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.
Karst terrain is a type of topography that is formed by dissolution of bedrock in areas underlain by limestone, dolostone or, as in some western states, gypsum. Such terrain has underground drainage systems that are reflected on the surface as sinkholes, springs, disappearing streams or even caves.
The term karst, refers to the terrain and the term sinkhole is one of the types of drainage features reflected by that type of terrain.
Other subterranean events can cause holes, depressions or subsidence of the land surface that may mimic sinkhole activity. These include :
- subsurface expansive clay or organic layers which compress as water is removed
- collapsed or broken sewer and drain pipes or broken septic tanks
- improperly compacted soil after excavation work, and even buried trash, logs and other debris
Commonly, a reported depression is not verified by a licensed professional geologist to be a true sinkhole, and the cause of subsidence is not known. Such an event is called a subsidence incident.
There are three types of solution sinkholes
- 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”
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 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.
If you have a sinkhole on a large property and it is not actively developing or impacting some activity on the property it can be left alone. If there is a danger of people or animals falling into the depression it can be filled with clayey sand or fenced off. Do not fill it with organic material or something that could potentially decompose or release potential toxins into the underlying groundwater. Many people have sinkholes on their property that are just part of the natural landscape. If one suddenly appears then we suggest filling it with clayey sand as the clayey material will retard water movement. Water flowing into a sinkhole can cause it to expand and become more active. Never throw anything into a sinkhole that could possibly contaminate groundwater!
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? back to top
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.
Can a home inspector determine if there is a sinkhole on a property? Or determine if a property is more likely to have a sinkhole (e.g. land near water, etc.)? back to top
An inspector that is a licensed professional geologist and is trained to recognize sinkhole activity might be able to tell if a sinkhole exists on a piece of property. The problem exists when the sinkhole has not yet developed on the property and it gets built on. At a later date a sinkhole can occur and damage the property. It is possible, in some cases, to perform geological tests on a piece of property and assess the potential for sinkhole development. Some of the geological tests include ground penetrating radar (GPR), soil borings, and resistivity. All these method require experts and are costly to perform. It is not possible at this time to predict when and where EXACTLY a sinkhole will develop.
Sinkhole activity is greatest when there have been drought conditions for some period of time and then there is a return to normal rainfall. A large rain event can weigh the soil down and can cause sinkholes to form. When the water level in the underlying limestone aquifer is depressed (due to drought or consumptive uses) the void spaces that once held water now are filled with air. Water provides some buoyancy and can keep the overlying geologic material stable. When the void contains air it is less competent and when a large rain event weighs the overburden down it can collapse into the underlying air-filled void.
Do I need a permit to fill a sinkhole?
In general no permit is needed to fill a new dry sinkhole. Sinkholes occurring in wetland areas or sinkholes whose filling would alter surface water flows or contribute to water pollution may require an Environmental Resources Permit before filling. This permit application is typically filed with 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. 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 groundwater. Natural earth materials such as clean limestone rock, sand, and clayey sand are suitable.
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 risk prediction methodology, but since the underground cavities are largely undetectable without expensive ground-penetrating radar surveys, resistivity tests, or test drilling, little real progress towards this goal has been made. In recent years, at least one geotechnical company has developed a fee-based risk assessment registry based on an extensive private database of known sinkholes and local geologic conditions, which it provides to insurers. Some Florida insurance companies now utilize this registry for assisting in determining the sinkhole risk in specific areas.
Is there any way to have my property evaluated as to the risk of a sinkhole forming?
back to top 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.
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.
Is there a safe area of Florida in which to live with no chance of sinkholes? back to top
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 only way to ensure that you don’t purchase property that might be prone to sinkhole activity is to not buy property in a karst region. Karst refers to landforms that develop due to the dissolving away, over geologic time, of geologic materials near the surface. In most cases that material is limestone. Learn about the local geology in an area you are considering purchasing land in and find out if it is a karst region.
Is there a database showing all sinkholes in Florida?
No. The Florida Geological Survey maintains a database of reported subsidence incidents. This represents only those incidents officially reported by observers. Although the data may contain some true sinkholes, most have not been verified by professionals and they are collectively called “subsidence incidents”. The reported incidents tend to cluster in populated areas where they are readily seen and commonly effect roads and dwellings. However, numerous subsidence features may also occur in fields and forests, many of which go unseen and unreported. Also, the reported data only covers the period from 1954 to present. Many earlier subsidence incidents are unrecorded.
Where can I find available subsidence incident information for a specific area?
You can find this information in the Subsidence Incident Reports located on this website. Click the Excel Spreadsheet link. The data is arranged by county. Find your county and search under the column that is titled “address”. It is here that you will have the best chance of finding the street locations. There are over 2000 reported incidents in the database.
I am buying a new home and I want to know if there is a sinkhole disclosure law? back to top
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.
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.
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 utilize private sinkhole data to assign relative sinkhole risk (see question #12). Other companies may have more liberal policies, and you may wish to shop around for other insurance that may be available.
