How Do Sinkholes Form?
Sinkholes are by far the largest and most frequently encountered karst hazards. Sinkholes Subsidence is a natural or human-induced process that results in progressive lowering of land-surface elevations. Sinkhole subsidence is a consequence of karst development and may occur over long-term (including geologic) and short-term timescales.
Sinkholes are physically manifested as closed and internally drained topographic depressions, of generally circular shape, that develop where soil or other overburden material subsides or collapses into subsurface voids. Sinkholes can form as a result of both natural (karst-related) processes and as a direct or indirect consequence of human activities.
Human activities that can cause sinkholes include groundwater withdrawals, alteration or diversion of surface runoff, subsurface mining; subsurface erosion, piping, or compaction of unconsolidated soils or sediments along buried pipelines or beneath highways and roads; and decaying buried organic debris (e.g., tree roots, buried trash, and other debris).
Sinkholes also form in nonkarst areas where leaking water or sewer pipes and other human activities create or result in subsidence, compaction, or subsurface erosion (i.e., piping) of soil, gravel, or other fill materials. Pipeline leakage and flooding affect highway roadbeds, pipelines, and other utility trenches, and often confuse nongeologists because the “sinkholes” created by these localized collapses may not be, and often are not, related in any way to karst or karst processes.
Sinkholes are classified by geologists using numerous descriptive terms depending on the types of geologic materials and processes or sequence of processes involved in their formation. Sinkholes may be grouped into two broad categories: subsidence and collapse.
Subsidence and collapse sinkholes often occur together in the same karst area, and many sinkholes form as a combination of the two processes.
- Subsidence sinkholes form by the relatively slow and gradual subsurface dissolution of soluble bedrock and piping of unconsolidated cover materials (soil, alluvium) into fractures and conduits enlarged by solution in the epikarst, a zone of intensified weathering and dissolution at the soil-bedrock interface.
- Collapse sinkholes form suddenly by failure of the roof or arch of soil, bedrock, or other surface and subsurface materials located above subsurface karst voids and caves. Collapse sinkholes that form over voids in unconsolidated materials—soil, sediment, or brecciated bedrock—are common and are referred to as cover-collapse sinkholes.
- Cover-collapse sinkholes are typically steep-walled circular or funnel-shaped depressions having diameters that range from a few feet to hundreds of feet.
The Kentucky Geological Survey began developing a catalog of case histories of cover-collapse occurrences in 1997 and has documented 354 occurrences throughout the state; an average of 24 new reports are received each year.
If you discover a sinkhole on your property, report it to your local public works department, sewer district, or other local officials as the sinkhole could be the result of a sewer collapse or other drainage issue.
What Is the Sinkhole Risk in Kentucky?
Karst hazards that could have an impact on Kentucky’s citizens and infrastructure include sinkholes, flooding, and groundwater and surface-water contamination. Sinkholes are by far the largest and most frequently encountered karst hazards. Kentucky is ranked fifth nationally among states affected by sinkhole hazards.
Subsidence sinkholes in Kentucky are generally recognizable as broad, shallow, bowl-shaped depressions. These sinkholes are largely responsible for the rolling topography that characterizes much of the Bluegrass and Western Pennyroyal Regions. Diameters can range from several tens to hundreds of feet, and shapes can be circular, elongate or irregular and complex. See the graphic below from the Karst and Sinkhole Risk Assessment.
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.
Link for Reporting A Cover-Collapse Sinkhole
State Hazard Mitigation Plan – Karst and Sinkhole Risk Assessment
Link to the State hazard Mitigation Plan – Karst and Sinkhole Risk Assessment for more information and maps.
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