A&M Contracting | Education



SINKHOLES and the AQUIFER Sinkholes originate beneath the surface when groundwater moves through the limestone and erodes large voids, or cavities, in the bedrock. When water fills a cavity, it supports the walls and ceiling, but if the water-table drops, the limestone cavity is exposed to further erosional processes that eventually result in the collapse of the cavity, causing a surface indenture, or sinkhole. The sinkhole becomes a primary site of recharge, where surface water can enter the aquifer and replenish the groundwater supply.
SURFACE CHARACTERISTICS On the surface, sinkholes may develop progressively as subtle, bowl-shaped depressions, or they may collapse suddenly into steeply sided, water-filled craters. The shape of the sinkhole, and the speed that it forms, depend on the size of the subsurface cavity and the thickness of the overburden (sediments or organic matter that rest on the limestone bedrock).
Three general types of sinkholes occur in Florida: collapse, solution, and subsidence.
COLLAPSE SINKHOLES Collapse sinkholes are the most common type in Florida. They happen suddenly where the overburden is thick with soils and heavy clay. Collapse sinkholes are deep, steeply-sided holes in the ground. They are frequently triggered by fluctuations in the water-table. As water levels fluctuate, the roof of the cavity is stressed and weakened. When the water-table drops too far, the cavity walls are unsupported and the ceiling becomes too weak to hold the heavy overburden. Eventually, the ceiling collapses and a sinkhole is formed. A conical debris mound left on the sinkhole floor is all that remains of the cavity ceiling. If the water-table rises, the collapse sinkhole can fill with water, and overflow like a spring. An off-set sinkhole will have an upstream and downstream conduit as water flows into the sink and siphons underground. If the water-table drops below the sinkhole, it will remain dry and accumulate sediments and vegetation.
SOLUTION SINKHOLES If the overburden is thin or absent, the surface of the limestone bedrock is broken down by erosion from wind and surface water. A bowl-shaped depression, or solution sinkhole, naturally forms slowly and continuously as chemical and physical processes erode the rock.
SUBSIDENCE SINKHOLES Subsidence sinkholes form gradually where the overburden is thin. The dissolving limestone is replaced by sand granules that fall into the depression and fill the holes. They appear as a concave depression in the ground. Subsidence sinkholes are usually only a few feet in diameter and depth because the development of the cavities in the limestone is retarded since they are filled with clay and sand. As the sediments fill the depression, they restrict the flow of water through the bottom and the hole begins to retain water. As water accumulates, a lake is formed.
SINKHOLES and LAKE FORMATION Many ponds and lakes in Florida are a result of sinkhole formation. The characteristics of a sinkhole lake can give clues as to how it was formed. A circular lake indicates that the lake evolved from a collapse sinkhole. A shallow circular lake results from impermeable sediments washing into a subsidence sinkhole. If a lake rests above groundwater level, it is above a confining bed.
	Source: USGS

Sinkhole Questions

Why do sinkholes form?
My yard is settling---do I have a sinkhole?
I think I have a sinkhole in my yard. What should I do?
How long does it take for the sinkhole to stop growing?
How do I fill in a sinkhole?
A sinkhole just opened in the middle of my street ---who should I call?
A sinkhole opened in my next door neighbor's yard...should I be concerned?
Will watering our lawn lower the water table level and cause sinkholes in our neighborhood?
Is there a government agency that will come and inspect my sinkhole?
Is there a government agency available to help fix a hole on my property?
Do I need a permit to fill a sinkhole?
What is the sinkhole risk factor associated with my area?
Is there any way to have my property evaluated as to the risk of a sinkhole forming?

Sinkhole Questions Answered:

Why do sinkholes form? back to top

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.

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, therefore, 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. The Florida Geological Survey maintains and provides a downloadable database of reported subsidence incidents statewide. While this data may include some true sinkholes, the majority of the incidents have not been field-checked and the cause of subsidence is not verified.

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? back to top

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? back to top

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 appropriateSubsidence Incident Report Form and submitted to the Florida Geological Survey

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

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? back to top

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? back to top

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? back to top

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? back to top

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? back to top

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.

Do I need a permit to fill a sinkhole? back to top

In general no permit is needed to fill a new sinkhole on private property unless it contains groundwater. 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 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? back to top

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.