http://plants.ifas.ufl.edu/guide/sinkholes.html

 

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.

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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).

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Three general types of sinkholes occur in Florida: collapse, solution, and subsidence.

Source: USGS

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.

Source: USGS

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.

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Source: USGS

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.

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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. Although a sinkhole can form without warning, specific signs can signal potential development:

• Slumping or falling fencepost, trees, foundations, etc.
• Sudden formation of small ponds
• Wilting vegetation
• Discolored well water
• Structural cracks in walls, floors, etc.

WHAT TO DO if a SINKHOLE DEVELOPS on your PROPERTY

• Notify your local Water Management District
• Fence or rope the hole off
• Keep children away!
• Protect the area from garbage and waste
• Contact your homeowners insurance company

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SINKHOLES and RECREATION
In addition to providing essential pathways for water to replenish the aquifer, a variety of sinkholes are protected and maintained as Florida state parks where they provide frost-free habitats for cold sensitive species and offer many opportunities for sports and recreation.

 

 

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 http://www.inspectapedia.com/vision/sinkholes.htm

Sinkhole Detection and Warning Signs

Visual Indicators of Extra Risk of Sinkhole Formation

Although a sinkhole can form without warning, specific signs can signal potential development: [7]

Site and Neighborhood Observations - ordered from general-area to site-specific to property-specific

• Indications on maps of the locations of likely sinkholes. [8]
• Areas known to be of thin supporting layers of sand and clay soil (look at any local excavation projects) (Sinkholes develop more frequently north of Tampa Bay where the limestone base is closest to the land surface and the supporting sand and clay layers are thin. [9] )
• Areas known to be soil over salt domes See Texas Sinkholes for an example.
• An actual sinkhole is present on or near the subject property (duh!)
• Slumping or falling fenceposts, trees, foundations on or near the property
• Previously-buried fenceposts, foundations, trees, become exposed [because of sinking ground]
• Small rills, gullies, or bare soil areas develop [soil particles being carried away to sinkhole]
• Cracked earth, a circular pattern of ground cracks outlining the sinking area. Sudden earth cracking should be taken as a very serious sinkhole or earth collapse risk. See Texas Sinkholes for an example.
• Undercut stream banks and fallen trees along a drainage way
• Sudden formation of small ponds [of rainfall forming where water has not collected before]
• Wilting vegetation [small circular areas, because moisture that normally supports the vegetation is draining into a developing sinkhole - wilting is not always a sinkhole indicator]
• Circular or oval depressions in cultivated fields that may or may not pond standing water after rain events
• Areas of cultivated fields which are not being plowed
• Areas of oil drilling or underground mining which remove large volumes of liquids or solids from the earth
• Areas of oil drilling where wastewater produced by drilling is pumped back into the ground may cause dissolution of salt domes and lead to local sinkhole collapses See Texas Sinkholes for a possible example.
• Gradual, localized ground settlement [does not always indicate a sinkhole]
• Sudden ground openings
• Sudden ground settlement
• Interrupted electrical or plumbing service to a building or neighborhood due to undermined, settled buried mechanical lines.

Visual Indicators of Extra Risk of Sinkhole Formation (continued)

Building and Water Supply Observations

• Silt buildup, fresh mud deposits, muddy water [in wells? in a pond or stream?] Muddy or cloudy well water from nearby wells can indicate an early stage of sinkhole development.
• Structural cracks in walls, floors [10]
• Doors or windows that don't close properly [traced to building foundation movement]

Temporal Sinkhole Triggers

• Following a period of heavy or prolonged rain (washing-in supporting soils)
• Following a period of drought (lowering the water tables, leaving cavities)
• Following a period of housing development (adding pressure on supporting soils)
• Over pumping existing water supply wells, or drilling of additional wells in an area (lowering the aquifer)
• Diverting surface water from a large area and concentrating it in a single point
• Artificially creating ponds of surface water

Engineering Methods for Detecting Sinkholes

[11]

• Soil borings or other direct testing - Borings can be reduced by reconnaissance scannings using the following methods:
• Electromagnetics (EM) and DC Resistivity: detect variations in subsurface electrical properties related to anomalously thick or wet soils (electrical conductivity highs similar to our use of moisture meters in homes), or voids in the electrically conductive clay soil mantle (electrical conductivity lows)
• Spontaneous Potential (SP): detects naturally-occurring minute electrical currents or potentials commonly associated with concentrated vertical water infiltration (Streaming potentials)
• Micro-gravity: detects minute variation in gravity (subsurface voids create missing mass and lower gravity)
• Seismic Refraction: profiles the top-of-rock which may display conical depressions of a type associated with subsidence sinks or deep gouges or cutters which represent sinkhole-prone lineaments.
• Ground-penetrating radar [12]

What to do if a Sinkhole is observed or suspected at a property during a home inspection

This constitutes an immediate potential safety concern. ASHI Standards require you make appropriate notifications.

• Notify all parties: occupants, owners, real estate agents, buyers
• Notify the local Water Management District
• Fence or rope the hole off or arrange for this action to be taken immediately
• Keep children away!
• Protect the area from garbage and waste
• The property owner should be advised to contact their homeowners insurance company
• You may inform the parties that there are engineering firms specializing in detection and evaluation of potential or evident sinkholes
• Record in your report the notifications and actions you took

Sinkhole Warning - How Much Time Do You Have ?

A rapid sinkhole caused by well drilling or other sudden alterations to the terrain may not give any warning signs. Otherwise, the collapse process usually occurs gradually enough that a person may leave the affected area safely. The final breakthrough can develop over a period of a few minutes to a few hours. [13]