The Toads Favorite Fire Pads - Sinkhole Information

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Sinkhole Information

Sinkholes are a common naturally occurring geologic phenomenon and one of the predominant land forms in Florida. Many of the lakes in Florida are relic sinkholes. They are typically circular but can develop in other shapes depending on the soils and geology. Sinkholes can be classified as geologic hazards sometimes causing extensive damage to structures and roads resulting in costly repairs. Sinkholes can also threaten water supplies by draining unfiltered water from streams, lakes and wetlands directly into the aquifer (underground water supply).

Some of the most asked questions about sinkholes are:

What is a sinkhole?

Sinkholes are depressions or holes in the land surface that occur throughout central Florida. They can be shallow or deep, small or large, but all are a 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, can all contribute to sinkhole development.

What does a sinkhole look like?

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 slopping or they may be vertical. There are generally three types of sinkholes.

Limestone Solution Sinkholes - Along the western portion of Lake County, limestone is exposed at the surface or is covered by a thin layer of soil. This leaves 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 soils, more clay is present in the soils. 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. See the graphic above. 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 can cause a sinkhole to form?

Rainfall absorbs carbon dioxide and reacts with decaying vegetation, creating slightly acidic water. When this water reaches the limestone aquifer, it moves through spaces and cracks slowly dissolving the limestone and creating a network of cavities and voids. As the limestone dissolves, pores and cracks are enlarged and carry even more acidic water. Water not only contributes to the chemical dissolving of the limestone but it also affects the support or lack of support provided to a cavity when the water level changes. A sinkhole is formed when the land surface above a cavity collapses or sinks into the cavity or when surface materials are carried downward into the voids.

A natural drought or the pumping too much groundwater can leave underground cavities empty. This can make conditions favorable for sinkholes to form. Also, heavy rains following a drought often cause enough pressure on the ground to create sinkholes.

Sinkholes can be triggered by human activities such as:

Over pumping of groundwater
Diverting surface water from a large area and concentrating it in a smaller area
Artificially creating ponds of surface water
Drilling new water wells
Construction of roadways or structures

Sinkholes are hazardous because they can destroy highways and buildings. Sinkholes also can cause water quality problems. During a collapse, surface waters may leak into the aquifer, our underground source of drinking water.

Where do sinkholes occur?

Sinkholes can occur in all parts of Lake County although some areas are more prone to sinkhole than others.

What are Sinkhole Warning Signs?

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.

If it is not a sinkhole, what else can cause depressions or structural damage?

There are many other circumstances that can cause depressions or sinkhole-like damage to structures.

Buried debris from past construction activities that has settled or rotted over time
Old trees that were cut down with the roots left to rot in the ground
Improper compaction of the soil during filling and/or construction
Structures built on organic soils (muck and peat) or clays
Damaged septic tanks or other underground tanks
Leaky water lines
Broken sewer or stormwater pipes
Wells with improperly installed casings

What to do if a sinkhole opens 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.
Damage to your house or other structure (but not to your land) is generally covered by homeowner's insurance policies. If your home is threatened or damaged, contact your insurance company.
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 soils testing firm to evaluate the cause at you own expense. A list of consultants is available from Water Resource Management.
If lake or river levels are affected, or you think ground-water quality is endangered by a sinkhole, please report it to the Water Management District.
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.

Frequently Asked Questions on Sinkhole

1. Why do sinkholes form?

Sinkholes form in karst terrain principally from the collapse of surface sediments into underground 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. Well drilling data suggests that much of the underlying bedrock in Florida is riddled with cavities of differing size and depth. However, relatively few ever collapse and directly effect roads or dwellings.

Karst terrains develop in areas underlain by carbonate rocks such as limestone. They often have drainage systems that are reflected on the surface as sinkholes, springs, disappearing streams or even caves. The term karst, therefore, refers to the terrain and the term sinkhole is one of the types of drainage features reflected by that type of terrain.

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.

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.

2. My yard is settling…do I have a sinkhole?

Maybe, but a number of other factors can cause sinking or settling. 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. If the settling is affecting a dwelling, further testing by a licensed engineer with 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.

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

Small sinkholes 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 many counties the local Emergency Management Offices render assistance when a home is endangered. [In Lake County the Emergency Management number is (352) 343-9420.] Personnel from your local Water Management District may also assist in sinkhole assessment, especially if the sinkhole potentially impacts local ground water. The sinkhole should be reported on the appropriate form and submitted to the Florida Geological Survey

4. How long does it take for the 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, a bowl-shaped depression forms at the surface, typically over longer periods of time (sometimes as long as years).

5. 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.

6. 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. [In Lake County call (352) 253-4980.]

7. A sinkhole opened in my next door neighbor's yard….should I be concerned?

Although sinkholes in Florida tend to occur along linear fracture trends (aligned northwest to southeast and northeast to southwest), 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.

8. 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 ground-water pumpage. Private lawn wells are typically not sufficient to impact the water table enough to cause sinkholes.

9. 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 sinkhole report. The Florida Geological Survey maintains a database of reported sinkholes 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 have staff available to check local sinkholes, particularly if they contain water. If a sinkhole is threatening your home, immediately contact your insurance company. In many counties staff from the local Emergency Management offices will advise homeowners on safety and evacuation of homes impacted by sinkholes. [In Lake County the Emergency Management number is (352) 343-9420.]

