The Nature of Vision
Inspecting Complex Systems, Why We See or Do Not See Things We Are Looking For - Or Should Be Looking For
VISUAL PERCEPTION ERRORS - CONTENTS: Visual perception errors in building, environmental & forensic investigations, Limits to the visual perception of errors, mistakes, defects, faults, or trouble in physical systems can in turn explain When and Why Inspectors "See" or "Don't See" Things Which are Present. Inspection methods for any complex system - Advanced Home Inspector Education.
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Visual perception errors in building, environmental & forensic investigations: this article discusses errors in visual perception during examination of complex systems and structures such as buildings or mechanical systems.
The purpose of this article is to improve the chances that any inspector of any complex system,
charged with the visual examination of the system will find both the visually apparent and
the visually present but more subtle clues which signal important costly
repairs, unsafe conditions, or other targets of the inspection.
believe that if our eyes are open, we are seeing. Cognitive scientists once
thought our visual perception acted
much like a videotape recorder. We know now that this is not the case.
Perception studies are demonstrating how little people actually see when they
are not paying attention.1.
Surprises in the Study of Vision and Inspection of Buildings or Other Complex Systems
Study in a variety of fields (eye and brain, the psychology of error, the
physics of light and optics, microscopy, and aerobiology) has led to some
surprises and to recognition that forensic experts and inspectors should not be
overconfident in their ability to "see" what they are looking "for." Even the
ability to see what we are looking "at" is demonstrated to be incomplete.
"Inspection" as used here refers to a
primarily-visual approach which may make limited use of special equipment such
as moisture meters or electrical circuit polarity testers, but which does not
involve demolition or similar "invasive" methods to evaluate the condition of a
building or similar complex system and its subsystems such as building
Also see InspectAPedia.com for free in-depth
research on building defect recognition and repair, including both physical
defects on buildings and environmental concerns such as mold and allergens.
Several areas of research can inform forensic inspectors and may improve their ability to recognize, record, and thus act on defects:
Neurology and brain function, particularly with respect to "seeing"
Physics and the study of light, particularly optical physics and the
detection of objects, both macroscopically (by eye) and microscopically (by
Psychology of seeing, or "not seeing," the psychology of errors - why we
make mistakes or fail to recognize something important, and why we do not,
indeed cannot, focus our attention so as to "see" everything that is in
our visual field.
An enormous body of research pertinent to the human ability to consciously see, recognize, and record information exists among these areas. I have not attempted to recap everything about these areas, but I will
cite a few ideas they suggest and which pertain to the use of an inspection
methodology employing techniques to improve recognition of subtle clues of
What is needed in order to "see" any object or object-feature?
Presence: Information or Object Must Be Present
The object or feature must be present in the visual
field of the observer so that all the observer needs to do is direct his or her
eye there to be [potentially] able to see it. The size and other requisite aspects
of the object or feature are discussed below.
For purposes of this discussion
we refer to normal sighted-human vision, unaided by special equipment such as
microscopes or telescopes, though the observer may [indeed should] make use of
corrective eyeglasses or contact lenses if their vision has been found in need
of those aids. In my longer X-ray Vision article my view is that "clues"
which suggest an otherwise "hidden" defect are in fact visual "features" of
that very defect or condition and can so can be recognized.
Light: Light Reflects From Object to Eye
Light reflected from a surface towards
the eye of an observer carries incomplete information about the object and can
be distorted. In any case it will not contain all of even the surface
information about the object due to limitations of illumination, optical
distortion, and of course the inability to "see around corners."
The human eye
is sensitive to light (electromagnetic radiation) from around 350 to 750nm in
wavelength. It is speculated that we evolved to be sensitive to radiation at
these wavelengths because they are so common in sunlight. Some other animals
have eyes which are sensitive to electromagnetic radiation at lower wavelengths
(such as infra red).
In the process of "inspecting" objects and systems in the field, inspectors
use a combination of local natural light, artificial indoor lighting, and
flashlights. The flashlight is a more important tool than some inspectors
realize. the level or intensity of illumination, angle of illumination, and
color of the light source can make the difference between a humans ability to
"see" or not see a feature.
