SINKHOLES, WARNING SIGNS
Sinkholes in California
Sinkholes in Florida
Sinkholes in Estonia-The Witches' Well
What is Karst?
What is a Sinkhole?
Sinkholes and the Aquifer
Four types of sinkholes
Three types in Florida
Sinkholes and Lake Formation
Sinkholes and Urban Development
Sinkhole Detection, Warning Signs
Visual Indicators of Sinkhole Formation
Site and Neighborhood Observations
Visual Indicators of Extra Risk
Building and Water Supply Obs
Temporal Sinkhole Triggers
Engineering Methods for Detecting Sinkholes
What to do if a Sinkhole is observed or suspected
Sinkhole Warning - How Much Time Do You Have?
Sink Hole & Related Engineering References
STRUCTURAL INSPECTIONS & DEFECTS
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How big are sinkholes? Most sinkholes are 10 to 12 feet in diameter. A discussion of foundation repair methods such as driven piers, helical piers, or other structural repair methods may seem in order, but if a sink hole is big enough to swallow a home, the first order of business for areas where those problem soils are found (California sinkholes, Florida sinkholes, Pennsylvania sinholes over mines, Texas sinkholes, often over salt domes and possibly affected by wastewater disposal back into the ground during oil drilling, others) is to recognize the signs that sinkholes have plagued a neighborhood and/or that a sinkhole is presently developing at a particular home. Sinkholes hundreds of feet in diameter have occurred in Florida and Texas - big enough to swallow a home. The "December Giant" sinkhole in Montevallo, Alabama was 520 x 125' and 60' deep. The Dasietta Texas sinkhole reached 525' x 600' and a depth of 150', collapsing an aera of roughly 1/10 of a square mile within two days of its first appearance. Significant to property inspectors, the first signs that a sinkhole was developing in Dasietta Texas was the opening up of cracks in the ground and in the roadway on the morning of the collapse. Because a sinkhole can develop suddenlyh and expand rapidly, the sudden appearance of cracks in the earth should be taken as a serious safety hazard at any location, moreso in an area where sinkholes are known to occur. 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. 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. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website. |
This document explains 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.Daniel Friedman - Florida Suncoast ASHI Educational Seminar - 1 May 2004, updated 2007, 2008
Portions of this text are extracted, quoted, or paraphrased from references provided; a key resource author was Sarah Cervone at Reference-1.
The text document is SinkholesFL.doc =inspect-ny.com/vision/sinkholes.htm © 2009-2004 Daniel Friedman All Rights Reserved
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, http://InspectAPedia.com/structure/x-ray.htm
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/vision.htm.Comments and content suggestions are invited.
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.
Sinkholes in California
"California: Sinkhole under house kills man. A man was killed on Friday when a huge hole opened beneath his house in Alta, and after two days of recovery efforts, workers reached the body on Sunday. The authorities identified the man as Jason Chellew, 32, a schoolteacher. Mr. Chellew was in his living room about 9:30 p.m. on Friday when the floor opened beneath him, the authorities said. The area in the Sierra Nevada foothills was heavily mined for gold in the late 1800's. A mine collapse could have caused the accident, officials said." (AP) - New York Times Tuesday 4/25/2006 p. A25 National Briefing
Sinkholes in Texas
Notes on the Dasietta Texas Sinkhole Reported in May 2008 - the "Sinkhole de Mayo"
The Texas Dasietta Sinkhole which appeared suddenly and apparently with no warning on May 8, 2008 in Dasietta Texas. New York Times reported that after only about two days the Dasietta Texas sinkhole covered an area of 600 x 525 feet, or about 1/10 of a square mile. At the time of the May 9th report, experts didn't know if the sinkhole had stopped growing, but Carl Norma n, a geologist at U.Houston reported in the Times that the sink hole could become stable or it could collapse further in six months, doubling in size. The salt dome over which the Dasietta Texas sinkhole is located was estimated to be six miles in diameter.
It was posed that while this sinkhole collapse could be due to entirely natural causes, the the practice of disposal of waste saltwater produced by oil drilling operations by pumping the wastewater into the ground might be contributing to or even causing the Dasietta sinkhole by dissolving underground salt. -- Ref: "Sinkhole and Town: Now You See It ...", New York Times, p. A-15, 9 May 2008.
Property inspectors working in Texas in areas of construction over salt domes or where oil drilling may be both extracting oil from and inserting wastewater into the soil need to be particularly vigilant and should caution home owners or home buyers about the risk of sudden sinkhole development.
