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Defining Sinkholes and Their Causes

Posted by Nicole Ghedini | January 25, 2017   

To help you understand the basics of a sinkhole, we have put together this brief blog post to define exactly what sinkholes are, what causes them, and the ways in which you can determine if you have one.

Sinkholes Defined

Simply put, a sinkhole is a hole in the ground created by erosion and water drainage. A sinkhole can be formed as the result of natural processes or triggered by human activities.

The Causes behind Sinkhole Formation

In the case of naturally occurring sinkholes, rock is dissolved by groundwater, leading to the collapse of ground into the resulting caverns and spaces where the rock once existed. This process can take place slowly over time or be sudden and catastrophic. In many instances, water will fill the caverns and spaces before ground can make its way into them.

Sinkholes can also occur as a result of human activity. Typically, in these instances, groundwater subsides after over-extraction of water from such activities as deploying a well or clearing subsurface water during construction of buildings. (graphic courtesy of Lake County, Florida.)

Sinkhole Causes in Depth

Sinkholes form in karst terrain. Karst terrain is a type of topography that forms when bedrock dissolves in areas where limestone, dolostone, or gypsum underlie the ground. Such terrain has underground drainage systems that manifest themselves on the surface as sinkholes, springs, disappearing streams, or even caves. Therefore, a sinkhole is just one type of drainage feature that can occur in karst terrain. 

Where karst terrain exists, slightly acidic ground water slowly dissolves cavities and caves in the stone over the course of many years. When a cavity grows so large that its ceiling can no longer support the weight of overlying sediments, the earth collapses into the cavity. In the less catastrophic type of sinkhole, a bowl-shaped depression forms at the surface, usually over a considerable period of time, as surface sediments ravel downward into small cavities in the bedrock. 

Three types of sinkholes exist in Florida. These include ‘solution sinkholes’, ‘cover-subsidence sinkholes’, and ‘cover-collapse sinkholes’.

Solution sinkholes usually occur where there is little or no sediment cover over the limestone. The rock is readily dissolved away at the ground surface or along joints and/or other openings.

Cover-subsidence sinkholes, by contrast, are located where thick, permeable sediments cover the limestone. In these cases, the void in the rock is filled by sediments slumping downward from above. Eventually, the ground surface will often show a slight, circular depression. If a relatively thick layer of impermeable sediments covers the limestone, it may be difficult to see surface evidence of a subsurface collapse.

Cover-collapse sinkholes occur where sediments that overlie the void in the rock suddenly collapse due to external triggering mechanisms, such as heavy rainfall, drought, or mechanical loading.

Determining If You Have a Sinkhole

There are numerous warning signs that often show up around forming sinkholes, including the following:

  • A low spot in the yard
  • Fresh exposures on fence posts and/or foundations
  • Slumping or sagging fence posts
  • Difficulty closing doors and/or windows
  • Cracks in walls, floors, pavement, and/or ground surface

Finally, there are numerous factors (other than sinkholes) that can lead to holes, depressions, or subsidence of the ground surface in your yard. These include the existence of broken pipes, buried trash, drying clay layers in the earth, and more. To know for sure if you have a sinkhole, you will need to obtain the results of a test by a licensed engineer with a professional geologist on staff to perform an evaluation of your property.

Contact us if we can be of assistance.

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