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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. Here a series of articles explains the cause of sinkholes, how they can be spotted, sometimes before a sudden collapse, and what to do if you know or suspect that a sinkhole is developing on or close to your property.
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In general terms a sinkhole is any sudden subsidence of the earth. In more technical terms, a true "sinkhole" is a subsidence or soil collapse caused by a combination of water eroding supporting soils, rock, or limestone, and an underlying geological structure or material that is particularly vulnerable to dissolving in water.
But consistent with their cause and the underlying soil and geology, the actual cause, and the size and dangers of sinkholes vary widely, from a small subsidence over an abandoned septic tank or swimming pool, to enormous sinkholes that swallow towns.
Here we describe this variety of sinkholes and soil subsidences, their locations, causes, sizes, and impact or dangers.
Synonyms and similar terms for sink holes include: shake hole, swallow hole, swallet, doline, cenote, moulin, and glacier mill.
Urban or suburban sinkholes due to burst water mains, sewer lines, or storm drains can occur almost anywhere. Here we focus on other sinkhole types and causes including sinkholes due to geological formations, types of rock (karst formations), or due to human activities such as mining. We also discuss here sinking buildings from causes other than sinkholes.
"Sinkholes" that are not verified by a licensed professional geologist or geotechnical engineer to be a true sinkhole are also described in various publications as subsidence incidents.
Readers should see SINKHOLES - IMMEDIATE SAFETY ACTIONS, and also see FOUNDATION CRACKS & DAMAGE GUIDE and CESSPOOL SAFETY WARNINGS. Additional septic system safety warnings are at SEPTIC & CESSPOOL SAFETY.
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.
The photograph (above-left) of a sinkhole opening in a residential yard in Pennsylvania is from Kochanov, W.E. and illustrates the child hazard or even adult sinkhole hazard that can be formed by storm water drainage.
Daniel Friedman - Florida Suncoast ASHI Educational Seminar - 1 May 2004, updated 2007, 2008Portions 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 =InspectAPedia.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
What are sinkholes and why do they occur?
A sinkhole is a subsidence feature. Subsidence is the downward movement of surface material; it involves little or no horizontal movement.
Subsidence occurs naturally due to the physical and chemical weathering of certain types of bedrock. Subsidence can also occur as a result of underground mining, excessive pumping of groundwater, or subsurface erosion due to the failure of existing utility lines. All of these examples of subsidence can produce surface features that appear similar, but not all are naturally occurring. Some are solely the result of human activities. - Kochanov, W.E.
Because the causes of sinkholes vary by geographic area, soil conditions, and human activity, we discuss the character, cause, signs, and repair of sinkholes in different areas of the U.S., Canada, and other countries in this article. Links at the left of this page and at page bottom continue sinkhole discussion in more detail by geographic area, sink hole type, and other factors. Four types of karst sinkholes are described at Four types of sinkholes.
Smaller Soil Subsidences Also Called "Sinkholes"
When an expert such as a geotechnical engineer uses the term "sinkhole" s/he refers to generally large soil subsidences that we described just above and caused by the events we just described. But in more popular speech among homeowners, attorneys, and contractors, smaller site subsidences may also be referred to as "sinkholes" and may be caused by events such as collapsing soils or smaller sized but still dangerous sinkholes over
Watch out: these smaller collapses are still potentially very dangerous, even fatal, should an adult or child fall into an opening, pit, or subsidence. 
How big are sinkholes? Most sinkholes are 10 to 12 feet in diameter. Sinkholes hundreds of feet in diameter have occurred in Eastern Canada, 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 era of roughly 1/10 of a square mile within two days of its first appearance. The September 1999 Lake Jackson sinkhole near Tallahassee Florida drained the lake into a 50-foot deep sinkhole. See Ghost Lakes.
Sinkholes of enormous size and depth do occur in some locations, such as the 330 foot deep sinkhole in Guatemala City (2010), and the Rissa, Norway 1978 soil collapse covering more than 330,000 square meters.
Sinkhole repairs: 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 sinkholes 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 have been reported world-wide, including including California, Canada, Quebec, Estonia, Guatemala, Maryland, New York, Pennsylvania, Texas, Virginia.
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 suddenly and expand rapidly, the sudden appearance of cracks in the earth should be taken as a serious safety hazard at any location, more so 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.
The Neversink TAG Pit, located in Jackson County, Alabama is a Karst formation or limestone-formation sink hole about 40 feet in diameter at its top and 100 wide at its bottom. feet deep. Details are at What is Karst?. Since 199t the Neversink pit has been owned and operated by the Southeastern Cave Conservancy and Karst Preserves. Contact SCCI.org.
