Can "X-Ray Vision" (Advanced Visual Inspection Methodology) Indicate Imminent or History of Sinkholes at a Home Inspection in Florida or Elsewhere
InspectAPedia^® -

What are sinkholes?
What causes sinkholes? Can a sinkhole be predicted?
How do sink holes affect buildings?

This article discusses how to recognize that a sinkhole may be imminent or starting at a property. It also summarizes 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.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers nor with topics or services discussed at this website.

Most sinkholes are 10 to 12 feet in diameter. Sinkholes hundreds of feet in diameter have occurred in Florida - big enough to swallow a home. 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 2012 InspectAPedia.com, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website.

Visual Inspection to Detect Sinkholes in Florida

Daniel Friedman - Florida Suncoast ASHI Educational Seminar - 1 May 2004

Portions of this text are extracted, quoted, or paraphrased from references provided; a key resource author was Sarah Cervone at Reference-1. Updated 2004, 2005, 2010 – This document is SinkholesFL.doc at InspectAPedia.com/vision/sinkholes.htm - © 2004 - 2010 All Rights Reserved. Also 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,".
X-Ray Vision.doc =InspectAPedia.com/structure/xray.html

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. Further review comments and content suggestions are welcome to Author for content suggestions, and more information on building inspection are at More Information below.

FLORIDA HAS MORE SINKHOLES than any other state among the United States. They are an obvious feature of Florida's natural karst topography.

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
Stoping 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

Sinkholes and the Aquifer[4]

[Excerpting from References 1 and 2]

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

Four types of sinkholes, all beginning with a "solution cavity"[5]

1. Solution – surface depressions, not complete collapse

2. Cover-subsidence – loose, overlying sand slides into solution cavity

3. Collapse – roof of an underground channel suddenly collapses, forming a steep-sided cavity

4. Cover-collapse – 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

Three general types occur in Florida: collapse, solution, and subsidence[6]

Collpase sinkholes

Source: USGS

Collapse sinkholes

. most common type 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

Subsidence sinkhole

Source: USGS

. overburden is thin.

. form slowly

. dissolving limestone is replaced by sand granules that fall into the depression and fill the holes

. concave depression

. only a few feet in diameter and depth (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

Sinkholes and Lake Formation

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])
An actual sinkhole is present on or near the subject property (duh!)
Slumping or falling fence posts, trees, foundations on or near the property
Previously-buried fence posts, 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.
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
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

Discolored well water
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]
References:

1. [primary resource] 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

2. Center for Cave and Karst Studies or the Kentucky Climate Center, both at Western Kentucky University.

3. "Detecting Sinkholes with Geophysics," Enviroscan, Inc., Lancaster PA 717-396-8922 email@enviroscan.com www.enviroscan.com 2003

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.htm SW Florida Sinkhole Information

14. http://www.dep.state.fl.us/geology/gisdatamaps/index.htm Sink 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

[1] 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

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

HTML Comment Box is loading comments...

Recommend / Share this Article

...

Technical Reviewers & References

InspectAPedia.com^® - Daniel Friedman - Publisher & Editor.
InspectAPedia Bookstore lists recommended books, organized by topic & available for purchase. Most of our articles also include a list of recommended books for the specific article topic as well as other references, and information sources.
Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
Additional technical contributors & reference sources for this article are listed below.

Use links just below or 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.

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

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop, and from the InspectAPedia bookstore. The 2010 edition of the Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

