Sinkholes: An overview from Fla. DEP

TALLAHASSEE, Fla. – Aug. 16, 2013 – Another Florida sinkhole made national headlines when a resort near Disney World broke apart. Because the phenomenon scares most homeowners, the Florida Department of Environmental Protection’s Florida Geological Survey compiled the following information. It provides a single point source for general knowledge about the nature of sinkholes in Florida, along with information on what should be done if a sinkhole happens nearby.

Facts about sinkholes in Florida

• The entire state sits on top of several thousand feet of limestone, which can form with natural void spaces called porosity. In limestone where the void spaces are connected, the rock is permeable. In most ways, this is positive: Porous and permeable limestone makes great aquifers and provides millions of gallons of fresh drinking water for residents and agriculture. However, acidic groundwater can change the limestone, leading to sinkholes.

• Sinkholes form from the collapse of surface sediments into underground voids. In Florida one may see solution sinkholes, cover-subsidence sinkholes or cover-collapse sinkholes. The first two types will show very little topographical disturbance to the naked eye, while the third is the type that shows an abrupt change in topography and the one most people consider when they hear the word “sinkhole.”

Questions about sinkholes

My yard is settling … Do I have a sinkhole?
Maybe. But a number of other factors can cause holes, depressions or subsidence of the ground surface. Expansive clay layers in the earth may shrink upon drying, buried organic material, poorly-compacted soil after excavation work, buried trash or logs and broken pipes all may cause depressions to form at the ground surface. These settling events, when not verified as true sinkholes by professionals, are collectively called “subsidence incidents.”

If the settling affects a dwelling, further testing by a licensed engineer with a licensed geologist on staff or a licensed geology firm may be in order. Property insurance may pay for testing, but in many cases insurance may not cover damage from settling due to causes other than sinkholes.

A sinkhole opened in my neighborhood … should I be concerned?
Although sinkholes in Florida sometimes occur in sets, most are isolated events. The bedrock underlying the state is honeycombed with cavities of varying size, most of which will not collapse in our lifetimes. A quick inspection of your property for any sinking or soft areas might be prudent. Unless the sinkhole is very large and extends to your property, there’s likely to be little reason for concern.

Should a sinkhole open in an area near you, the hole should be immediately cordoned off and clearly marked to protect traffic. Contact local law enforcement to report the hazard and call your city or county road department to initiate repair work. If the road is private, repair of the hole is usually the responsibility of the landowner or property owners’ association.

Is there a safe area of Florida with no chance of sinkholes?
Technically, no. Since carbonate rocks underlie the entire state, sinkholes could theoretically form anywhere. However, there are definite regions where sinkhole risk is considerably higher. In general, areas of the state where limestone is close to surface, or areas with deeper limestone but with certain types of water table elevation or characteristics have increased sinkhole activity.

More information about Florida sinkholes should be developed over the next three years. The Department announced recently that the Florida Geological Survey, in conjunction with the Florida Division of Emergency Management, has received a $1.1 million grant from the Federal Emergency Management Agency (FEMA) to address sinkhole vulnerability.

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