Sinkholes and the aquifer Definition of Karst & Relation of Karst Formations to Sinkhole Collapses
Florida Sinkholes

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Karst formation sinkholes in Florida. This document defines karst and explains what karst sinkholes are and why they occur, describes their effects on buildings, and gives building and site inspection advice useful in identifying areas where there is an increased risk of sink holes at properties.

The presence of karst formations help predict where a sinkhole collapse may be ongoing or imminent. Recognizing indicators of potential sinkholes can reduce but not eliminate this risk. This limitation should be stated clearly by any home inspector in an area where sinkholes are known to occur or wherever one is suspected.

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What is Karst? What is the Role of Karst in Florida or other Sink Hole Formation?

Photograph of a sink hole swallowing a house in FloridaIn Florida, the underlying basis of sinkholes is the presence of porous [1] limestone layer below (often thin) topsoil. Karst is any land with sinkholes, springs, and streams that sink into subsurface caverns.

Article Contents

  • Definition of karst - meaning of karst
  • What are sinkholes?
  • Inspecting a property for signs of sink holes
  • Types of sink holes, signs of sink holes
  • Causes of sinkholes
  • Sink hole damage and risks
  • When to hire a geotechnical engineer for sinkhole or soil testing

What is a Sinkhole?

A sinkhole is created surface materials collapse or are dissolved into an underground cavern or stream. 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). [2]

The term sinkhole is applied by engineers to the following geological features [3]

  • Bedrock voids (most difficult to detect, but least likely of imminent collapse)
  • Depressions in the top-of-bedrock
  • Sloping voids in the soil column
  • Zones of wet, soupy soils (mud filled voids in the soil column)
  • Clay seams (mud filled voids in bedrock)
  • Actual surface collapse features

If a sinkhole is already visible near an inspected property or if signs of a sinkhole are observed this information should be cited by the inspector as a potential safety concern and significant expense requiring immediate professional action.

The bare minimum that a property owner needs to know about sinkholes or any other sudden subsidence of soils at a property is that these conditions might be very dangerous. Someone falling into a sink hole or into a collapsing septic tank could be seriously injured or even die. If a suspicious hole, subsidence, or depression appears at a property the owner should rope off and prevent access to the area to prevent anyone from falling into the opening, and then should seek prompt assistance from a qualified expert, geotechnical engineer, septic contractor, excavator, or the like.


Additional septic system safety warnings are at SEPTIC & CESSPOOL SAFETY

For assistance in making repairs to sinkholes or sinkhole damage see SINKHOLE DAMAGE REPAIRS.

Sinkholes and the Aquifer

Sinkholes and the aquifer

Source: USGS - [Excerpting from References 1 and 2]

Characteristics of Sinkholes:

  • originate beneath the surface
  • 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
  • the water table drops
  • the limestone cavity is exposed to erosion
  • the cavity collapses
  • causing a sinkhole to develop, possibly suddenly

Four types of sinkholes, all beginning with a solution cavity [5]

  1. Solution sinkholes - surface depressions, not complete collapse
  2. Cover-subsidence sinkholes - loose, overlying sand slides into solution cavity
  3. Collapse sinkholes - roof of an underground channel suddenly collapses, forming a steep-sided cavity
  4. Cover-collapse sinkholes - thick layer of sand over clay over limestone. Limestone dissolves, clay keeps the sand from collapsing-in, then suddenly fails, leading to sudden and very violent collapse: the most dangerous. An example of collapse sinkholes is the collapse of underground mines which can lead to a fracture and collapse of the ground surface above.

