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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What are Sinkholes, How Big Are Sinkholes, & Where do Sinkholes Occur?

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
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.

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

• an abandoned buried oil storage tank - OIL TANK ABANDONING PROCEDURE
• a septic tank, cesspool or drywell - CESSPOOL SAFETY WARNINGS
• buried gasoline or other fuel storage tanks
• buried debris, garbage, trees, tree-stumps.
• dug wells
• areas of soil wash-out due to burst water mains or leaking water or sewer piping

Watch out: these smaller collapses are still potentially very dangerous, even fatal, should an adult or child fall into an opening, pit, or subsidence. [3][4]

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.

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 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 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 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.

Sinkholes in Florida

Source: USGS

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.

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.

On September 16, 1999, much of the central portion of Lake Jackson, a large lake on Tallahassee’s northern side, drained down an eight-foot-diameter sinkhole known as Porter Hole. Sinkholes in Lake Jackson open and drain portions of the lake approximately every 25 years. Following the September 1999 event, the FGS, in cooperation with the Northwest Florida Water Management District, Leon County, and theA picture of Geologists entering Porter Sinkhole in Tallahassee, Florida Florida State University Department of Geological Sciences began investigating this phenomenon. This investigation will improve our understanding of the connection between Lake Jackson and the Floridan aquifer system, which provides most of the area’s drinking water.

... The sink consists of a series of convoluted passages through the limestone, with various conduits intersecting the main sink. The FGS has drilled four vertical core holes to depths from 105 feet to 119 feet below the lakebed. Interpretation and analysis of the cores continues and will be published by the FGS upon completion.

See Ghost Lakes & Sinkholes.

Sinkholes in Guatemala

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.)

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 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 Maryland

Where do Sinkholes Occur in Maryland?

As pointed out by the Maryland Geological survey, Maryland karst terrains are limited to areas underlain by carbonate rocks.

In Maryland these areas most associated with collapse sinkholes are the Hagerstown Valley (HV), the Frederick Valley (FV), and the Wakefield Valley (WV). To a lesser degree, collapse sinkholes are found in Green Spring Valley (gs), Worthington Valley (wo), and Long Green Valley (lg). The Maryland karst location map (above) and excerpts from the referenced text are courtesy of Maryland Geological Survey. [MGS].

Quoting from the MGS:

In a type of karst known as cutter-and-pinnacle karst, the contact between bedrock and soil overburden is very irregular (see Fig. 2 and 3 for example). Water preferentially dissolves bedrock along some planar feature, such as bedding, joints, or fractures, whichever is the easier path. Roughly vertical, solutionally widened joints are called cutters, or grikes.

Cutters are generally filled with soil. The bedrock that remains between cutters may be reduced to relatively narrow "ridges" of rock, called pinnacles, particularly where cutters are closely spaced. Cutter-and-pinnacle karst (or simply "pinnacle karst" for short) is common in many of the carbonate valleys in Maryland (Map above).

The problem develops when a building foundation lies on cutters and pinnacles. The weight of the building will compact the soil to some extent, and the building will settle. That is normal, and does not pose a problem as long as the building settles uniformly.

However, in pinnacle karst, part of the foundation may be supported by a bedrock pinnacle and part may be supported by a cutter (soil-filled).

The result can be differential settling of the building, which may produce cracks in the walls, foundation, and floor (sketch at left). This may compromise the structural soundness of the bearing walls and, therefore, place the safety of the whole structure in doubt.

The Maryland Geological Survey makes a helpful distinction between collapse or subsidence due to subsurface erosion (piping), and sudden collapse sinkholes (closed depressions).

Piping Sinkholes in Maryland

Piping is subsurface erosion of soil by percolating waters to produce pipe-like conduits underground. Piping can affect materials ranging from clay-size particles (less than 0.002 mm) to gravels (several centimeters), but is most common in fine-grained soils such as fine sand, silt, and coarse clay. The resulting "pipes" are commonly a few millimeters to a few centimeters in size, but can grow to a meter or more in diameter. They may lie very close to the ground surface or extend several meters below ground.

