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This article describes the visual signs that a sinkhole is likely or is already developing in an area: sinkhole warning signs. We also discuss 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.
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- Daniel Friedman - Florida Suncoast ASHI Educational Seminar - 1 May 2004, updated 2007, 2008, 2014
Portions of this text are extracted, quoted, or paraphrased from references provided; a key resource author was Sarah Cervone at Reference-1.
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.
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.
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 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 possible, probable, or an imminent risk. Synonyms and similar terms for sink holes include: shake hole, swallow hole, swallet, doline, cenote, moulin, and glacier mill.
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.
Is it a sinkhole or some other kind of soil collapse? Does a soil collapse threaten nearby buildings? Is a soil collapse dangerous?
Examples of other possible buried components that can lead to sudden or gradual soil subsidence, apparent sink holes, and collapses are given below. These, too, can be very dangerous, as we describe in the case of collapsing cesspools or even buried trees and stumps. But their probable impact on nearby structures may be less than from a classic sinkhole provided the collapse is not already close - within 50 feet - of a building.
Be careful: excavation or soil subsidence near a building can lead to a dangerous foundation collapse. Here are some other collapsing ground hazards:
Note on cesspool collapse hazard - question about sudden back yard cave in
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.
Although a sinkhole can form without warning, specific signs can signal potential development: 
Site and Neighborhood Observations - ordered from general-area to site-specific to property-specific
This constitutes an immediate potential safety concern. ASHI Standards require you make appropriate notifications.
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 leading to a sudden sink hole collapse can develop over a period of a few minutes to a few hours.  or select a topic from the More Reading links shown below.
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