Three general types occur in Florida: collapse, solution, and subsidence [6]

Collpase sinkholes

Source: USGS

Collapse sinkholes

  • most common type of sinkhole in Florida
  • Happen suddenly
  • Where the overburden is thick soil and heavy clay
  • Deep, steeply-sided holes
  • 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. 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

  • overburden is thin or absent
  • Forms slowly and continuously
  • Surface of the limestone bedrock is broken down by erosion from wind and surface water
  • Bowl-shaped depression, or solution sinkhole
  • Chemical and physical processes erode the rock

Subsidence sinkholes

Subsidence sinkhole

Source: USGS

  • overburden is thin at subsidence sinkholes
  • subsidence sinkholes form slowly
  • at subsidence sinkholes dissolving limestone is replaced by sand granules that fall into the depression and fill the holes
  • subsidence sinkholes form a concave depression
  • subsidence sinkholes may be only a few feet in diameter and depth (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

Sinkholes and lake formation

Source: USGS

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.

Sinkholes and Urban Development

Sinkhole formation is aggravated and accelerated by urbanization. Development increases water usage, alters drainage pathways, overloads the ground surface, and redistributes soil. According to the Federal Emergency Management Agency, the number of human-induced sinkholes have doubled since 1930, insurance claims for damages as a result of sinkholes has increased 1200% from 1987 to 1991, costing nearly $100 million.

To avoid the destruction of property and the contamination of groundwater, it is important to monitor potential sinkhole formation.

Sinkhole Repair Services in Florida

Watch out: Readers trying to diagnose and deal with sudden soil subsidence or yard collapses should see SINKHOLES - IMMEDIATE SAFETY ACTIONS.

Companies identify themselves as sinkhole damage repair experts in Floria are listed at SINKHOLE DAMAGE REPAIRS or select a topic from the More Reading links shown below.

More Reading

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Frequently Asked Questions (FAQs) about Karst Sinkholes

Question: Were some sinkholes made from the glaciers? Especially in Florida?

Were some sinkholes made from the glaciers? Especially in Florida? - J.K., Florida

Rely: Probably not

We have not found a direct association between glaciers and sinkholes, especially in Florida, though in other geographic zones one can state a potential relationship between groundwater movement, rivers, and sinkholes.

So a short answer is that while one could make a speculative argument that water flowing through and removing fines of glacial till in alluvial valleys could play a part in certain sinkhole formations, a predominance of karst-based sinkholes argues against a role of glaciers in sinkholes, at least in north central Florida.

A more detailed look at the relationship between glaciers and sink holes:

Different types of sinkholes: first let's recap how sinkholes form. A contemporary or active sinkhole is formed typically in karst systems as groundwater dissolves minerals creating openings that then subside or collapse.

Cech[1] comments on glaciated terrain and the role of glaciers in deposited sedimentary layers of glacial material and in some cases material that contained groundwater. Paraphrasing:

During glacial activity, (the most recent deglaciation began 15,000 years ago) underlying terrain was scraped, carved, cut as the glacier ice sheet base pushed ice and rocks along , depositing ice and rocks as the ice retreated. This material is glacial till, [Cech p. 112] comprised of boulders, gravel, sand, and silt. During melting some of these smaller particles were further moved by the glacial outwash.

Cech, also himself referring to Charles Lyell [1797-1875] notes that there is a relationship between the valleys or glacial till-filled valleys (in some cases hundreds of feet thick) and later groundwater movement. As he puts it

Rivers that flow through an alluvial valley are often hydraulically linked to groundwater. This physical connection creates opportunities for surface water in a river to recharge groundwater or for groundwater to replenish flows in a river as baseflow. The direction of water movement between groundwater and surface water is dependent on gradients, climactic conditions, and water volume ... - Thomas V. Cech [1]

So there is a potential relation between rivers and groundwater in some locales that may be traced back to glacial activity.

But to move from there to a closer tie between an active sinkhole and the glaciers seems to me a bit more difficult. If we exclude sinkholes associated with collapses due to mining or similar subsidences, and focus first on the most common sinkhole formation: karst topography plus water, the mechanism that results in a sinkhole is the dissolution of limestone below ground.

Cech and others explain that low pH rainwater entering the groundwater system dissolves (reacts with) the carbonate limestone, opening conduits through the aquifer. When enough limestone has been removed the surface collapses. Thus the two most common sinkholes (especially in north central Florida) are caused by either solution (tending to be slow in formation) or collapse sinkholes (tending to be sudden) through karst formations, not the sudden subsidence of a cavern left by glacial activity.