The ultimate level of detection of visual features being illuminated by
light is a function of the wavelength of the light source, since particles
smaller than the wavelength may fail to refract or bend the light waves
sufficiently to permit detection. In forensic microscopy the wavelength of the
light source is a factor but more important is the ability of the microscopes
lens system to direct light rays at the particle being examined. The particle
refracts, or "bends" light rays, producing a visible image which is focused by
the lens system.
Eye: Light Reaches Eye
The eye, the cornea and
lens of the eye itself is the image-forming system for human inspectors working
without a microscope or binoculars.
The image of the observed object is formed
at the back of the eye on the retina where special cells in turn fire neurons
which pass the visual image map to the visual cortex of the brain. The human
eye can resolve particles down to a few hundredths of an inch or so in
Brain: Brain Interprets Information
The brain plays a considerable additional role in "seeing"
beyond mere image mapping. Additional image data processing occurs in the brain
in order to recognize what is being observed.
What a human "sees" is a
representation of the objective world built-up in the brain from a combination
of sensory inputs and significant additional processing in the brain. While I'm
taking some literary license to say so, it would appear most likely that the
brain constructs the "image" in the observers consciousness from a combination
of visual inputs from the eye, a map of the eyes input in the visual cortex,
and processing of that data along with
stored maps of "expected images."
human brains are designed to "make sense" of various sensory inputs, we may do
so even if "making sense" means "making it up." Thus our observations are
vulnerable to illusion.
Inattentional Blindness: Observer (Home Inspector) Must Pay Conscious Attention
In addition to the need for an object to be observed, a light source, the
observers eye and brain to form an image, vision also requires a level of
attention by the observer. The eye and brain are not a video camera faithfully
recording all visual data towards which aimed.
The inspector may fail to see objects even when they
are present and readily apparent in the center of the visual field. Perhaps
less difficult to grasp is research about forms of blindness or an inability to
"see" - change blindness and inattentional blindness
in particular. In the forming conscious visual information in the brain, inattentional
blindness refers to an observers blindness to the presence of an object right
in the visual field, even though the observer is awake and directing her eyes
directly towards it.
Because "seeing" at a conscious level requires both the processing of
visual signals from the eye and a processing of the input by the brain to form
an "image," research has demonstrated that conscious "attention" is absolutely
required to "see" anything. In other words, a person can be "looking" right at
something, but if his attention is not focused there as well, he will not see
it. Perhaps not at all. 
The observer is likely to fail to see something significant, even in the center
of the visual field, if s/he is not looking for that particular thing - if its
not in the observers "attention set."
Inspector learn quickly the importance of a comprehensive visual scan of
everything. An inspector wont "inspect" a room by only looking at one wall.
But now we understand that merely directing the eyes at every surface may be
What's tricky about this is that the inspector believes s/he has
"looked-at" a surface, but may not have "attended" to it.
Photo Manipulation Experiment in Change Blindness in Home Inspection Reports
In a similar experiment I (D Friedman) provided a few clients with home inspection reports that included photographs of the homes we had inspected together. Despite spending three or four hours "looking" at a home, and in most cases despite multiple visits to the home by the clients, not one of the clients observed that the report included multiple inconsistent photographs of the front of the home.
In the first photograph I used photo-retouching software to add or delete windows from the front of the home. In the second photograph later in the report I used an un-edited photograph that accurately described the number of windows. Only when I asked clients to carefully compare the two photographs side by side did they report the difference in the number of windows presented.
The information that enters our eyes and is transmitted to our brains is not necessarily identical to the data that enters our consciousness. Experts explain that change blindness occurs at least in part because more information falls on our eyes than our brain can efficiently process. In other words, our brains discard much of the visual information that is available in order to avoid brain overload.
Keeping change blindness in mind and developing strategies for coping with this source of inaccuracy in visual awareness can help any inspector perform better.
Kristins Cards: A Demonstration of Inattentional Blindness
The ease with which IB occurs is nicely
demonstrated by Kristins Cards at http://www.cs.bris.ac.uk/~cater/PhD/Magic/cardtrick.html -- UPDTATE: regrets, this website is obsolete and I have not found a current version online. I'm working on reproducing this exhibition.
I previously left the explanation of the viewers inability to "see" something
important to Kristen. Because that online display has been lost, below I will explain this illusion of inattentional blindness:
This "card trick" shows a set of cards, say by projection onto a movie screen, and invites the audience to remember them, and to select ONE of the cards to pay attention to. "Pick your card and watch it" is more or less the instruction.
