A geologic hazard is a naturally occurring geologic condition that may result in property damage or may be a threat to the safety of people. The geologic hazards associated with karst are: sinkhole flooding, groundwater vulnerability, radon, and cover-collapse sinkholes described on this page.
What Is a Cover-Collapse Sinkhole?
Cover-collapse sinkholes occur in the soil or other loose material overlying soluble bedrock. A sinkhole is any naturally occurring depression in the surface of the ground from which rainfall is drained internally. Sinkholes that suddently appear form in two ways. In the first way, the bedrock roof of a cave becomes too thin to support the weight of the bedrock and the soil material above it. The cave roof then collapses, forming a bedrock-collapse sinkhole. Bedrock collapse is rare and the least likely way a sinkhole can form, although it is commonly incorrectly assumed to be the way all sinkholes form. The second way sinkholes can form is much more common and much less dramatic. The sinkhole begins to form when a fracture in the limestone bedrock is enlarged by water dissolving the limestone. As the bedrock is dissolved and carried away underground, the soil gently slumps or erodes into the developing sinkhole. Once the underlying conduits become large enough, insoluble soil and rock particles are carried away too. Dissolution sinkholes form over long periods of time, with occasional episodes of more rapid subsidence or collapse. It is the collapse of the loose cover over the bedrock or soil that causes the problem. Sometimes the collapse will occur in an area with no indication of previous subsidence. In other words, a new sinkhole has formed. Sometime there was a preexisting sinkhole, but it had been filled in and covered over during land clearing or construction. Factories or homes built over filled sinkholes may be damaged as the fill continues to be transported out of the sinkhole and cover collapse reoccurs.
Why Does Cover Collapse Occur?
Many important aquifers are composed of granular materials such as sand and gravel or weakly cemented bedrock. Such aquifers occur in Kentucky, particularly along the Ohio River and in the Jackson Purchase Region. Groundwater flow in these aquifers is through the pores between the grains of sand or gravel or through narrow fractures in solid bedrock. Small openings act as a filter, physically or chemically removing most suspended matter, which holds back the soil cover. Water movement through the fracture system of a fractured bedrock aquifer or the pore spaces of a granular aquifer is slow, when compared to movement in a karst aquifer. The flow velocity in a granular aquifer is too slow, and the pores too small, to transport grains of soil.
The drainage patterns of karst conduits resemble the branching pattern formed by streams flowing above ground across insoluble rocks. Water recharge to karst aquifers occurs either directly, through swallow holes and sinkholes, or indirectly, through the pores in the soil overlying the limestone bedrock. The conduit of a karst aquifer is somewhat like a roofed creek bed and responds to rainfall like a storm sewer. Water flow is fast and turbulent, and significant amounts of sediment can be carried through the relatively large conduits. As water flows through the soil into the underlying bedrock, it enlarges joints (cracks) in the bedrock to form grikes . The grikes drain into small conduits, which in turn drain to a conduit large enough to be called a cave, which discharges to a spring . When the overlying soil is repeatedly wetted and dried, small amounts of soil are dislodged and carried away by the cave conduit draining the sinkhole. Gradually, a void develops in the soil overlying the enlarged grike or conduit. The collapse occurs only in the overlying soil cover, not in the limestone bedrock. Below is a diagram of the steps in the formation of a typical cover-collapse sinkhole.
Cover-collapse sinkholes can vary in size from 1 or 2 feet deep and wide, to tens of feet deep and wide. Unlike large collapses, like those in Florida where the cover over the limestone is very thick, cover-collapse sinkholes in Kentucky are unlikely to swallow entire houses or businesses. Cover-collapse sinkholes in Kentucky do severely damage buildings, drain farm ponds, damage roads, and wreck farming equipment. The thickness and cohesiveness of the soil cover determine the size of a cover-collapse sinkhole. In Kentucky the thickness of soil, sand or clay, and bedrock fragments over limestone bedrock is typically less than 25 feet, so cover-collapse sinkholes more than 20 feet in diameter are uncommon. Some precautions can be taken, however.
How Can Cover-Collapse Sinkholes Be Avoided?
The only guaranteed method of avoiding karst geologic hazards is to avoid living on karst. But because such a large percentage of Kentucky is karst, that is clearly impractical for most people. The most effective way to avoid cover collapse sinkhole damage is to avoid buying or building a structure on a sinkhole that has been filled. Ask the seller if any sinkholes have been filled, and, if so, where, how, and by whom. Look for previous damage to foundations and check doorframes and windows for squareness. Check the surrounding lot for shallow depressions and arch-shaped cracks in the soil. Should a cover collapse occur on your property, it is possible to repair the damage.
Repairing a Cover-Collapse Sinkhole
If a cover-collapse sinkhole develops under a building, the foundation of the building should be shored up as quickly as possible to avoid major foundation damage. Several engineering techniques are available to transfer the load of the building to competent bedrock. The sinkhole can then be filled at a less urgent pace using a graded-filter technique (Reitz and Eskridge, 1977, Sowers, 1996), the foundation reinforced, and the soil graded. The purpose of the graded filter is to allow water to seep into the ground while the soil is held back. In the case of farm ponds and lagoons, the graded filter construction is essentially the same, but the final layers are fine gravel, coarse sand, and fine sand. The uppermost layer is bentonite clay or volclay, which blocks water seepage. Always consult a professional geologist who is experienced in identifying karst subsidence and an engineer experienced in sinkhole remediation when dealing with any structure threatened by a cover-collapse sinkhole.
For More Information Contact
Head, Water Resources Section
Kentucky Geological Survey
228 Mining and Mineral Resources Bldg.
University of Kentucky
Lexington, KY 40506-0107
- Reitz, H.M., and Eskridge, D.S., 1977, Construction methods which recognize the mechanics of sinkhole development, in Dilamarter, R.R., and Csallany, S.C., eds., Hydrologic problems in karst regions: Bowling Green, Western Kentucky University, Department of Geology and Geography, p. 432-438.
- Sowers, G.F., 1996, Building on sinkholes: Design and construction of foundations in karst terrain: American Society of Civil Engineers, p. 115.