Sinkholes are a common naturally occurring geologic phenomenon and one of the predominant land forms in the State of Florida. Many of Florida's lakes are a result of sinkholes. Sinkholes are typically circular but can develop in other shapes depending on the geology and soils of the area.
Sinkholes can be classified as geologic hazards sometimes causing extensive damage to structures, roads, homes and commercial buildings resulting in costly repairs. They can also threaten local water supplies by draining unfiltered water from wetlands, lakes and streams directly into Florida's underground water supply, aquifer.
Definition: Sinkholes are holes or depressions 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. Sinkholes can happen because of a lack of rainfall, lowered water levels, or excessive rainfall in a short period of time.
In the state of Florida there are 3 types of sinkholes that are common in our area. Three general types of sinkholes occur in Florida: collapse, solution, and subsidence.
Sinkholes do not have to be classified into one specific category. Many sinkholes are a combination of any or all of the 3 categories and may form in several phases. All of these types can cause damage that requires foundation repair.
- Collapse Sinkholes
- Solution Sinkholes
- Subsidence Sinkholes
Florida Sinkhole Zones
Yellow - Region of exposed or thinly-covered carbonate rocks. Broad, shallow solution sinkholes dominate, with less common collapse sinkholes in areas with thicker overburden sediments.
Green - Region of incohesive, permeable sand ranging from 20 to 200 feet thick. Small cover subsidence sinkholes dominate, with less-common collapse sinkholes forming in areas with clayeye overburden sediments.
Blue - Region of cohesive, low-permeability clayey sediments 30 to 200 feet thick. Abruptly-forming collapse sinkholes dominate. The size of these sinkholes depends upon the thickness and bearing properties of the overburden sediments.
Pink - Region of deeply-buried carbonate rocks. Overburden sediments and primarily cohesive clayey sands and interbedded carbonates in excess of 200 feet thick. sinkholes are uncommon, but rare deep collapse types and small subsidence sinkholes formed in shallow shell beds or carbonate lenses are possible.
Dissolution is the ultimate cause of all sinkholes. Dissolution happens when rain water falls and saturates the ground. The water causes voids to develop in pre-existing openings in the ground, such as, along joints, fractures, and bedding planes causing a small depression to form gradually over time. This downward erosion flows into a pre-existing cavity below.
Everyone is aware of this rapidly occurring type of sinkhole. Collapse sinkholes are noticeable, because as the name suggests, the surface of the ground gives way to the underground cavity. Usually, collapse sinkholes happen when the overburden (trees, soil, etc.) is heavy, and when water level fluctuations have weakened the sides and ceiling of the cavity. Collapse sinkholes are usually the most damaging type of sinkhole. Fortunately, collapse sinkholes are also the most infrequently occurring sinkholes.
Cover-Collapse sinkholes develop quickly over a period of hours and can cause major damage. These are the types of sinkholes which make the news. These usually happen where the ground has a large amount of clay. Water flows underground and fills a cavity. As it continues the ground forms a structural arch. The cavity moves upward by progressive roof collapse. Then the cavity breaks through to the surface and creates sudden, dramatic sinkholes. These types of sinkholes have been known to swallow homes, swimming pools, buildings, roadways, and bridges.
Cover Subsidence forms gradually overtime. Underneath all our structures lies water deep below the surface or empty spaces. They are caused by water which once flowed there or roots from plant materials which grew above ground and are now dead. Rain water or surface water flows into the underlying ground and settles into these vacated spaces. Overtime the small downward erosion forms small surface depressions. Gradually more water will continue to accumulate and fill the void below. This may go undetected for long periods of time due to the fact that it happens so slowly. Small cracks in your properties structure form and you may think nothing of it. Overtime these cracks my get bigger and you may begin to wonder if they were always so large. Until one day, you notice other parts of the structure cracking or sloping and you realize this is not ordinary settling.
This type of sinkhole usually occurs gradually. In the absence of topsoil and vegetation, the exposed limestone layer is slowly washed away by rainwater, surface water, and even wind. The sinkhole forms slowly and deliberately, and can usually be spotted or avoided before sinkhole property damage occurs.