Then this illusion changes and displays a "new" set of cards. The audience is asked if the card they were concentrating on has "disappeared". Everyone will pretty much say yes, their card has vanished.
What most audiences fail to detect and report is that all of the cards have changed between the two displays.
Church points out that while this is an entertaining illustration, Kristin's Cards may not be the best example of inattentional blindness because we are directing the audience to attend a particular card. Good inattentional blindness examples demonstrate that the observer(s) fail to notice changes, even substantial changes, in two successive images of the "same" scene.
Factors Affecting Inattentional Blindness
"Seeing" involves conscious mental effort
and focus, a task from which observers can be distracted or which can fail due
to IB. Four factors affect
inattentional blindness: conspicuity [both sensory and cognitive], mental
workload [too much or too little], expectation [attention set], and capacity.
An inspector is more likely to see what
s/he is looking for, and outside of that set, to see that which is
familiar or large.
Home inspectors are not "seeing"
everything that is present, even when its right in the center of their field
An inspector is less likely to see
something that s/he is not expecting, even if it is
important and visually distinct by characteristics such as shape, color, or
An inspectors "attention set" of
expectations filters out other stimuli. An inspector is less likely to
"see" a defect in a structure or system being examined if s/he does not already
have an in-depth and visceral understanding of the construction and operation
of the structure or system, leading to a broad and deep "attention set."
Change Blindness and its Effects on the Inspection of Complex Systems
Researchers in this area describes "change blindness" - the inability to "see"
a component change between two rapid presentations of an image, and "inattention
blindness [IB]which describes an inability to register and use
visual information if attention is not properly focused. "Inattention
blindness" is most pertinent to inspectors and recalls earlier work on the
psychology of errors, and capture errors.
An interesting article on Change Blindness appeared in the New York Times reported on a talk by Jeremy Wolfe (Harvard Medical School, May 2008). Wolfe addressed a symposium, (held at the Italian academy for Advanced studies in America, at Columbia University) on the relationship between the themes of Art and Neuroscience, and he explained how often people "see" inaccurately. For inspectors of complex systems (home inspectors or even more lofty space shuttle inspectors) the significance of Wolfe's report was the underscoring of the "frequent inability of our visual system to detect alterations to something staring us straight in the face."
This visual error occurs not only in small subtle details but also in significant items. Angier's article reports that the audience failed to notice entire stories disappearing from buildings!
The Natalie Angier's article and Jeremy Wolfe's presentation emphasize that change blindness is an important component of the list of caveats that face any inspector who needs to form an rapid, accurate impression of the system she is inspecting.
Wolfe defines two classes of brain information processing that control our conscious attention to details among the soup of items that appear in the visual field:
Bottom-up attentiveness: something grabs our attention, such as a moving object or a bright color against a different background. These items are usually noticed consciously.
Top-down attentiveness: the viewer makes a conscious act to attend to something in the visual field even though it is not contrasted by conditions such as color or movement.
-- "Blind to Change, Even as It Stares Us in the Face", Natalie Angier, New York Times, p. F2, 1 April 2008
Improving Vision: Tuning Up Your Ability to See During an Inspection
Improving Top-Down Attentiveness to Items in the Visual Field
In addition to taking advantage of an understanding of the mechanisms that our eyes and brain use to record and attend information (bottom-up attentiveness and top-down attentiveness for example) there is another step we can take to improve our top-down attentiveness:
Better training in content or significance recognition: It is possible that we can train ourselves to better manage what information our brains discard and what information our brains bring to consciousness by tuning our attention to details that pertain to specific needs, and by broadening our training in specific fields of diagnostic inspection so that more visual data leaps to our attention because we have learned that that data may be important for the task at hand.
Better training in attention management: because research shows (and Wolfe reports) that the human brain can only attend well to a limited number of items at a time, even simplistic mechanical or rote inspection procedures that force the inspector to spend a portion of his/her time attending to specific questions might improve inspection performance. For example, let's not just "inspect the heating system" rather, let's "look for evidence that the chimney may be blocked, having been trained to identify a larger set of clues that can indicate that condition. (Back pressure burns on the boiler, odors, soot, stains in the living space, occupant health complaints, etc.)