Sinkholes in Florida
Florida has More Sinkholes than any other state among the United States. They are an obvious feature of Florida's natural karst topography.
Sinkholes in Estonia - the Witch's Well in Tuhala
Sinkholes occur in many parts of the world, and often from similar underlying geological formations. The New York Times described the Witch's Well in Tuhala, Estonia as caused by a combination of a large field of porous karst combined with water from fifteen undergorund rivers, a "maze of caverns", and periodic flooding ground water during periods of heavy rains. Sinkholes in the Tuhala area include the "Horse's hole" (1978) and the "Mother-in-Law's hole" and the Times also reported that streams appear and disappear "like phantoms." (See Hannu Oittinen's photo in the NY Times article and other photos in our references below.)
Although some local people enjoy the magic of curative waters roiling unbidden to the surface of the Witches Well in Tuhhala when underground witches are fighting, or by other accounts, when the witches are flogging themselves with birch branches in an under-earth sauna, the Witch's well in Tuhala, Estonia is an episodic upsurge well through which nearby river and marsh water is pushed to the surface when rainwater floods the porous karst field below the well.
Because the up-flow of water in this Tuhala well is caused by increased water pressure from high water levels in the nearby undergruond Tuhala river rather than by pressure from groundwater, the Witches' well is not a true artesian well. Water flows up through the Witches' well at hundreds of liters per minute during these eruptions.
The Witches' well is normally fed by the Kuhala underground river (1.5 km away) through the karst field and through nearby bogs. On occasion, during years of heavy spring floods and sometimes in other seasons, when the Tuhala river flow is increasedby local rains to exceed 5000 liters/second, it causes the Witches' well to send water flowing to the surface.
Water from the Witches' well is muddy brown as it is fed from the Tuhala river through nearby marshes to porous karst below the well. For thousands of years people living in the area have believed that water from these erupting springs is holy, able to cure blindness, remove freckles, or increase longevity. (Tuhala, a very old city that includes 500-year old farmhouses, has been occupied for roughly 3,000 years, and includes eleven prehistoric camps or settlements, cult stones, and Estonia's oldest log road (Heinasoo bog).)
The history of water usage in Estonia is given by a citation in our references. Characteristic of worldwide growing problems with severe loss of ground water due to pumping-out by water mining companies, this concern faces Tuhala as well, where a nearby quarry may lead to significant drops in ground water that may be more serious for owners than simply the loss of the fun of the Witches' well eruptions.
What is Karst?
In 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.
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 [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
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]
- Solution sinkholes - surface depressions, not complete collapse
- Cover-subsidence sinkholes - loose, overlying sand slides into solution cavity
- Collapse sinkholes - roof of an underground channel suddenly collapses, forming a steep-sided cavity
- 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]
 |
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
 |
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
 |
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 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]
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- "A Hole in the Ground Erupts, to Estonia's Delight", New York Times, 9 December 2008 p. 10.
- History of water usage in Estonia: (5.7 MB PDF) jaagupi.parnu.ee/freshwater/doc/the_history_of_water_usage_systems_in_estonia.pdf
Sink Hole & Related Engineering References
1. [primary resource] Sarah Cervone, [web page] data from the APIRSdatabase, 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
3.
Detecting Sinkholes with Geophysics,
Enviroscan, Inc.,
4. http://members.aol.com/caveconser/page1.htmand 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
-
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
3. http://www.swfwmd.state.fl.us/emer/sinkhole/sinkpage.htmSW Florida Sinkhole Information
14. http://www.dep.state.fl.us/geology/gisdatamaps/index.htmSink 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
17. http://InspectAPedia.com/structure/foundation.htm - Inspecting Foundations for Structural Defects
19. http://sjr.state.fl.us
[1] see 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.
[8] Reference 7: Geologists have a good idea where sinkholes are likely to form geographically, but it's much more difficult to accurately predict specifically where[and when] sinkholes will occur."
[9] Reference 6
[10] At a previous ASHI Florida conference slides of a masonry block building repaired by AB Chance Helical Pier Co. showed step cracks at the top of the house front wall which were diagnosed as settlement at the opposite end of the house - structural rigidity caused cracking to telegraph to the front; detectable by noting out-of-plumb mortar joints!
[11] Reference 3
[12] Reference 7
[13] Reference 19
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