In this type of sinkhole, loose sand gradually replaces diminishing limestone bedrock. As the underground cavity loses stability and becomes increasingly weak, the surface of the ground begins to sink. Again, this type of sinkhole occurs slowly, forming a bowl-shaped depression in the surface. Because of the accumulation of water-blocking materials where permeable bedrock used to exist, groundwater can build up in a subsidence sinkhole, creating spongy surface texture or even a small pond. Over time, entire lakes may even develop from subsidence sinkholes.
Dissolution sinkholes are formed by slow dissolutional lowering of the limestone outcrop or rockhead, aided by undermining and small scale collapse. They are normal features of a karst terrain that have evolved over geological timescales, and the larger features are major landforms. An old feature, maybe 1000 m across and 10 m deep, must still have fissured and potentially unstable rock mass somewhere beneath its lowest point. Comparable dissolution features are potholes and shafts, but these are formed at discrete stream sinks and swallow holes, whereas the conical sinkholes are formed largely by disseminated percolation water.
Collapse sinkholes are formed by instant or progressive failure and collapse of the limestone roof over a large cavern or over a group of smaller caves. Intact limestone is strong, and large-scale cavern collapse is rare (most limestone gorges are not collapsed caves). Though large collapse sinkholes are not common, small-scale collapse contributes to surface and rockhead degradation in karst, and there is a continuum of morphologies between the collapse and dissolution sinkhole types.
Caprock sinkholes are comparable to collapse sinkholes, except that there is undermining and collapse of an insoluble caprock over a karstic cavity in underlying limestone. They occur only in terrains of palaeokarst or interstratal karst with major caves in a buried limestone, and may therefore be features of an insoluble rock outcrop (Thomas, 1974).
Dropout sinkholes are formed in cohesive soil cover, where percolating rainwater has washed the soil into stable fissures and caves in the underlying limestone (Fig. 6). Rapid failure of the ground surface occurs when the soil collapses into a void that has been slowly enlarging and stoping upwards while soil was washed into the limestone fissures beneath (Drumm et al, 1990; Tharp, 1999). They are also known as cover collapse sinkholes.
Suffosion sinkholes are formed in non-cohesive soil cover, where percolating rainwater has washed the soil into stable fissures and caves in the underlying limestone. Slow subsidence of the ground surface occurs as the soil slumps and settles in its upper layers while it is removed from below by washing into the underlying limestone - the process of suffosion; a sinkhole may take years to evolve in granular sand. They are also known as cover subsidence sinkholes. A continuum of processes and morphologies exists between the dropout and suffosion sinkholes, which form at varying rates in soils ranging from cohesive clays to non-cohesive sands. Both processes may occur sequentially at the same site in changing rainfall and flow conditions, and the dropout process may be regarded as very rapid suffosion. Dropout and suffosion sinkholes are commonly and sensibly described collectively as subsidence sinkholes and form the main sinkhole hazard in civil engineering (Waltham, 1989; Beck and Sinclair, 1986; Newton, 1987).
Subsidence sinkholes are also known as cover sinkholes, alluvial sinkholes, ravelling sinkholes or shakeholes.
Buried sinkholes occur where ancient dissolution or collapse sinkholes are filled with soil, debris or sediment due to a change of environment. Surface subsidence may then occur due to compaction of the soil fill, and may be aggravated where some of the soil is washed out at depth (Bezuidenhout and Enslin, 1970; Brink, 1984). Buried sinkholes constitute an extreme form of rockhead relief, and may deprive foundations of stable footings; they may be isolated features or components of a pinnacled rockhead. They include filled sinkholes, soil-filled pipes and small breccia pipes that have no surface expression. Large breccia pipes formed over deeply buried evaporites (Ford and Williams, 1989; Lu and Cooper, 1997) are beyond the scope of this paper. Slow settlement of the fill within buried sinkholes, perhaps induced by water table decline, creates shallow surface depressions known in South Africa as compaction sinkholes (Jennings, 1966).