A Checklist of Coping Strategies to Improve Visual Attention during Inspections
An inspector can improve his/her ability to see. Simply realizing this fact opens the way to improvement in inspection acuity and completeness.
Manage distractions as a cause of
inattentional blindness: use coping strategies (be in control of the
inspection, use both methodical and routine-breaking scanning)
Use but limit object fixation, a cause of inattentional blindness common to inspectors [take an observation
to completion and redirect attention to a new scan]
Expand the "attention set" [know how its constructed, how it works etc.] but
recognize that training has the danger of masking off the unexpected.
Inattentional blindness vanishes if the observer expects the item.
Don't assume you know it all: Minimize the danger of "being an expert"
having a too-fixed set of expectations and being unable to "see" new or
changed inputs occurring on old, familiar systems)
Study the proper use of light (level of
illumination, angle of illumination, reflection, resolution, color) to maximize
the ability to "see"
"Keeping fresh" as an experienced visual-inspector of complex systems
Study new topics in new fields: forensic engineering, microscopy, yoga,
Zen, [insight and attitudes about "paying attention" ]
Inspect with another professional, comparing approaches to field work
Invite client [or another third party] participation, inviting client
questions during the process; the client, even as a non-expert, may notice and question clues that lead to something the inspector has not "seen"; even extraneous questions "off topic" can prevent an inspector from becoming too routinized in the inspection procedure
Look for the "surprise" that is present at every building or system
Vary the inspection routine, sequence, or other methods and tools
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"Blind to Change, Even as It Stares Us in the Face", Natalie Angier, New York Times, p. F2, 1 April 2008
References: Vision, Seeing, and Inattentional Blindness
"A Neurobiologists Notebook: The Minds Eye, what the blind see," Oliver
Sacks, New Yorker Magazine, July 28, 2003 p. 46-59 [Three different
responses to "blindness," increased-detailed visual mapping in some "blind"
 Ward points out [Reference 2] that "Most people
believe that if our eyes are open, we are seeing. Cognitive scientists once
thought our visual perception acted much like a videotape recorder. We know
now that this is not the case. perception studies are demonstrating how
little people actually see when they are not paying attention. Inattentional
blindness refers to a situation in which a stimulus that is not attended is
not perceived, even though a person is looking directly at it.
Mack and Rock
found that a puzzling and surprising aspect of all the experiments examining
the perception of a small number of critical stimuli under conditions of
inattention was that, on average, 25% of the observers failed to detect their
presence"" leading to a paradox that "in order to see something with any detail
in the environment, observers must first direct their attention towards an
object. However if something is not yet perceived, how can observers direct
their attention towards it?" Part of
the answer lies in the observation that we perceive more than we notice.
Driving is an example. So we do perceive environmental stimuli outside of our
awareness. The paradox falls apart when one distinguishes between conscious,
attended stimuli and unconscious, unattended stimuli.
determine which environmental stimuli are attended and thus consciously
perceived? Conspicuity. "Blindness" to visual stimuli may occur, then because
they lack conspicuity. But such blindness can also occur when the observer is
focusing on something else, (distraction or a capture error) even though the
stimuli are right there in the center of his/her field of vision! Such
blindness can also occur from too little mental activity, such as performing a
task by rote.
An observer can be blind to even conspicuous stimuli if s/he is
not expecting it. Navy pilots in training failed to see a large airplane set on
the carrier deck in the landing zone just before touchdown. "When an observer
has an attentional set for an object or for certain characteristics of objects,
only things in the attentional set will capture their attention when presented
in the perceptual field." Similarly, observers were more likely to notice an
unexpected object if it was more similar to the stimuli they were currently
Other references for inattentional blindness
Stanislas Dehaene and Jean-Pierre Changeux. Ongoing spontaneous activity controls access to consciousness: a neuronal model for inattentional blindness.. PLoS Biol, 3(5):e141, May 2005.
