by: Jeff L. Coyne, P.E.

Sinkholes may form in certain areas (karst areas) of the world where soluble rock exists below the land’s surface.  The process of dissolving rocks (dissolution) occurs over the course of several years from the movement of slightly acidic groundwater traveling through small joints, fractures, and even caves.  Sinkholes are not the only features associated with karst topography; other common features include caves, springs, and disappearing and seasonal streams.  The United States is underlain with rock that dissolves in groundwater, and therefore some areas are susceptible to sinkholes.  The United States Geological Survey (USGS) indicates that sinkholes are predominantly found in Florida, Texas, Alabama, Georgia, Tennessee, Kentucky, Missouri, and Pennsylvania.

Sinkhole collapses occur when the ground (overburden) above the dissolved rock loses its support and falls into a void. 

Sinkhole collapses are generally unexpected although the formation of the actual cavity in the rock takes place over the course of several years.  Often the collapse is triggered by heavy rainfall, droughts, pumping of wells, or anything that may affect the groundwater level.  The collapse of the land’s surface can be catastrophic when roads or structures are located above a sinkhole collapse.  

Damages from sinkholes and their repairs should be performed by qualified professionals.  Most states listed above have adopted laws stating that sinkhole insurance claims should be inspected by qualified engineers (i.e. geotechnical engineers) or geologists.  Sinkholes that cause damage to structures may also warrant inspections by a structural engineer.   A geotechnical engineer/geologist and a structural engineer should work together to determine or approve repair methods to limit the possibility of another sinkhole collapse occurring at the same location. 

The most commonly used repair method for sinkhole collapses involves excavating down to the sinkhole’s “throat”, wrapping a filter fabric around the opening, filling the hole with a combination of large and small rocks, wrapping the surface of the rock with the filter fabric, and then covering it with compacting soil.  The purpose of these repairs is to not alter the groundwater’s path by allowing water to flow into th

e rock, but also to prevent future settlement (subsidence) or collapse.       

 Subsidence due to collapsed cisterns, buried trash, expansive clays, and abandoned mines are sometimes wrongly blamed as sinkhole activity.  Evidence is generally found during studies that can rule out sinkhole activity.  Buried cisterns are typically capped with concrete or metal that will deteriorate over time. Evidence of brick- or metal-lined walls is generally found in old cisterns as well as clay pipes discharging into the cistern’s cavity.  Buried trash and organic material will often decompose over a period of several years and cause the overlying ground to subside.  Buried trash is not easily detected from the surface and sinkholes are sometimes filled with trash on farms and in rural areas.  For these reasons, a subsurface investigation (i.e. geotechnical study) may be required to determine whether subsidence is due to buried trash or not.  Expansive clays may shrink or swell with the changes in moisture content of the soil causing subsidence of foundations.  Gutter and downspout drainage, poorly graded lots that allow water to pond near a foundation, chronic plumbing leaks, and significant drought conditions can in the swelling or shrinkage of expansive clays.