The Florida Office of Insurance Regulation provides a listing of insurance companies writing policies in the different Florida counties at: http://www.shopandcomparerates.com/HOCompareRates300.htm
Our 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 #21. Certainly the availability of insurance is a major factor to most homebuyers. Current Florida law requires that insurance companies provide catastrophic ground collapse coverage, but unless specific criteria are met, this would not include sinkhole damage. Specific sinkhole coverage is now an option. 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 have relied heavily upon the regional maps showing zones of sinkhole occurrences based on the local geology and historical sinkhole activity, or on private sinkhole data. 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.
My insurance company has done sinkhole testing at my house, but has not released the reports to us. Is there a way to see if a sinkhole report has been filed on my home address?
The FGS SIRs database is comprised of purely voluntary data. Most insurance providers do not send us their sinkhole testing reports or sinkhole insurance claims locations. Since your insurance company had sinkhole testing performed at your residence you are entitled to a copy of the report. By law (627.7073, F.S.: Sinkhole Reports),
“(1) Upon completion of testing as provided in s. 627.7072, the professional engineer or professional geologist shall issue a report and certification to the insurer and the policyholder as provided in this section.” If for some reason you have not received your required copy of the report there are three avenues you can take to possibly obtain it: 1) contact your insurer,
2) contact the professional engineer/geologist of record,
3) contact your county Clerk of Court as 627.7073 (2)(a) also requires that the insurer file a copy of the report with the Clerk of Court in the respective county in which the insurance claim occurred.
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 Financial Services has established a HELP LINE. The phone number is 1-877-693-5236. Or you may contact them online.
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.
Since its inception in 1907, the Florida Geological Survey has gathered data on Florida karst (sinkholes, caves, springs, etc.) This information is primarily used to more fully understand the unique relationship between karst and the state’s groundwater resources and aquifer systems.
In 1982, the Florida Sinkhole Research Institute was created at the University of Central Florida in Orlando. At that time all FGS sinkhole data files were transferred to the FSRI. One of their programs was to compile and tabulate this information and convert it to a computer database. When the Florida legislature discontinued FSRI’s funding in the early 1990’s, the FSRI nearly ceased operations entirely. After that the database was returned to the FGS and reformatted. Selected portions of the reformatted data were published in 1994 as FGS Open File Report 58, “FLORIDA SINKHOLE INDEX.” The data is currently available as subsidence incident reports, in a MS Excel spreadsheet format via the FGS Internet web site. As for FSRI, they are still located at the University Of Central Florida (UCF) and their current director is Dr. Shiou-San Kuo. He can be reached at UCF’s Department of Civil and Environmental Engineering at (407) 823-2280.
To better understand karst processes and the features associated with it, the FGS published Special Publication 29, “Karst in Florida”. The author, Mr. Ed Lane, did an excellent job of explaining the various aspects of Florida Karst in an easy to understand manner for the non-scientific community, and as a tool for teachers to use in the classroom. Other FGS publications that discuss karst in Florida include OFR-58, mentioned above, and Map Series 110 which explains sinkhole types, their distribution and development.
The 1992 Florida Legislature mandated that a study of sinkhole insurance issues be conducted. The study was completed by the Florida State University Center for Insurance Research, under the direction of the Florida Department of Insurance. The report, Insurance Study of Sinkholes, was submitted to the Department in December of 1992 and subsequently to the appropriate Legislative Committees.
Two chapters of that report were reproduced by the Florida Geological Survey as Open File Report 72, in response to interest from governmental agencies, the public, and the professional community. Chapter V, deals with “Claims Standards.” It was determined during the course of the study that a listing of typical standards used by Professional Geologists or Professional Geotechnical Engineers was needed to offer guidance regarding what a competent geological assessment of a site should consider to determine if karst processes are responsible for observed features. Chapter V is entitled “ Examination of the Establishment of Minimum Standards for the Evaluation of Sinkhole Claims.”
Chapter VI addresses the State’s need for an ongoing research resource to understand and characterize sinkhole occurrences and to create a central clearinghouse for the collection of sinkhole data and for its dissemination to the public. The Chapter entitled “Need for an Ongoing Research Resource” includes input from four state university geology departments and the Florida Geological Survey.
The intention of the reproduction of these chapters into an Open File Report was to make it easier for the public to obtain the results of the “Sinkhole Standards Summit” which was organized by the authors and attended by geologic experts from throughout the state. Their resulting consensus is presented in Chapter V of the report. This and all of the FGS publications are available at selected libraries throughout the state or from our library at the address shown below. Most may be ordered online through the FGS publications website, and a large number are viewable online from links in the FGS online List of Publications.
FGS staff are on call at all times to receive calls from the State Emergency Warning Point, which is part of the Department of Community Affairs. The Warning Point acts as a clearinghouse for emergency situations of all types including sinkhole activity throughout the Florida. Additionally, selected members of the FGS staff respond to a multitude of requests from the public, state and federal agencies, and consultants regarding sinkhole development or potential.