Why do sinkholes form?

Sinkholes form in karst terrain principally from the collapse of surface sediments into underground voids and cavities in the limestone bedrock. Slightly acidic ground water slowly dissolves cavities and caves in the limestone over a period of many years. When the cavity enlarges to the point 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. Well drilling data suggests that much of the underlying bedrock in Florida contains cavities of differing size and depth. However, relatively few ever collapse and directly effect roads or dwellings.

In Florida you may see solution sinkholes, cover-subsidence sinkholes or cover-collapse sinkholes. The first of these three, 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 or other openings. Cover subsidence sinkholes are located where thick permeable sediments cover the limestone. In this case the void in the rock is filled by sediments slumping downward from above. Eventually, the ground surface often shows a gentle circular depression. If a relatively thick layer of impermeable sediments covers the limestone there may not be a surface expression of a subsurface collapse Cover-collapse sinkholes occur where sediments that overlie the void in the rock suddenly collapse due to triggering mechanisms such as heavy rainfall, drought, or mechanical loading.

Generally speaking karst terrains are not newsworthy items. Typically, it is only when a road or house happens to be located above developing karst features such as a sinkhole that headlines are made. Since much of Florida is karstic in nature, these same processes are continually taking place. As such, there is a certain degree of risk in living on karst. However, most people accept the risk as one price to pay for living in the sunshine state.

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 is affecting a dwelling, further testing by a licensed engineer with a professional geologist on staff or a professional 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.

I think I have a sinkhole in my yard. What should I do?

Small holes often require only filling with clean sand or soil. If the hole is under or very near a structure or swimming pool, your property owner’s insurance may cover assessment and repair. Mark and secure the hole and keep children and pets away. If the hole is directly impacting a house, and sinking, sagging, or cracking walls are apparent, stay out of the dwelling. Call your property insurance adjuster and report it immediately. In some communities local government agencies may assist in evacuating the home, assessing damage and reporting the sinkhole. In some counties the local Emergency Management Offices (see contact list below) render assistance when a home is endangered. Personnel from your local Water Management District may also assist in sinkhole assessment, especially if the hole potentially impacts local ground water. The incident should be reported on the appropriate Subsidence Incident Report Form and submitted to the Florida Geological Survey

How long does it take for a sinkhole to stop growing?

When an underground cavity enlarges to the point that its ceiling can no longer support the weight of overlying sediments, the earth suddenly collapses into the cavity. A circular hole typically forms and grows over a period of minutes to hours. Slumping of the sediments along the sides of the sinkhole may take approximately a day’s time to stop. Erosion of the edge of the sinkhole may continue for several days, and heavy rainfall can prolong the stabilization. In the less catastrophic cover subsidence type of sinkhole, sediments slowly settle into underground voids in the bedrock. A bowl-shaped depression forms at the surface, typically over longer periods of time (sometimes as long as years).

How do I fill in a sinkhole?

Since anything buried in the earth potentially affects the groundwater, use only native earth materials or concrete for the fill. Broken limestone rip-rap or a concrete plug in the bottom of the sinkhole often helps create a stable foundation for the fill. Above that, add clayey sand to form a barrier that will help to prevent water from seeping downward through the hole and enlarging it further. Lastly, add sand and top soil, and landscape to surrounding conditions. Additional fill may be necessary over time, but most holes eventually stabilize.

A sinkhole just opened in the middle of my street…who should I call?

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/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.

A sinkhole opened in my next door neighbor’s yard….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.

Will watering our lawn lower the water table level and thus, cause sinkholes to develop in our neighborhood?

Probable triggering mechanisms for sinkhole collapse may include drought, new construction, blasting, heavy ground loading, heavy rainfall, and heavy groundwater pumpage. Private lawn wells are typically not sufficient to impact the water table enough to cause sinkholes.

Is there a government agency that will come and inspect my sinkhole?

There is currently no agency with responsibility and authority for sinkhole inspections in Florida. Often the Florida Geological Survey (FGS) receives calls from homeowners all over the state who have had the unfortunate experience of sinkhole. We do not have sufficient staff to visit all new sinkholes but do encourage the submittal of a subsidence incident report. The Florida Geological Survey maintains a database of reported subsidence incidences which is available through the FGS web site. We will be happy to discuss your individual situation and make suggestions to you so that you will be informed as to how to handle the situation. In some parts of Florida, the local water management districts may have staff available to check local suspected sinkholes, particularly if they contain water. If a sinkhole is threatening your home, immediately contact your insurance company. In some counties staff from the local Emergency Management offices may advise homeowners on safety and evacuation of homes impacted by sinkholes.

Is there a government agency available to help fix a hole on my property?

No. Sinkholes on private property are the responsibility of the property owner. In some cases the owner’s property insurance may cover evaluation and repair of confirmed sinkholes. Actual coverage may vary according to circumstances and insurance company policy.

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