This article discusses how to recognize that a sinkhole may be imminent or starting at a property.

It also summarizes what 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.

Most sinkholes are 10 to 12 feet in diameter. Sinkholes hundreds of feet in diameter have occurred in Florida - big enough to swallow a home. What about cases 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.

Watch out:  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.

Visual Inspection to Detect Sinkholes in Florida

Daniel Friedman - Florida Suncoast ASHI Educational Seminar - 1 May 2004, updated 2012

Portions of this text are  extracted, quoted, or paraphrased from references provided; a key resource author was Sarah Cervone at Reference-1. Updated 2004, 2005, 2010,2015

FLORIDA HAS MORE SINKHOLES than any other state among the United States. They are an obvious feature of Florida's natural karst topography.

What is "Karst"?

[1] limestone layer below (often thin) topsoil. Karst is any land with sinkholes, springs, and streams that sink into subsurface caverns.

[2] The term "sinkhole" is applied by engineers to [3]

• Bedrock voids (most difficult to detect, but least likely of imminent collapse)
• Depressions in the top-of-bedrock
• Stoping 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

Source: USGS

Sinkholes and the Aquifer[4]

[Excerpting from References 1 and 2]

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

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

1.      Solution – surface depressions, not complete collapse

2.      Cover-subsidence – loose, overlying sand slides into solution cavity

3.      Collapse – roof of an underground channel suddenly collapses, forming a steep-sided cavity

4.    Cover-collapse – 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.

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

Source: USGS

Collapse sinkholes

. most common type 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

Source: USGS

. overburden is thin.

. form slowly

. dissolving limestone is replaced by sand granules that fall into the depression and fill the holes

. concave depression

. only a few feet in diameter and depth (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

Source: USGS

Sinkholes and Lake Formation

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 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])
• An actual sinkhole is present on or near the subject property (duh!)
• Slumping or falling fence posts, trees, foundations on or near the property
• Previously-buried fence posts, 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.
• 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
• 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

• Discolored well water
• 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]

References:

1. [primary resource] Sarah Cervone, [web page] data from the APIRS database, Graphics by Ann Murray, Sara Reinhart and Vic Ramey, Vic Ramey is the editor. DEP review by Jeff Schardt and Judy Ludlow. The web page is a collaboration of the Center for Aquatic and Invasive Plants, University of Florida, and the Bureau of Invasive Plant Management, Florida Department of Environmental Protection contact: varamey@nersp.nerdc.ufl.edu

2. Center for Cave and Karst Studies or the Kentucky Climate Center, both at Western Kentucky University.

3. "Detecting Sinkholes with Geophysics," Enviroscan, Inc., Lancaster PA  717-396-8922 email@enviroscan.comwww.enviroscan.com 2003

4. http://members.aol.com/caveconser/page1.htm  and http://members.aol.com/caveconser/page2.htm

5. http://www.nd.edu/~techrev/Archive/Spring2000/a2.html

6. http://www.swfwmd.state.fl.us/about/isspapers/sinkholes.html

7. http://aquat1.ifas.ufl.edu/guide/sinkholes.html -- Sinkholes in FL, surface characteristics, types, warning signs, what to do, recreation

8. http://kyclim.wku.edu/BRADD/sinkholes/intro.html   Sinkhole explanation and warning signs  - Kentucky

9. http://fl.water.usgs.gov/Tampa/    -- Center for Coastal and Watershed Studies - Tampa Florida

10.     http://fl.water.usgs.gov/Pubs_products/online.html   -- bibliography including sinkhole studies

11.     http://coastal.er.usgs.gov/publications/ofr/00-180/index.html  -- sink hole maps for NE Florida - index page

12.     http://gulfsci.usgs.gov/tampabay/index.html  - Tampa Bay Study

13.     http://www.swfwmd.state.fl.us/emer/sinkhole/sinkpage.htm  SW Florida Sinkhole Information

14.      http://www.dep.state.fl.us/geology/gisdatamaps/index.htm  Sink Hole Maps - FL

15.     http://www.dep.state.fl.us/geology/gisdatamaps/sinkhole_database.htm  - Sink Hole Locations - database for FL (Excel)

16.     http://coastal.er.usgs.gov/publications/ofr/00-180/intro/intro.html   specific to Florida northeast

17.    https://InspectAPedia.com/structure/Foundation_Damage_Repair_Guide.php   - Inspecting Foundations for Structural Defects

18.    http://216.239.39.104/search?q=cache:ZeYj0XgJ38oJ:www.gamineral.org/_docs/Apr03p7-12.pdf+sinkhole+clues+signs&hl=en&ie=UTF-8

19. http://sjr.state.fl.us                                                                   

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[1] Reference 6

[2] Reference 1

[3] Reference 4

[4] Reference 2

[5] Reference 5

[6] Reference 1

[7]  This list compiles clues from multiple sources and references.

[9] Reference 6

This document is SinkholesFL.doc at InspectAPedia.com/vision/Sinkholes_Subsidences.php - © 2017 - 2004 All Rights Reserved. Also see "Developing your X-Ray Vision - A Promotion Theory for Forensic Observation of Residential Construction - Levels of Fear, and how to use them to find and report significant, hidden problems,".
X-Ray Vision.doc =InspectAPedia.com/structure/xray.html

Also see The Nature of Vision - Inspecting Complex Systems - When and Why Inspectors "See" or "Don't See" Things Which are Present - InspectAPedia.com/vision/Visual_Perception_Errors.php. Further review comments and content suggestions are welcome to Author for content suggestions, and more information on building inspection are

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