Piping can become a problem in areas of cutter-and-pinnacle karst, as well as in some non-karst areas. As shown in in the MGS sketch below, what begins as piping can develop into cavities in the soil overburden. Piping tends to become accelerated when the water table is lowered by over-pumping ground water, when the amount of infiltrating water increases, or both. (The "water table" marks the top of the zone of saturation, in which all pores and voids in bedrock and soil are filled with water.)

What can cause increased volume of water that infiltrates the soil overburden? Long periods of rainfall can be a factor, but man's activities also are significant. Buildings with large roof areas, parking lots, streets and highways change the runoff and infiltration characteristics of soil by decreasing widespread, diffuse infiltration and channeling surface runoff to areas where more concentrated infiltration can occur.

The MGS illustration (above left)shows how runoff can be concentrated in the subsurface to create subsurface cavities. This is especially common in soil-filled cutters.

Collapse sinkholes in Maryland

In Maryland, collapse sinkholes occur mainly in four areas: the limestones of the Hagerstown Valley in Washington County and the Frederick Valley in Frederick County, marble in the Wakefield Valley in Carroll County and, to a lesser degree, in marble valleys of Baltimore County (map above). Collapse sinkholes seem to be most prevalent in the Frederick Valley and the Wakefield Valley. [Sketch left, courtesy of MGS].

As used in the Maryland MGS discussion, the term sinkhole refers exclusively to one type of closed depressions in karst landscapes. One type of sinkhole is the collapse sinkhole, so named because it forms suddenly when the land surface collapses into underground voids, or cavities. Collapse sinkholes are often fairly circular with steeply sloping sides. They can be so small as to be barely noticeable to 50 meters or more in width and depth. Once formed, they can also grow larger. [We illustrate and discuss collapse sinkholes also at Sinkholes in Pennsylvania.]

In some karst terranes, collapse sinkholes form when the roof of a cave or cavern collapses. Such is the case in some collapses in Florida (Sinclair, 1982). [See Sinkholes in Florida] However, most collapse sinkholes seem associated with cavities in the soil overlying the carbonate rock. Some prefer the term cover collapse sinkhole to denote that collapse occurs in cavities in the soil overburden, or cover, rather than in the carbonate bedrock below. This is the general case for collapse sinkholes in areas of pinnacle karst in Maryland.

Other types of sinkholes form slowly by the dissolving of carbonate rock at or very near the surface. They tend to have gently sloping sides, and they seldom pose a hazard by collapsing. Like collapse sinkholes, however, they can pose environmental problems related to pollution, because they provide a point where polluted surface runoff can directly flow into the ground water.

In the United States, according to one study, the states most impacted by collapse sinkholes are Alabama, Florida, Georgia, Missouri, Pennsylvania, and Tennessee (Newton, 1987).

More information on Maryland sinkholes:

Foundation Engineering Problems and Hazards in Karst Terranes, James P. Reger, Maryland Geological Survey, web search 06/05/2010, original source: http://www.mgs.md.gov/esic/fs/fs11.html

Sinkholes in Missouri

Sinkholes in New York

See Sinkholes in New York for examples of a sudden soil subsidence in New York. Details of this case and our advice on what to do about this sudden yard collapse are provided.

Sinkholes in Pennsylvania

Sudden subsidences and sinkholes in some areas of Pennsylvania have been associated with coal mining activities combined with surface and subsurface water drainage or pumping. De-watering Sinkholes Sinkholes that are manmade also include subsidences caused by de-watering due to pumping and removal of subsurface water either for consumption or for drainage. De-watering sinkholes have been widely reported including in Maryland, Pennsylvania, and Florida. - See Newton, J. G. and Sinclair, W. C.. Illustrations and portions of this text are from Kochanov, W.E. Sinkhole Sizes in Pennsylvania On average, sinkholes in Pennsylvania range from 4 to 20 feet in diameter and 4 to 20 feet in depth. If a sinkhole develops over an area due to mining, its size and depth may be significantly different than these "natural" sinkholes not directly related to human activity. 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. Where do Sinkholes Occur in Pennsylvania? The PA DCNR illustration (above) demonstrates that in Pennsylvania sinkholes occur in certain areas, not state wide, and track principally areas where carbonate bedrock is found. [This illustration does not track the effect of coal mining on sinkholes.]

