This article describes sinkholes caused by Quick clay or Leda clay hazards in Quebec & Ontario and also discusses the relationship between sinkholes and clay soils causing landslides, soil subsidences, or sudden sinkholes in Canada, Norway, Sweden.
This article series 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.
Synonyms and similar terms for sink holes include: shake hole, swallow hole, swallet, doline, cenote, moulin, and glacier mill, unstable clay soil, Leda clay, or quick clay.
The Lemieux landslide photo (left) of an earlier unstable clay soil or quick clay landslide is from the Canadian Department of Natural Resources.
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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).
Readers who find evidence of an active sinkhole should see SINKHOLES - IMMEDIATE SAFETY ACTIONS.
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.
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 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.
Skölda et als. reported on the chemistry of unstable clay soils in 2005. In 1950 in Surte, Southwest Sweden, unstable "quick clay" soils led to a catastrophic soil collapse as well. Our photo (left) of the 1950 landslide in Surte is from Wikimedia Commons.
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.
According to Canada's Department of Natural Resources, "The most disastrous Leda clay landslide in eastern Canada occurred in 1908 at Notre-Dame-de-la-Salette, Quebec, with the loss of 33 lives."
This "quick clay" or "Leda clay" found in Quebec and Eastern Ontario is a unique marine clay that can sudden liquefy when disturbed. Quick clay / Leda clay may appear to be solid ground, but it is composed of as much as 80 percent water whose clay particles are held together primarily by the surface tension of water itself.
As for some of the other types and locations of sinkholes discussed here, the presence of un-stable quick clay in Eastern Canada can be detected by soil testing but not by casual inspection of the top layer of (more stable) ground soils. However the long history of more than 100 years of documented sudden subsidences in areas of Quebec has made local experts aware of the risk of this dangerous soil.
Similar unstable clay soils or quick clay found in Rissa, Norway, led to an April 1978 soil collapse covering more than 330,000 squre meters.
Significant quick clay or Leda clay collapses in Eastern Canada have been documented in 1908, 1955, 1971, [and April 1978 in Rissa, Norway, 330,000 sq. meters] as well as soil tests (and town relocations) in 1989, 1991, 1993, and the 2010 catastrophe and deaths reported above.
The map (above left, from Canada's Natural Resources department in Ottowa) shows the locations of landslides due to Leda clay deposited when the Champlain Sea retreated to its present size, and the blue area on the map shows the maximum extent to which the Champlain Sea previously extended into Eastern Canada.
Part of the explanation underlying the different character of quick clay (Leda clay) in this area of Canada, referred to as marine clay, is the presence of salt (from sea water) that provides sea-salt ions of NaCl acting as an adhesive between the clay particles.
If only the salt were present, the marine clay formed by this process would be quite stable, as it is elsewhere in the world.
The illustration (left, from Canada Natural Resources), shows the "anatomy of a Leda Clay landslide".
But following the retreat of the last glaciers in this area (roughly 10,000 years ago) rainwater in this area (possibly very low in mineral content), perhaps combined with a high silt content of the clay that allowed rainwater to penetrate to the clay layer, resulted in an un-stable clay soil chemistry. -- Wikipedia
Wikipedia adds that "These landslides are progressive, meaning they usually start at a river, and progress upwards at slow walking speed. They have been known to penetrate kilometers inland, and consume everything in their path."
It is not difficult to understand that a soil relying on water's surface tension can easily become unstable in response to even the smallest shock or by a larger one such as an earthquake, even a distant one. The disturbed clay changes form to a watery gel, losing its previously (and false) stable soil characteristic.
Watch out: Readers trying to diagnose and deal with sudden soil subsidence or yard collapses should
see SINKHOLES - IMMEDIATE SAFETY ACTIONS.
Companies identify themselves as sinkhole damage repair experts in Floria are listed
at SINKHOLE DAMAGE REPAIRS or select a topic from the closely-related articles below, or see the
complete ARTICLE INDEX.
(Oct 30, 2012) Felipe said:
Why are quickclays in quebec more unstable now than when they formed?
Changes in water content are probably a factor. See Larson (2002) and Smalley (1976) cited below. Other researches are evaluating effects of climate change on quick clay stability.
...
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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
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.
Quick clay, a soil that changes from normal firm ground to a liquid mass when it is disturbed, has been involved in most of the large and serious clay slides in Sweden, Norway and Canada. The location, time of occurrence and size of quick clay slides are difficult to predict and large slides may cause great devastation.
Some geochemical studies of Swedish quick clay were done in the 1960s and early 1970s, but no systematic studies of the interrelationships of pore water chemistry, mineralogy, geotechnical properties and other parameters on quick clays in Sweden have been published.
Such studies are of national and general interest because of the many combinations of rock flour source areas and sedimentation conditions that occurred across central Sweden and into the Baltic Sea area during deglaciation.
In this study, geotechnical properties related to the in situ chemistry at one quick clay site were extensively studied, and spot sampling was conducted at two other locations in Southwest Sweden. In this area the clay minerals mainly are non-expanding phyllosilicate minerals (illite) and primary minerals (quartz, feldspar), which is consistent with previous studies of quick clay mineralogy.
Extensive leaching has occurred at all three locations. At the extensively studied site, Surte, the lowest salinity was found at the greatest depth, inferring that the leaching by fresh water was accomplished by water movement upward and laterally through the sediment from the underlying bedrock.
This is consistent with the local setting where bedrock hills rise sharply to over 100 m above the marine sediment surface. An artesian pressure would also be anticipated at this location.
There is a correlation (negative) between sensitivity and salinity but there is an indication that the maximum salinity or electrical conductivity consistent with the quick clay behaviour is higher than reported elsewhere. However, for high sensitivities the salinity is about the same as reported elsewhere. In the deepest part of the borehole, there is a higher content of Fe and Al in the pore water, indicating reduced state.
Further work is needed to confirm the difference in salinity and to investigate the possible interplay of salinity and potential dispersing agents such as the role of anoxic conditions, in this region. F
urther work is especially needed in the locations where the sediment accumulation occurred under lower salinity conditions. At all three locations, high remoulded shear strength and low sensitivity have been seen near the surface together with a decrease in pore water cation concentrations.