The "Blue Hole" seawater sinkhole opened in Belize when seawater began entering a seep forming a very large round subsidence.
"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 Canada: Unstable Leda Clay & Risk of Sinkholes or Landslides in Quebec & Eastern Ontario, Canada, Norway, & Sweden
See Sinkholes in Quebec, Leda Clay for our full article on unstable clay soils in Quebec & Eastern Ontario.
Unstable clay soils found in some areas of Quebec and eastern Ontario (also found in Rissa, Norway) can "spontaneously liquefy with little or no provocation", leading to sudden catastrophic sinkhole formation and soil collapse reported the New York Times 13 May 2010).
The Times article reports the tragic death of the Richard Préfontaine family when on May 11, 2010 their home suddenly fell into a mud crater 100 feet deep hole approximately 900 feet by 1700 feet in size. More than 250 such collapses have been identified in this area of Canada.
The Lemieux landslide photo (left) of an earlier unstable clay soil or quick clay landslide is from the Canadian Department of Natural Resources.
The May 2010 Times article explained that because the unstable clay formed in salt water the molecular structure of its particles is unstable (compared with clays formed as layers in fresh water). When an event breaks the molecular bonds between clay particles the clay can spontaneously liquefy. The same Times article reported another clay liquefication collapse in St. Jean Vianney, Quebec in 1971, when 31 people died and 40 homes were destroyed, and continued that the town of Lemieux, Ontario (east of Ottawa) was relocated in 1991 due to concern for unstable clay soils that two years later collapsed over a 42-acre area.
See Sinkholes in Quebec, Leda Clay for a detailed article on unstable clay soils in Quebec & Eastern Ontario.
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 underground 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 underground 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 increased by 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.
Florida has More Sinkholes than any other state among the United States.
They are an obvious feature of Florida's natural karst topography. The Floridian aquifer, a karst formation, covers about 100,000 square miles, and provides drinking water to residents of that state.
A sinkhole in Winter Park Florida opened beneath the city swimming pool in 1981 and formed an enormous subsidence. A second, round and more classical sinkhole opened in Mulberry, Florida.
What the Florida Karst and sinkhole discussion sometimes fails to include is the increase in sinkholes in Florida due to increased development and the pumping of groundwater in that state. A water table that falls (or rises) by significant amounts will cause an increase in the formation of sinkholes.
Beginning at What is Karst? we discuss Florida sink holes, detection, cause, remedy, in detail. Florida's sinkholes also include phantom lakes or "ghost lakes" such as Lake Jackson. Quoting the Florida DEP & the Florida Geological Survey - FGS.
Associated Press reported (2 June 2010) that an enormous sinkhole opened suddenly in Guatemala City, Guatemala. The hole, nearly perfectly round, was estimated at 66 feet in diameter and 100 feet deep. (Wikipedia reported that the sinkhole is 200 feet deep.) Other reports assert that heavy tropical rains from the storm Agatha triggered the collapse of this sinkhole.
National Geographic and Wikipedia both report that Guatemala City, the location of the giant sinkhole is also the location of a 2007 sinkhole that was 330 feet deep. Guatemala city is built on soils comprised of volcanic ash that has not solidified into rock.
We wondered if volcanic ash or pumice (Guatemala city is built atop a thick layer of loose pumice) may in periods of flooding (such as 2010 tropical storm Agatha) become unstable in a manner similar to the sudden liquification of high-water-content clay discussed at Sinkholes in Quebec, Leda Clay. But Bonis' explanation (below) seems the most credible to date.
It is apparent and the media report implied that the sudden development of this sinkhole in Guatemala was related to weather conditions, including heavy rains, occurring in the area when tropical storm Agatha reached the Guatemala/Mexico border shortly before. In other areas (Guatemala, Honduras, Mexico) that tropical storm caused landslides and led to at least 150 deaths in Guatemala alone. But more than sudden rainfall was at work in causing this sinkhole.
The very round shape of that sinkhole indeed suggests that there was a pre-existing condition or geological formation, perhaps related to a history of water drainage in that location. Two Guatemalan sinkhole explanations are provided here:
Wikipedia reported (June 2010) that four volcanoes are visible from the city, two of them active. The nearest and most active is Pacaya, which at times expels a considerable amount of ash. That source also reports that "Geologist Sam Bonis suggests that the recurring "sinkholes" in Guatemala are, as others have suggested, caused by sewer leaks eroding Guatemala's soft soil rich in volcanic ash. As such Bonis does not consider the features sinkholes proper, instead calling them a "piping feature" of Guatemala's aged sewer system."