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.
Decks and Porches, the JLC Guide to, Best Practices for Outdoor Spaces, Steve Bliss (Editor), The Journal of Light Construction, Williston VT, 2010 ISBN 10: 1-928580-42-4, ISBN 13: 978-1-928580-42-3, available from Amazon.com
The Journal of Light Construction has generously given reprint permission to InspectAPedia.com for certain articles found at this website. All rights and contents to those materials are ©Journal of Light Construction and may not be reproduced in any form.
Appliances and Home Electronics, - energy savings, U.S. Department of Energy
Avongard foundation crack progress chart for structural crack monitoring
Basement Moisture Control, U.S. Department of Energy
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
The Circular Staircase, Mary Roberts Rinehart
Construction Drawings and Details, Rosemary Kilmer
Crawl Space Moisture Control, U.S. Department of Energy
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.
Design of Wood Structures - ASD, Donald E. Breyer, Kenneth Fridley, Kelly Cob een, David Pollock, McGraw Hill, 2003, ISBN-10: 0071379320, ISBN-13: 978-0071379328
This book is an update of a long-established text dating from at least 1988 (DJF); Quoting:
This book is gives a good grasp of seismic design for wood structures. Many of the examples especially near the end are good practice for the Californa PE Special Seismic Exam design questions. It gives a good grasp of how seismic forces move through a building and how to calculate those forces at various locations. THE CLASSIC TEXT ON WOOD DESIGN UPDATED TO INCLUDE THE LATEST CODES AND DATA. Reflects the most recent provisions of the 2003 International Building Code and 2001 National Design Specification for Wood Construction. Continuing the sterling standard set by earlier editions, this indispensable reference clearly explains the best wood design techniques for the safe handling of gravity and lateral loads. Carefully revised and updated to include the new 2003 International Building Code, ASCE 7-02 Minimum Design Loads for Buildings and Other Structures, the 2001 National Design Specification for Wood Construction, and the most recent Allowable Stress Design.
Building Failures, Diagnosis & Avoidance, 2d Ed., W.H. Ransom, E.& F. Spon, New York, 1987 ISBN 0-419-14270-3
Domestic Building Surveys, Andrew R. Williams, Kindle book, Amazon.com
Defects and Deterioration in Buildings: A Practical Guide to the Science and Technology of Material Failure, Barry Richardson, Spon Press; 2d Ed (2001), ISBN-10: 041925210X, ISBN-13: 978-0419252108. Quoting:
A professional reference designed to assist surveyors, engineers, architects and contractors in diagnosing existing problems and avoiding them in new buildings. Fully revised and updated, this edition, in new clearer format, covers developments in building defects, and problems such as sick building syndrome. Well liked for its mixture of theory and practice the new edition will complement Hinks and Cook's student textbook on defects at the practitioner level.
Guide to Domestic Building Surveys, Jack Bower, Butterworth Architecture, London, 1988, ISBN 0-408-50000 X
"Avoiding Foundation Failures," Robert Marshall, Journal of Light Construction, July, 1996 (Highly recommend this article-DF)
"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
Energy Recover Ventilation Systems for Buildings, U.S. Department of Energy
Energy Savings Methods: Whole House Systems Approach, U.S. Department of Energy
Historic Preservation Technology: A Primer, Robert A. Young, Wiley (March 21, 2008) ISBN-10: 0471788368 ISBN-13: 978-0471788362
Log Homes: Minimizing Air Leakage in Log Homes, U.S. Department of Energy
Log Homes: Controlling Moisture in Log Homes, U.S. Department of Energy
Log Homes: Log Home Design, U.S. Department of Energy
Moisture Control in Buildings, U.S. Department of Energy
Moisture Control in Walls, U.S. Department of Energy
Quality Standards for the Professional Remodeling Industry, National Association of Home Builders Remodelers Council, NAHB Research Foundation, 1987.
Quality Standards for the Professional Remodeler, N.U. Ahmed, # Home Builder Pr (February 1991), ISBN-10: 0867183594, ISBN-13: 978-0867183597
Natural Ventilation for Buildings, U.S. Department of Energy
R-Value of Wood, U.S. Department of Energy
Spot Ventilation for houses, U.S. Department of Energy
Slab on Grade Foundation Moisture and Air Leakage, U.S. Department of Energy
Straw Bale Home Design, U.S. Department of Energy provides information on strawbale home construction - original source at http://www.energysavers.gov/your_home/designing_remodeling/index.cfm/mytopic=10350
More Straw Bale Building: A Complete Guide to Designing and Building with Straw (Mother Earth News Wiser Living Series), Chris Magwood, Peter Mack, New Society Publishers (February 1, 2005), ISBN-10: 0865715181 ISBN-13: 978-0865715189 - Quoting:
Straw bale houses are easy to build, affordable, super energy efficient, environmentally friendly, attractive, and can be designed to match the builder’s personal space needs, esthetics and budget. Despite mushrooming interest in the technique, however, most straw bale books focus on “selling” the dream of straw bale building, but don’t adequately address the most critical issues faced by bale house builders. Moreover, since many developments in this field are recent, few books are completely up to date with the latest techniques.
More Straw Bale Building is designed to fill this gap. A completely rewritten edition of the 20,000-copy best--selling original, it leads the potential builder through the entire process of building a bale structure, tackling all the practical issues: finding and choosing bales; developing sound building plans; roofing; electrical, plumbing, and heating systems; building code compliance; and special concerns for builders in northern climates.
"Vapor Barriers or Vapor Diffusion Retarders", U.S. DOE: how vapor barriers work, types of vapor diffusion barriers, installing vapor barrier
Ventilation for energy efficient buildings, Purpose, Strategies, etc.,
Weather-Resistive Barriers, how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
Whole House Ventilation Systems, U.S. Department of Energy
Whole-House Balanced Ventilation Systems, U.S. Department of Energy
Whole-House Exhaust Ventilation Systems, U.S. Department of Energy
Whole-House Supply Ventilation Systems, U.S. Department of Energy
...

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

Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair

Ask a Question or Search InspectAPedia

Visual Inspection to Detect Sinkholes in Florida

Sinkholes and the Aquifer[4]

Three general types occur in Florida: collapse, solution, and subsidence[6]

Subsidence sinkholes

Sinkholes and Lake Formation

Sinkholes and Urban Development

Sinkhole Detection and Warning Signs

Visual Indicators of Extra Risk of Sinkhole Formation

Visual Indicators of Extra Risk of Sinkhole Formation (continued)

Building and Water Supply Observations

Temporal Sinkhole Triggers

Engineering Methods for Detecting Sinkholes:

What to do if a Sinkhole is observed or suspected at a property during a home inspection

Sinkhole Warning – How Much Time Do You Have ?

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

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

Sinkholes and Sudden Land Subsidence References, Products, Consultants

Sink Hole & Related Engineering References