Continuing from Kochanov, Case histories of sinkhole occurrence reveal that sinkholes occur only in certain parts of Pennsylvania. By examining these records, we learn that sinkholes are found in areas underlain by carbonate bedrock. Large areas of central and eastern Pennsylvania are underlain by this type of bedrock [see the figure above]. We can now add this information to the definition of a sinkhole. A sinkhole is a subsidence feature in an area underlain by carbonate bedrock. It can form rapidly and is characterized by a distinct break in the land surface and the downward. The PA DCNR photograph of a Pennsylvania sinkhole (above) shows the throat and arch of a typical occurrence. In Pennsylvania sinkholes formed as Kochanov describes, one side of the sinkhole is steep (the upper portion of the photo), and the other side is more gently sloped. The arch that has formed over the sinkhole's drainage throat forms the sinkhole roof.

If a sinkhole roof collapses, this structure can convert to a sudden and larger subsidence or sinkhole (as occurs in Florida and in Pennsylvania). Depending on the initial sinkhole tunnel, throat, size, soil rock and water conditions, the result may be a shallow depression, or a larger, more catastrophic soil collapse such as shown in this second photo from Kochanov and the PA DCNR. Multiple sinkholes close together in an area may also collapse to form a series of depressions or a single larger sinkhole or depression as this PA DCNR photo shows. See Kochanov, W. E., 1999, Sinkholes in Pennsylvania: Pennsylvania, Geological Survey, 4th ser., Educational Series 11, 33 p.

What are Ghost Lakes and how do they Relate to Sinkholes?

Ghost lakes form in some areas over limestone that has previously been partially dissolved to leave a network of water-draining passages in rock. On occasion water passages may become clogged, perhaps with surface soils and debris, leading to lakes that form, then later disappear if the drainage clogs open again.

The September 1999 Lake Jackson sinkhole near Tallahassee Florida drained the lake into a 50-foot deep sinkhole. Before it was drained by this sinkhole, the lake covered six square miles. The Florida Geological Survey (FGS) indicates that Lake Jackson drains and refills every twenty-five years.

OPINION-DF: As Kochanov explains, acidic rainwater is formed by the interaction of atmospheric moisture with carbon dioxide to form carbonic acid. As global warming is associated with increased atmospheric levels of carbon dioxide (CO2), we might expect an increase in the acidity of rainfall that in turn may increase the incidence of sinkholes where carbonate bedrock (limestone) is prevalent.

Causes of Soil Subsidence & Building Movement Not Due to Sinkholes

See the full article on this topic at Sinking by Other Problems, Not Sinkholes. Excerpts are below.

Kochanov, W.E. lists and we elaborate here a variety of subsidences that are not caused by naturally-occurring sinkholes, followed by examples of sinking houses not caused by sinkholes, and examples of cesspool or septic system collapses.

• Natural soil settlement under and around new construction, potentially causing severe damage if soils were improperly compacted below a foundation, for example.
• Subsidence and structural movement due to building expansions built over old cisterns, wells, or cesspools.
• Local drainage areas or springs
• Burrowing animals (groundhogs)
• Abandoned mines, underground mining activities
• Broken water mains or sewer mains can undermine streets and sidewalks, leading to sudden catastrophic collapse of streets, sidewalks, or nearby properties, at times consuming people or even vehicles and causing injury and even death [3]

Understanding the cause of a sinkhole assists in knowing what to expect in the future and in planning for building or sink hole repairs. An important key to identifying a sinkhole area is to consider the location of carbonate bedrock.

Watch out: significant building movement may be caused by a variety of problems other than sinkholes. Some of these are equally dangerous, and expert evaluation is important. Consult with a structural engineer who is specifically familiar with building and foundation movement and failures. Examples of conditions that can raise serious, urgent concerns for safety and risk of sudden, catastrophic building collapse include at least the following:

• Cracks and bowing in structural brick walls. Not all masonry wall cracks are dangerous, but if the bond courses are broken in a structural brick wall (for example if there is bulging), the building may be in very dangerous, unstable condition and at risk of sudden collapse.
• Sudden significant building movement
• Large cracks, bulges, bowed walls that may indicate sufficient movement to have damaged structural connections
• Damage from flooding, earthquake, etc.
• Nearby excavation that undermines the foundation of an existing building
• Soil undermining by burst water main

Watch Out: Immediately rope off the area of any soil subsidence or suspected old septic tank or cesspool area, and mark it plainly as unsafe so that a wandering neighbor, adult or child, does not go near nor fall into this hole. It could be quite dangerous. See CESSPOOL SAFETY WARNINGS for examples of potentially fatal cesspool collapse hazards.

