Sinkhole Hazard Overview

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.


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.


Link to the State Hazard Mitigation PlanKarst and Sinkhole Risk Assessment for more information and maps.