We'd expect sinkhole activity to increase when there is a period of less rain, leading to drops in water tables and the leaving of voids that may collapse, and we'd also expect sinkhole activity to increase when there is a period of prolonged heavy rainfall, causing rising water tables and causing further dissolution of the underlying limestone formations.

Sinkoles may also be increased due to local drilling of wells into the existing underground water cavities in the karst, or by pumping groundwater from nearby, or by diverting drainage to areas where karst cavities already exist.

More interesting sink hole types from Cech [1] p. 109] are

  • siphon sinkholes that occur in a riverbed when a sinkhole opens in the riverbed causing drainage of a portion of the river's volume
  • swallow sinkholes that happen when a sinkhole of sufficient size opens in a river bed and is able to capture all or most of the river water volume- the river "goes dry" suddenly
  • karst sinkholes open within a cave or conduit: water flowing in the conduit removes the overburden soils "... revealing a view into the aquifer. It is a combination of a spring and a sink in the same feature."

Conclusions about the Role of Glaciers in Florida Sinkhole Formations

From these descriptions one might infer that some sinkholes could occur in a valley that was previously filled by glacial till if river water or ground water are able to remove sufficient volume of fine soil particles, clay, or silt, even in an area where karst might not be present.

But in Florida where you'd have trouble finding an alluvial valley, sinkholes associated with karst formations are principally due to the chemistry of karst, plus rainwater, not glacial activity. Karst is limestone, formed in "beach ridges" for example near the border of northern Florida and southern Georgia "at elevations between 42 and 49 meters above mean sea level" - the product of sea life. Opdyke et als point out that

Marine fossils of Pleistocene age are known to occur in beach ridges near the border of northern Florida and southern Georgia at elevations of between 42 and 49 m above mean sea level. No evidence exists for a massive melt-off of glacial ice, which would be required to raise sea level to these elevations. Florida, therefore, must have been uplifted epeirogenically during the Pleistocene. Measurement of dissolved solids in

Florida's springs demonstrates that the karst area is losing a minimum of 1.2 x 106 m3/yr of limestone through spring flow, the equivalent of 1 m of surficial limestone every 38,000 yr. This loss has led to an isostatic uplift of the north-central part of the Florida peninsula of at least 36 m during Pleistocene and Holocene time, which agrees with observed elevations of marine terraces. - Opdyke et als [2]

Question: Are sinkholes still active in Florida?

Are sinkholes an ongoing problem in Florida or just old history? - Anon 3/3/13

Reply: Sinkhole formation is an ongoning phenomenon in Florida and in many other areas of the world: 2013 Seffet Florida sinkhole fatality report

Anon, while an individual sinkhole may stabilize and stop "sinking", the formation of new, often terribly sudden sinkholes are a continuing event in many areas of the world. On February 28, 2013, a sudden sinkhole opening in Seffner, Florida resulted in a death and nearly the death of a second person. A sudden sinkhole formation caused the bedroom portion of a home to fall into the earth, burying Jeff Bush who was in the home at that time. Other occupants of the home escaped injury, but as the New York Times reported in early March of 2013, the home and areas around it were marked off by engineers out of concern that ther "fluid mass" of earth in the area might at any moment engulf the rest of the structure.

The Times reported experts on the scene describing the Seffner Florida sinkhole as a particularly dangerous formation that in its first 24-hours had increased in size to 20 feet deep and 30 feet wide and was still expanding.

"Sinkholes are so common in Florida that the state requires home insurers to provide coverage against the danger" the Times reorted, continuing to describe a 400-foot Florida sinkhole that developed in 1981 and noting that more than 500 sinkholes have been reported in Hillsboro County - the location of the Seffner Florida 2013 sinkhole catastrophe.. [3]

What we have not found (and continue to research) about the 2013 Seffert Florida sinkhole is whether or not there were early warning signs (SINKHOLE DETECTION, WARNING SIGNS) that might have saved Mr. Bush.


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