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 Norman, 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 Virginia

The Virginia Department of Mines, Minerals, and Energy reports that sinkholes have been reported in that state since colonial times.

“In the spring of the year 1780, the earth at the bottom of this sink suddenly gave way and fell into the cavity below, forming a circular aperture about the ordinary circumference of a common artificial well . . . There being no artificial or natural means to prevent the earth immediately about the well from falling in, the aperture is greatly enlarged, forming a sloping bank, by which a man on foot can easily descend within eight or ten feet of the water .... The whole depth of the cavity is thirty or thirty-five feet” (Kercheval, 1850, p. 273).

As in several of the other locations of sinkholes around the U.S., Canada, and other countries, in Virginia sinkholes are associated with karst formations. [See What is Karst?] The sinkhole photograph above (from Virginia's DMME) shows a sinkhole that formed in Russell County, Virginia. According to the DMME, [quoting]

In Virginia the formation and modification of sinkholes (also known as sinks, dolines and dolinas) is a natural process in areas underlain by limestone and other soluble rock. The location and rate at which sinkholes form can be affected by man’s activities. Sinkholes are basin-like, funnel shaped, or vertical-sided depressions in the land surface. In general, sinkholes form by the subsidence of unconsolidated materials or soils into voids created by the dissolution of the underlying soluble bedrock.

The rock exposed in a collapsed sinkhole is usually weathered and rounded, but some sinkholes contain freshly broken rock along their steep sides. Freshly broken rock may indicate that the sinkhole has formed by the collapse of a cave (naturally occurring) or a mine (man-made).

Where sinkholes and caves have formed by the dissolution of soluble rock, such as limestone, dolomite, and gypsum, surface water is uncommon and streams may sink into the ground. This type of topography, formed by dissolution, is referred to as karst terrain. In karst terrain, sinkholes are input points where surface water enters the groundwater system.

Location of Sinkholes in Virginia

The VA DMME provides the map (left) showing the location of sinkholes by underlying cause and probability. Quoting:

Sinkholes are most prevalent in karst terrains, but they occur throughout Virginia. Areas which may contain sinkholes:

1. Subsidence/sinkholes are related to mining activity or soil piping;
2. Karst, sinkholes are related to the dissolution of limestone and dolomite (refer to Hubbard, 1983 and 1988, for more detail) or soil piping;
3. Sinkholes are very rare and are related to soil piping;
4. Sinkholes are rare and are related to soil piping or dissolution of sparse carbonate rock;
5. Sinkholes are related to the dissolution of shell concentrations in sand and soil piping.

More information about sinkholes in Virginia, published by the DMME, is at Sinkholes in VA.

Sinkholes in Berezniki Russia - 24-hour Monitoring

In 2012 the New York Times reported that the city of Berezniki, Russia has a severe and sudden sinkhole problem that defies corrective measures and has prompted the dity to staff a 24-hour sink hole watch, using video monitors of Berezniki areas to observe, report, and issue warnings to the city's residents when a sinkhole begins to appear.[5]

The Times reporter, Andrew E.Kramer, who has reported on sinkholes in Russia before [3], explained that extensive and deep (750 - 1,500 feet) potash (fertilizer) mining operations below Berezniki, a city of 154,000 that began as a labor camp, placed mines within walking distance of the original city center. The presence of deep mines below the city combined with lack of application of mine construction safety standards, mine supports that depend on pillars of material that is soluble and thus can collapse during flooding, led to trouble when fresh water began flowing into the mine from a spring in 2006.

