The Causes of Sinkholes

Sinkholes are a major source of property loss in Florida, though they can occur anywhere, especially in the South Eastern United States.  They are defined as a section of ground that collapses into a cavity that has been opened up in the rock below by the action of moving groundwater.  But just what causes a sinkhole to finally give way?

 The entire state of Florida lies on top of a layer of what is called carbonaceous rock.  This means that beneath a layer of clay and soil that covers the state to widely varying degrees, there are dolomite and limestone rocks resembling honeycombs that are being constantly dissolved and reshaped by the action of ground water movement, especially when that water is slightly acidic.

 The soils in the state are loosely defined as being less than 30 ft. thick, between 30 and 200 ft., and over 200 feet.  The amount of soil over the sinkhole determines what sort of sinkhole will open up and how large it will be when it finally collapses.  This also has a lot to do with whether the human activity in the area is sufficient to cause a sinkhole to open up.

 For instance, in very deep soils, it is very possible that some solid part of the soil above actually is bridging the gap in the rock below.  This means that in areas where there are thick and hard soils, the chasm below has to get pretty big before the surface above will fall in – sometimes hundreds of feet.  For this reason, while sinkholes tend to be far less common in areas with thick soils, those that do open up can swallow homes and streets whole, forming circular ponds.  It is believed that most of the natural lakes in central Florida are actually sinkholes that filled with water over time.

 It would be difficult to cause such a monster to appear by sprinkling your lawn.  However, if a new aquifer was tapped by the city or a significant drought affected the area, such an event could be triggered.

 Sometimes a sinkhole is a result of nearby construction.  The sheer weight and often-intense vibrations from such activity can cause already weak rock beneath the soil surface to suddenly rupture or weaken significantly to rupture later.  This may occur on the building site, next door or in the general area along lateral lines.  However, building sites in Florida are rarely, if ever, checked for sinkhole activity before construction begins.

 Any activity that changes the way water moves through the soil profile has the potential to cause sinkhole formation.  So, water being removed or even redirected from the aquifer can change the level or direction of flow, causing particularly porous rock that has thus far escaped erosion to be quickly eroded away.  In soils that are particularly thin this could be as a result of a single weather event or even just the afore mentioned lawn-watering

 Increasing the weight of the surface above can also have an impact on sinkhole formation.  Sometimes there are rock cavities below that have strong enough caps to keep from collapsing under the normal weight of the land above.  However, when soils become very waterlogged, they can tip the balance and trigger the sinkhole event.  Consider that a gallon of water weighs 8 pounds – far more than a similar volume of most dry soils, and you have an idea of how much more a waterlogged soil can weigh in comparison.  This is especially true in soils with high clay content, since such soils can hold more water than sandy soils.

 So, sinkholes and the weather are both forces of water and energy.  Weather that suddenly drops a lot of water on a dry soil differs from steady rains in how they can effect sinkhole formation.  Flash floods will form virtual rivers underground that can quickly erode away a small channel that was already forming.  On the other hand, steady rains can increase the weight of the soil, increasing the pressure on what was otherwise a relatively stable sinkhole prone chasm.

 Sometimes the weight of buildings and even structures such swimming pools or driveways can add just enough weight to the surface to trigger sinkhole activity.  Other changes might include new landscaping or even the swelling and shrinking of the soils themselves as the result of severe weather swings that are increasingly common in the area.

 While there are many known causes, the formation of sinkholes is still unpredictable.  While an expert can give you a probability of how likely you are to have a sinkhole open up on your property and perhaps even where on the property is most likely to suffer the trouble, no one can honestly tell you they know the hour and day one will strike.

 Changes in sinkhole laws as they pertain to insurance coverage have recently made it harder for property owners to claim legitimate sinkhole losses.  If you are concerned about sinkhole activity damaging your property, be sure to look for the warning signs.  If you see them, make a claim with your insurer immediately, because after June 1, 2007, as a result of new laws and new insurance policy language, traditional sinkhole coverage will no longer be available under your homeowners policy, unless you pay an additional (and likely very large) insurance premium.

Back to top