a plastic liner or clay, followed by eight to 12 inches of top soil. Ideally the sinkhole should be slightly mounded to keep water away. The larger rocks will support the material above them and the smaller material and mounding will prevent water infiltration.

John Panuska is a distinguished faculty associate in the Department of Biological Systems Engineering and a UW-Extension natural resources specialist. David Hart is a professor of civil and environmental engineering and a hydrogeologist with UW-Extension and the Wisconsin Geological and Natural History Survey.

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