The "Grandfather", a Berezinski Russia sinkhole that opened 2007 is 340 x 430 yards in size and 780 feet deep. This sinkhole may, according to Kramer, be the largest manmade sinkhole in the world. It required the evacuation of rougly 2000 people. - The New York Times [5]

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SINKHOLES, WARNING SIGNS

• [1] Thomas V. Cech, Principles of water resources: history, development, management, and policy, John Wiley and Sons 2009 ISBN0470136316, 9780470136317

• [2] 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

• [3] "Russian Child Disappears Into Pipe After Falling Into a Sinkhole With His Mother", Andrew E. Kramer, The New York Times, 10 January 2012, p. A9.
  
  
• [4] Buried oil tank collapse, subsidence, in condominium association meeting minutes, private correspondence, DF with attorney. 02/21/2012.
  
  
• [5] "A City Always on the Watch Against Being Sucked Into the Earth", Andrew E. Kramer, The New York Times, 11 April 2012, p. A10.

Additional reference citations within the above document:

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.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 Florida northeast

17. http://InspectAPedia.com/structure/foundation.htm - 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

Sinkholes and Sudden Land Subsidence References, Products, Consultants

• "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
• "Quebec Family Dies as Home Vanishes Into Crater, in Reminder of Hidden Menace", Ian Austen, New York Times, 13 May 2010 p. A8. See http://www.nytimes.com/
• "Quick Clay", Wikipedia search 5/13/2010 - http://en.wikipedia.org/wiki/Quick_clay
• Florida DEP - Department of Environmental Protection, & Florida Geological survey (http://www.dep.state.fl.us/geology/default.htm) on Florida sinkholes: Effects of Sinkholes on Water Conditions Hernando County, Florida, Brett Buff, GIS in Water Resources, 2008, Dr. David R. Maidment, Photos - Tom Scott, Florida Geographic Survey - Web Search 06/09/2010 - http://www.dep.state.fl.us/geology/geologictopics/jacksonsink.htm
  and - http://www.dep.state.fl.us/geology/geologictopics/sinkhole.htm
  also see
  Lane, Ed, 1986, Karst in Florida: Florida Geological Survey Special Publication 29, 100 p.
  
• Foundation Engineering Problems and Hazards in Karst Terranes, James P. Reger, Maryland Geological Survey, web search 06/05/2010, original source: http://www.mgs.md.gov/esic/fs/fs11.html
  Maryland Geological Survey, 2300 St. Paul Street, Baltimore, MD 21218
• "Frost Heaving Forces in Leda Clay", Penner, E., Division of Building Research, National Research Council of Canada, Canadian Geotechnical Journal, NRC Research Press, 1970-2, Vol 7, No 1, PP 8-16, National Research Council of Canada, Accession number 1970-023601, Quoting from original source

  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.

• "Geoscape Ottowa-Gatineau Landslides", Canada Department of Natural Resources, original source http://geoscape.nrcan.gc.ca/ottawa/landslides_e.php - quoting from that source:

  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.

• Kochanov, W. E., 1999, Sinkholes in Pennsylvania: Pennsylvania
  Geological Survey, 4th ser., Educational Series 11, 33 p., 3rd printing April 2005, Pennsylvania Department of Conservation and Natural Resources / Bureau of Topographic and Geologic Survey, DCNR Educational Series 11, Pennsylvania Geological Survey, Fourth Series, Harrisburg,
  1999 - web search 06/05/2010, original source: http://www.dcnr.state.pa.us/topogeo/hazards/es11.pdf - Quoting from the document introduction:
  The first 18 pages of this booklet contain an explanation of how sinkholes develop. In order to tell the sinkhole story, it is important to discuss a number of related geologic disciplines. The words used to describe sinkholes and these disciplines may be a bit unfamiliar. However, general explanations are given throughout the booklet to help clarify their meanings. Key words are printed in bold type for emphasis. The more important ones are defined in a Glossary that begins on page 29. The remaining sections, starting with “Sinkholes in the Urban Environment” (page 18), deal with sinkholes and their impact on our environment. This includes recognition of subsidence features and sinkhole repair.
• Thanks to reader Y.Z., Whitestone, NY for discussing a back yard collapse 4/1/2010
• [1] 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 [A primary resource for this article
• [2] Center for Cave and Karst Studies or the Kentucky Climate Center, both at Western Kentucky University
• Vanity Fair - web search 06/04/2010 http://www.vanityfair.com/online/daily/2010/06/what-caused-the-guatemala-sinkhole-and-why-is-it-so-round.html
• Sinkholes, [on file as /vision/Sinkholes_Virginia_DME.pdf ] - , Virginia Division of Mineral Resources,
• Virginia Department of Mines, Minerals and Energy, www.dmme.virginia.gov Virginia Department of Mines, Minerals and Energy Division of Mineral Resources 900 Natural Resources Drive, Suite 500 Charlottesville, VA 22903 Sales Office: (434) 951-6341 FAX : (434) 951-6365 Geologic Information: (434) 951-6342 http://www.dmme.virginia.gov/ divisionmineralresources.shtml - Web search 06/09/2010
• Wikipedia - web search 06/04/2010 - http://en.wikipedia.org/wiki/Guatemala_City

