How Sinkholes Develop

There are many detailed, scholarly reports that are available for those desiring a more thorough and highly technical discussion of this topic. What follows here is intended only to be a brief “consumer friendly” summation of sinkhole development.

Sinkholes are depressions or holes in the land surface. They can be shallow or deep, small or large, but all are the result of the dissolving of the underlying limestone. Hydrologic conditions, including lack of rainfall, lowered water levels, or conversely, excessive rainfall in a short period of time (especially after a drought) can all contribute to sinkhole development. New construction, new roads, and any diversion of water flow are also common culprits.

Solution activity within the limestone is greatest along localized fractures, joints, or bedding planes, since these features represent preferential paths that concentrate the flow of water into the formation. This activity is accelerated where the volume of flow increases. In Florida, especially, infiltration rates are very high; sometimes upwards of 75%, and this increased volume transports soil into the voids and speeds the dissolving of the limestone rock. The dissolution of the limestone can be accelerated even more if the rainwater permeates through vegetation. This makes it more acidic. Obviously, the closer to the surface the limestone is, the more likely that a sinkhole can occur, although there are many that are very deep. Despite many so-called insurance company experts, many sinkholes occur below layers of clay.

Sinkholes occur naturally, and it is estimated that the majority of all lakes in Florida are a result of sinkhole activity. Sinkholes are common where the rock below the land surface is limestone and other carbonate rock, which can naturally be dissolved by water. As the rock dissolves, the ceiling of the cavern becomes thinner. When the land above the cavern becomes too heavy for the ceiling to support, a depression or collapse of the land surface can occur. The damage can be minor or significant depending upon what structure is immediately above the sinkhole activity and how much ground is disturbed. Abrupt collapse sinkholes have become more common over the past several decades, proportionate to the increased activity of humans, which involves building, withdrawal of ground water, diversion of surface water, and etc. Even the “repair” of some sinkholes by grouting can cause more damage and create new sinkholes by the added weight to the soil and from the diversion of the current water flow.

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Spotting the Signs of a Sinkhole

 Just as there are several different types of sinkholes and all different types of homes and landscapes, sinkholes may appear very differently.  If you think you may have sinkhole damage to your home, the first thing that is done is determining whether or not it’s actually a sinkhole, as defined by Florida law.

 Roads, driveways, sidewalks or other pavements may be cracked or sitting at funny angles.  Floors might be off level in the house.  This can be checked by setting a ball in the middle of a room and watching if it rolls in a way it hadn’t before.  Such activity can easily crack the underlying concrete slab.

You may notice your doors no longer hang correctly in their frames and won’t close or lock.  Windows can even break in their frames or refuse to work properly anymore.  Large gaps in between the house and the door or window frames can appear -- sometimes large enough to see thorough or let critters in. Cracks in the wall at the corners of doors and windows are a common result of sinkhole activity.

Inside, if your water comes from a well, it might come out the tap discoloured when the soil movement causes pipes to break and let dirty water seep in.  If so, it’s very possible those pipes are leaking water back into the soil, and the problem might be getting worse by the moment. 

 If the sinkhole under your house or other structures is large enough, you may find some disturbing changes in the building itself.  These can manifest as large cracks that form in lines that point toward the middle of the hole or, zig-zags as along bricks, known as “stair-step” cracking. 

 Generally speaking, it’s a good idea to periodically check around garages, barns, swimming pools and any other development on your property.  A sinkhole can open up anywhere.  The condition of buildings, vegetation and the ground itself can be quickly surveyed by eye.  When professionals come in to scientifically verify a sinkhole, they may use one of several techniques to get a “picture” of the rock below.  Of these techniques, taking a simple drill core is the crudest and often yields poor results.  Other devices create a 3-D map of the property using radar or other sonic technology.

 One of the more obvious sign of sinkhole activity is the appearance of a round hole on your property where there wasn’t one before.  Often these are filled with the soil that fell in and water, but they may be filled with dissolved rock that looks like sludgy water and other debris, too. 

Sometimes, if the water table is higher than the lip of the new hole, the water can gush out like a spring, causing even more water to work on the soil beneath.  Such sinkholes are very dangerous, because they tend to get bigger for some time before they stabilize and can be repaired.

 However, even if you don’t have a gaping chasm on your property yet, there are signs that you can look for that will alert you to the conditions that mean sinkhole formation may be imminent.  These clues may be on the ground, affecting the vegetation and structures upon the land or already causing damage to the building itself.

 On the ground there are some obvious changes in the way water moves through your soil that impact the things living or just sticking in it.  For instance, you may find a circle of wilted plants.  As water drains way into the hole, it can starve the plants above of water – especially if they are annual plants with shallow root systems.  Sometimes trees will begin to lean over – sometimes dangerously.  It may happen so slowly you don’t notice until someone else points it out.  Often, upon inspection, you’ll find their roots are exposed as if they’re being pushed out of the ground.  This is an especially ominous sign if there is a ring of such damage around a central area of wilting vegetation. 

 Items that are in the ground, such as poles or fence posts may also begin to lean or tip over.  One may also find the ground loosened around such objects, as if someone had been pushing and pulling in an effort to remove it.  The ground itself may form observable cracks.  These will often appear in either a circular pattern or along radial lines. 

 The soil around your house or other building may become unsettled and prone to shifting.  This will be evident by a sudden change in the texture of the soil around the foundation and perhaps by a change in the vegetation found there.

 Small ponds of any size may form where they normally don’t.  Clear water is just groundwater filling a hole in, whereas mucky water may represent a type of sinkhole that contains a high concentration of the dissolved rock in the collected water.  This type is especially common where soils are very thin.  Such holes often continue to fill and sink over time, eventually collapsing.

 If you observe the signs of sinkhole activity on your property, contact a professional and your insurance company immediately.  Changes to Florida law that take effect after June 1, 2007, will make it much harder, if not impossible, to claim sinkhole damages after that time.  If your house appears to be succumbing to sinkhole activity, make a claim with your insurance company immediately.  Depending on the extent of the damage, it may be dangerous to remain in your home.  Keeping an eye out for such damage could mean the difference between safety and harm for your family.

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