Offering a second sink hole explanation that is a bit harder to swallow, Vanity Fair, consulting with David Bercovici and Mark Brandon, both professors of geology and geophysics at Yale University, reported that water collected and ate away at the "bedrock" and added the explanation that water had collected in a depression (presumably and unexplained, the depression itself was perfectly round) for some time, dissolving the underlying rock for some time before the sudden collapse.
Just why a depression under a paved city intersection in Guatemala City developed in the shape of a nearly-perfectly-round water collection point occurred below ground at that single location and at that depth could benefit from added detail. Nonetheless, the experts' observation that there was a pre-existing water drainage pathway that had been in fact developing for a long time, combined with recent flooding to produce the sudden and deep collapse is helpful in understanding this very round and very deep sinkhole event. One would expect further sudden sinkholes to appear under these conditions.
In sum, the most likely explanation of the Guatemala City sinkholes is the combination of the Bonis explanation, combined with sudden heavy rainfall that leads to the collapse of a previously-undermined area.
Sinkholes in Iceland
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Related Topics, found near the top of this page suggest articles closely related to this one.
 Thomas V. Cech, Principles of water resources: history, development, management, and policy, John Wiley and Sons 2009 ISBN0470136316, 9780470136317
 Opdyke et als, "Origin of the epeirogenic uplift of Pliocene-Pleistocene beach ridges in Florida and development of the Florida karst", N. D. Opdyke1, D. P. Spangler 1, D. L. Smith 1, D. S. Jones 1 and R. C. Lindquist 1, Geology; April 1984; v. 12; no. 4; p. 226-228; DOI: 10.1130/0091-7613(1984)12<226:OOTEUO>2.0.CO;2
© 1984 Geological Society of America
1 Department of Geology, University of Florida, Gainesville, Florida 32611
Additional reference citations within the above document:
4. http://members.aol.com/caveconser/page1.htm and http://members.aol.com/caveconser/page2.htm
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
13. 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
 see Reference 6
 Reference 1
 Reference 4
 Reference 2
 Reference 1
This list compiles clues from multiple sources and references.
 Reference 3
 Reference 7
 Reference 19
The frost heaving forces developed under a 1 ft. (30.5 cm) diameter steel plate were measured in the field throughout one winter. The steel plate was fixed at the ground surface with a rock-anchored reaction frame. heave gauges and thermocouples were installed at various depths to determine the position and temperature of the active heaving zone. The general trend was for the surface force to increase as the winter progressed. when the frost line approached the maximum depth the force was in excess of 30,000 lb (13,608 KG). Estimates of the heaving pressure at the frost line ranged from 7 to 12 psi (0.49 to 0.84 KG/cm) square during this period. The variation of surface heaving force was closely associated with weather conditions. Warming trends resulting in a temperature increase of the frozen layer caused the forces to decline.
Leda clay slopes in the Ottawa valley are vulnerable to catastrophic landslides. More than 250 landslides, historical and ancient, large and small, have been identified within 60 km of Ottawa. Some of these landslides caused deaths, injuries, and property damage, and their impact extended far beyond the site of the original failure. In spectacular flowslides, the sediment underlying large areas of flat land adjacent to unstable slopes liquefies. The debris may flow up to several kilometres, damming rivers and causing flooding, siltation, and water-quality problems or damaging infrastructure. Geologists and geotechnical engineers can identify potential landslide areas, and appropriate land-use zoning and protective engineering works can reduce the risk to property and people.
Deposits of Leda clay, a potentially unstable material, underlie extensive areas of the Ottawa-Gatineau region. Leda clay is composed of clay- and silt-sized particles of bedrock that were finely ground by glaciers and washed into the Champlain Sea. As the particles settled through the salty water, they were attracted to one another and formed loose clusters that fell to the seafloor. The resulting sediment had a loose but strong framework that was capable of retaining a large amount of water. Following the retreat of the sea, the salts that originally contributed to the bonding of the particles were slowly removed (leached) by fresh water filtering through the ground. If sufficiently disturbed, the leached Leda clay, a weak but water-rich sediment, may liquefy and become a 'quick clay'. Trigger disturbances include river erosion, increases in pore-water pressure (especially during periods of high rainfall or rapid snowmelt), earthquakes, and human activities such as excavation and construction.
After an initial failure removes the stiffer, weathered crust, the sensitive clay liquefies and collapses, flowing away from the scar. Failures continue in a domino-like fashion, rapidly eating back into the flat land lying behind the failed slope. The flowing mud may raft intact pieces of the stiffer surface material for great distances.