Sink Hole & Related Engineering References

• Newton, J. G., 1987, Development of sinkholes resulting from man's activities in the eastern United States: US Geological Survey Circular 968, 54 p.
• Sinclair, W. C., 1982, Sinkhole development resulting from ground-water withdrawal in the Tampa Area, Florida: U.S. Geological Survey Water-Resources Investigations 81-50, 19 p.
• White, W. B., 1988, Geomorphology and Hydrology of Karst Terrains: Oxford University Press, New York, 464 p.
• Williams, J. H. and Vineyard, J. D., 1976, Geologic indicators of subsidence and collapse in karst terrain in Missouri: Presentation at the 55th Annual Meeting, Transportation Research Board, Washington, D.C.
• Barry F. Beck, A. J. (1999). Hydrogeology and Engineering Geology of Sinkholes and Karst. Rotterdam, Netherlands: A. A. Balkema.
• Beck, B. F. (2003). Sinkholes and the Engineering and Environmental Impacts of Karst. Huntsville, Alabama: The American Society of Civil Engineers.
• Beck, B. F. (2005). Sinkholes and the Engineering and Envrionmental Impacts of Karst. San Antonio, Texas: The American Society of Civil Engineers.
• Tony Waltham, F. B. (2005). Sinkholes and Subsidence, Karst and Cavernous Rocks in Engineering and Construction. Chichester, United Kingdom: Praxis Publishing.
• Whitman D., G. T. (1999). Spatial Interrelationships Between Lake Elevations, Water Tables, and Sinkhole Occurence in Central Florida: A GIS Approach. Photogrammetric Engineering and Remote Sensing , 1169-1178.
• Cited References from this article:
• #3. Detecting Sinkholes with Geophysics, Enviroscan, Inc., Lancaster PA 717-396-8922 email@enviroscan.com www.enviroscan.com 2003
• ...

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

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• Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
• Avongard foundation crack progress chart for structural crack monitoring
• Building Pathology, Deterioration, Diagnostics, and Intervention, Samuel Y. Harris, P.E., AIA, Esq., ISBN 0-471-33172-4, John Wiley & Sons, 2001 [General building science-DF] ISBN-10: 0471331724 ISBN-13: 978-0471331728
• Building Pathology: Principles and Practice, David Watt, Wiley-Blackwell; 2 edition (March 7, 2008) ISBN-10: 1405161035 ISBN-13: 978-1405161039
• Diagnosing & Repairing House Structure Problems, Edgar O. Seaquist, McGraw Hill, 1980 ISBN 0-07-056013-7 (obsolete, incomplete, missing most diagnosis steps, but very good reading; out of print but used copies are available at Amazon.com, and reprints are available from some inspection tool suppliers). Ed Seaquist was among the first speakers invited to a series of educational conferences organized by D Friedman for ASHI, the American Society of Home Inspectors, where the topic of inspecting the in-service condition of building structures was first addressed.
• Building Failures, Diagnosis & Avoidance, 2d Ed., W.H. Ransom, E.& F. Spon, New York, 1987 ISBN 0-419-14270-3
• "A Foundation for Unstable Soils," Harris Hyman, P.E., Journal of Light Construction, May 1995
• "Backfilling Basics," Buck Bartley, Journal of Light Construction, October 1994
• "Inspecting Block Foundations," Donald V. Cohen, P.E., ASHI Reporter, December 1998. This article in turn cites the Fine Homebuilding article noted below.
• "When Block Foundations go Bad," Fine Homebuilding, June/July 1998