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That Sinking Feeling - Sinkhole 101

November 3, 2020
By WeatherBug
Sometimes, the low spot in the yard is just a depression from a rotting tree stump or potentially, an animal’s front door. However, it could also indicate a more dire situation — one that can literally swallow you up whole!

A common naturally occurring geologic phenomenon, sinkholes are classified as a geologic hazard that can cause extensive damage to structures and roads resulting in costly repairs. They even threaten water supplies by draining unfiltered water from streams, lakes and wetlands directly into the underground water supply! About 20% of our country is susceptible to a sinkhole event with the most damage from sinkholes occurring in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania.

Sinkholes can appear to have opened overnight, but the majority of them evolve over time, developing long before any surface evidence is detected. Areas where there is a lot of underground rock and clay soil are ideal for sinkholes. A common misunderstanding is that a sinkhole is the actual hole in the rock. But in reality, the sinkhole is what we see on the ground surface because of the hole in the rock below. The void in the rock takes hundreds or thousands of years to form.

Sinkhole Warning Signs

Below are a few warning signs that can signify that you have a sinkhole. If you’re experiencing a number of these signs, please contact your local emergency management organization and your insurance company:
  • Fresh exposures on fence posts, foundations or trees that result when the ground sinks around them;
  • Slumping, sagging or slanting fence posts, trees or other objects.
  • Doors and windows that fail to close properly or exhibit changed behavior such as doors remaining open where they had previously closed of their own accord.
  • Small ponds of rainfall forming where water has not collected before.
  • Wilting of small, circular areas of vegetation. This happens because the moisture that normally supports vegetation in the area is draining into the sinkhole that is developing below the surface.
  • Cloudy water is pumped from nearby wells where the water was previously clear.
  • Cracks in walls, floors, pavement and the ground surface. This is most noticeable in a concrete block structure and is different from a few hairline cracks normally seen between blocks.

Type of Sinkholes

Sinkholes can come in many different sizes and shapes.  They range from shallow depressions a few inches deep and several feet across to giants that can swallow multiple houses.  The sides of the sinkhole may be gently sloping or they may be vertical.  There are generally three types of sinkholes.
  • Limestone Solution Sinkholes – When limestone is exposed at the surface or is covered by a thin layer of soil, the limestone subject to both physical and chemical processes that break down the rock. When this breakdown occurs, it usually forms a saucer or bowl-shaped depression. Due to the natural dissolving of limestone, these sinkholes develop continuously, but slowly.
  • Cover-Subsidence Sinkholes – Where the sand layer may be as thick as 50 to 100 feet, with very little clay below it, the dissolving limestone is replaced by granules of sand that cascade down to fill the void. This type of sinkhole is referred to as a cover-subsidence sinkhole. These sinkholes are only a few feet in diameter and depth. Their small size is due to the fact that the cavities in the limestone cannot develop to appreciable size before they are filled with sand.
  • Cover-Collapse Sinkholes – Generally, the deeper the soil, more clay is present. This clay provides some cohesiveness to the soil material above it, allowing it to bridge an existing cavity in the limestone. If this “bridge” collapses, it results in what is called a cover-collapse sinkhole. The size of the sinkhole depends upon the size of the cavity. Cover -collapse sinkholes form the same way as cover-subsidence sinkholes but differ mainly in the bearing strength of the soil above the cavity and whether the sinkhole subsides slowly or collapses abruptly.

What To Do if a Sinkhole Occurs on Your Property:
  • Keep children and animals away!
  • Make sure the area is fenced or roped off clearly. In some areas a local enforcement agency may be able to assist. The property owner could be liable if someone is hurt in the sinkhole.
  • If your home is threatened or damaged, contact your insurance company. Damage to your house or other structure (but not to your land) is generally covered by homeowner’s insurance policies.
  • If damage to your house occurs, get out immediately! You may be able to go back and retrieve valuables once a professional has determined that the situation has stabilized.
  • You may want to consult with a soil-testing firm to evaluate the cause at you own expense.
  • If lake or river levels are affected, or you think ground-water quality is endangered by a sinkhole, please report it to your local water management company.
  • Monitor for signs that the sinkhole is enlarging, especially toward buildings, septic tanks, drain fields, or wells (flowing water into a sinkhole will continue or even accelerate its growth). You can monitor the hole by marking the sides with stakes or paint.  You can also use a thin hard metal rod that can be pushed into the soil. Areas near the sink will offer less resistance to the rod than the unaffected soil.
  • Monitor damage to structures. The width of cracks on houses can be measured and the size recorded by pencil or marker.  If damage is severe, water, gas and electricity should be turned off and the Fire Department notified.
  • Do not throw any waste into the sinkhole. Fill the hole only with clean sand.
  • Do not use the sinkhole as a drainage system. Pesticides and other wastes seep easily through the sinkhole and into the aquifer – your drinking water.

Generally, the size of a sinkhole will not increase after the first day or two except where the vertical banks collapse to form less steep slopes.  However, where there is water flowing into the hole it may continue to expand.