Abstract: Even in the absence of sensory inputs, cortical and thalamic neurons can show structured patterns of ongoing spontaneous activity, whose origins and functional significance are not well understood. We use computer simulations to explore the conditions under which spontaneous activity emerges from a simplified model of multiple interconnected thalamocortical columns linked by long-range, top-down excitatory axons, and to examine its interactions with stimulus-induced activation. Simulations help characterize two main states of activity. First, spontaneous gamma-band oscillations emerge at a precise threshold controlled by ascending neuromodulator systems. Second, within a spontaneously active network, we observe the sudden "ignition" of one out of many possible coherent states of high-level activity amidst cortical neurons with long-distance projections. During such an ignited state, spontaneous activity can block external sensory processing. We relate those properties to experimental observations on the neural bases of endogenous states of consciousness, and particularly the blocking of access to consciousness that occurs in the psychophysical phenomenon of "inattentional blindness," in which normal subjects intensely engaged in mental activity fail to notice salient but irrelevant sensory stimuli. Although highly simplified, the generic properties of a minimal network may help clarify some of the basic cerebral phenomena underlying the autonomy of consciousness.
Lionel Naccache, Stanislas Dehaene, Laurent Cohen, Marie-Odile Habert, Elodie Guichart-Gomez, Damien Galanaud, and Jean-Claude Willer. Effortless control: executive attention and conscious feeling of mental effort are dissociable.. Neuropsychologia, 43(9):1318-1328, 2005.
Abstract: Recruitment of executive attention is normally associated to a subjective feeling of mental effort. Here we investigate the nature of this coupling in a patient with a left mesio-frontal cortex lesion including the anterior cingulate cortex (ACC), and in a group of comparison subjects using a Stroop paradigm. We show that in normal subjects, subjective increases in effort associated with executive control correlate with higher skin-conductance responses (SCRs). However, our patient experienced no conscious feeling of mental effort and showed no SCR, in spite of exhibiting normal executive control, and residual right anterior cingulate activity measured with event-related potentials (ERPs). Finally, this patient demonstrated a pattern of impaired behavior and SCRs in the Iowa gambling task-elaborated by Damasio, Bechara and colleagues-replicating the findings reported by these authors for other patients with mesio-frontal lesions. Taken together, these results call for a theoretical refinement by revealing a decoupling between conscious cognitive control and consciously reportable feelings. Moreover, they reveal a fundamental distinction, observed here within the same patient, between the cognitive operations which are depending on normal somatic marker processing, and those which are withstanding to impairments of this system.
Claire Sergent, Sylvain Baillet, and Stanislas Dehaene. Timing of the brain events underlying access to consciousness during the attentional blink.. Nat Neurosci, September 2005.
Abstract: In the phenomenon of attentional blink, identical visual stimuli are sometimes fully perceived and sometimes not detected at all. This phenomenon thus provides an optimal situation to study the fate of stimuli not consciously perceived and the differences between conscious and nonconscious processing. We correlated behavioral visibility ratings and recordings of event-related potentials to study the temporal dynamics of access to consciousness. Intact early potentials (P1 and N1) were evoked by unseen words, suggesting that these brain events are not the primary correlates of conscious perception. However, we observed a rapid divergence around 270 ms, after which several brain events were evoked solely by seen words. Thus, we suggest that the transition toward access to consciousness relates to the optional triggering of a late wave of activation that spreads through a distributed network of cortical association areas.
Mariano Sigman, Hong Pan, Yihong Yan, Emily Stern, David Silbersweig, and Charles Gilbert. Top-Down reorganization of activity in the visual pathway after learning a shape identification task.. Neuron, 46(5):823-835, 2005. [PDF] [bibtex-entry]
Stanislas Dehaene, Jean-René Duhamel, Marc D. Hauser, and Giacomo Rizzolatti. From Monkey Brain to Human Brain. A Fyssen Foundation Symposium.. MIT Press, 2005
Claire Sergent. Dynamique de l'accès à la conscience : caractérisation comportementale et bases neurales de l'accès à la conscience lors du clignement attentionnel (attentional blink). Thesis/Dissertation, Université Paris XI, 2005.
Valérie Ventureyra. À la recherche de la langue perdue: étude psycholinguistique de l'attrition de la première langue chez des cor'eens adoptés en France. Thesis/Dissertation, École des hautes études en sciences sociales, Paris, January 2005.
SCSMI: Society for Cognitive Studies of the Moving Image. Kristin Thompson, Observations on film art
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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.
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.
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.
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
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.
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
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
 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: email@example.com [A primary resource for this article
 Center for Cave and Karst Studies or the
Center, both at
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
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.
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