10. 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 the sinkhole. Actual coverage may vary according to circumstances and insurance company policy.

11. Do I need a permit to fill a sinkhole?

In general no permit is needed to fill a new sinkhole on private property unless it contains ground water. Sinkholes intersecting the underlying aquifers (those containing water) may require an Environmental Resources Permit before filling. This permit is available through your local Water Management District in southern Florida, or from the respective district offices of the Florida Department of Environmental Protection in northern Florida. District staff will assist in assessing the need for a permit and the permit approval process. As many sinkholes are direct conduits to our drinking water aquifers, some care in selection of fill material is advisable. Do not fill a sinkhole with trash, chemicals, or other materials that could contaminate ground water. Natural earth materials such as clean limestone rock, sand, and clayey sand are suitable. [In Lake County it is illegal to fill a sinkhole with materials other than clean fill, i.e. soil or concrete rubble.]

12. What is the sinkhole risk factor associated with my area?

Unfortunately there is no ready reference on sinkhole prediction or risk assessment. The insurance companies have tried developing some sort of risk prediction methodology, but since the underground cavities are largely undetectable without expensive ground-penetrating radar surveys, resistivity tests, or test drilling, no real progress towards this goal has been made. About the most we can presently do is construct regional maps such as our, Sinkhole, Development and Distribution in Florida Map, showing zones of sinkhole occurrences based on the local geology and historical sinkhole activity.

13. Is there any way to have my property evaluated as to the risk of a sinkhole forming?

Professional geologists and geotechnical engineering consultants with professional geologists on staff can perform a variety of tests to attempt to locate buried cavities which might form sinkholes. These tests include ground penetrating radar surveys, electrical resistivity tests, and borings. However, test results may be affected by the local geology and elevation of the water table, and are not always conclusive. And in many cases the cost of a detailed survey is beyond the typical homeowner's budget.

14. I am buying a house with a repaired sinkhole under the foundation. Is this safe?

A number of engineering companies routinely repair sinkholes. Techniques vary from simple injection of grout into the hole to more advanced systems of engineered reinforced plugs, pins, and porous concrete. In general, if a repair has been certified by a licensed engineer, and completed to the satisfaction of the homeowner's insurance company, it is probably safe. However, as you are dealing with natural systems, there can be no guarantees that a repaired sinkhole will not cause future problems.

15. Is there a safe area of Florida in which to live with no chance of sinkholes?

Technically no. Since the entire state is underlain by carbonate rocks, 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 a conducive configuration of water table elevation, stratigraphy, and aquifer characteristics have increased sinkhole activity. The Florida sinkhole location map (link button) shows the distribution of reported sinkholes statewide, and gives a rough idea where some of the increased risk areas may be located. However, these are only the reported sinkholes. There are countless more sinks statewide than are shown on this map. The sinkhole type and occurrence map provides additional information on sinkholes statewide.

16. Is there a database showing all sinkholes in Florida?

No. The Florida Geological Survey maintains a database of reported sinkholes. This represents only those sinkholes officially reported by observers. The reported sinkholes tend to cluster in populated areas where they are readily seen and commonly effect roads and dwellings. However, numerous sinkholes also occur in fields and forests, many of which go unseen and unreported. Also, the reported date only covers the period from 1954 to present. Many earlier sinkholes are unrecorded.

18. I am buying a new home and I want to know if there is a sinkhole disclosure law?

Most real estate seller's disclosure forms used in Florida today include a sinkhole disclosure statement. Sometimes it is overlooked. If it is in question, be sure to ask.

19. Is a new construction site tested for sinkholes?

In most cases no. It is generally not required by building codes, and most building contractors do not provide testing on private home sites because of the additional expense. In some cases public building construction sites in sinkhole areas may be tested and reinforced as needed for safety and liability reasons.

20. I was denied homeowners insurance because there is a sinkhole within one half mile of my home. What can I do?

Currently, an insurance company has the right to not issue an insurance policy on the basis of sinkholes in the "area." The definition of "area" remains subjective, and the issue will likely only be resolved through specific legislation or by the general adoption of a standard by the insurance industry. Some companies have more liberal policies, and you may wish to shop around for other insurance that may be available.

21. My insurance company has informed us that the area where we are going to purchase property is listed as a sinkhole area. What does this mean? What can we do about it? Should we buy in that area?

See question #20. Certainly the availability of insurance is a major factor to most homebuyers. Insurance companies may vary on their individual requirements and you should shop around for the best insurance policy that may be available to you. Unfortunately there is no ready reference on sinkhole prediction or risk assessment. This has made accurate risk determinations difficult and has hampered the formulation of either legislation or an industry standard on this issue. As a result many insurance companies rely heavily upon the regional maps showing zones of sinkhole occurrences based on the local geology and historical sinkhole activity. Any decision to purchase a particular property is of course a highly individual one, involving not only insurance availability, but also your own personal tolerance for risk and your desire to live in a particular area..

22. Who may I call to obtain further information on insurance in Florida or to issue a complaint about my insurance company?

The Florida Department of Insurance has established a HELP LINE. The phone number is 1-800-342-2762.

23. What happened to the Florida Sinkhole Research Institute (FSRI)?

The Florida legislature discontinued FSRI's funding in the early 1990's, and its database was transferred to the Florida Geological Survey (FGS). A brief history is outlined below.

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