Detection & Warning Signs of Imminent Sinkhole Collapse

Visual & Other Clues Indicating the Risk of Developing Sinkholes

How big are sinkholes?

Most sinkholes are 10 to 12 feet in diameter. However as you can see from our photos above some are enormously larger.

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.

Watch Out : excavation or soil subsidence near a building can lead to a dangerous foundation collapse. Here are some other collapsing ground hazards:

• Collapsing cesspools - very dangerous - see CESSPOOL SAFETY WARNINGS
• Dug wells with unsafe covers - very dangerous
• Collapses and soil subsidence over buried trees and stumps - potentially very dangerous, especially to children and pets
• Yard collapses over abandoned drywells, cesspools, septic tanks - very dangerous -
  
  See SEPTIC & CESSPOOL SAFETY.
• Buried swimming pools, vehicles, even old garbage dumps that were later built-over (such as in Hyde Park, New York)
• Abandoned, buried building foundations not completely demolished and filled-in
• Cracks, gaps, ravines opened by earthquakes
• Risk of mudslides and collapsed homes in steep slope un-stable areas such as some areas in California

Note on cesspool collapse hazard - question about sudden back yard cave in

Earlier this evening, after a day of rainfall, our backyard caved in.

Currently there is a hole in the ground about 12x10ft and 6-7 feet in depth.

After the initial collapse, there was some growth in the diameter but that appears to have stabilized. The closest edge is about 6-7 feet away from the actual house.

[Photo at left shows the New York "sinkhole".]

See Sinkholes in New York for details of this case and our advice on what to do about this sudden yard collapse in New York.

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.

Real 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 or 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.
• Broken water mains or sewer mains can lead to street, sidewalk, and property sinkholes and sudden, very dangerous subsidences

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]
• Records of sewer or water main breaks

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

• 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 leading to a sudden sink hole collapse can develop over a period of a few minutes to a few hours. [13] or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

See Friedman, Daniel, "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, https://InspectAPedia.com/home_inspection/Building_Inspection_Techniques.php

Also see Friedman, Daniel, The Nature of Vision - Inspecting Complex Systems - When and Why Inspectors "See" or "Don't See" Things Which are Present - InspectApedia.com/vision/Visual_Perception_Errors.php. Comments and content suggestions are invited.