Quick clay, also known as Leda clay and Champlain Sea clay in Canada, is any of several distinctively sensitive glaciomarine clays found in Canada, Norway, Russia, Sweden, Finland, the United States and other locations around the world. The clay is so unstable that when a mass of quick clay is subjected to sufficient stress, the material behavior may change from that of a particulate material to that of a fluid.
Quick clay has a remolded strength which is much less than its strength upon initial loading. This is caused by its highly unstable clay particle structure.
Quick clay is typically originally deposited in a marine environment. In that environment, the positive charge of cations (such as Na2+) was able to bind clay particles with negative surface charge (typically silicates SOn-n) by balancing charge in the double layer. When the clay became uplifted and was no longer subjected to salt water conditions, rainwater infiltrated these clays and washed away the salts that allowed these clay particles to remain in a stable structure.
With shear stress, the lack of counterbalancing charge from salts in the quick clay results in clay particle repulsion and realignment of clay particles to a structure that is extremely weak and unstable. Quick clay regains strength rapidly, however, when salt is added, which allows clay particles to form complexes with one another.
Quick clay is found only in northern countries, such as Russia, Canada, Norway, Sweden, and Finland, and in Alaska, United States since they were glaciated during the Pleistocene epoch. In Canada, the clay is associated primarily with the Pleistocene-era Champlain Sea, in the modern Ottawa Valley, the St. Lawrence Valley, and the Saguenay River regions.
Quick clay has been the underlying cause of many deadly landslides. In Canada alone, it has been associated with more than 250 mapped landslides. Some of these are ancient, and may have been triggered by earthquakes.
Formation of quick clay
At the height of the past glaciation (about 20,000 years ago), the land was 'pushed' down by the weight of the ice (isostatic depression). All of the ground-up rock was deposited in the surrounding ocean, which had penetrated significantly inland. The loose deposition of the silt and clay particles in the marine environment, allowed an unusual flocculation to take place. Essentially, this formed a strongly bonded soil skeleton, which was 'glued' by highly mobile sea-salt ions.
At this point, there was only the formation of very strong marine clay, which is found all over the world and highly stable, but with its own unique geotechnical problems. When the glaciers retreated, the land mass rose (post-glacial rebound), the clay was exposed, and formed the soil mass for new vegetation. The rainwater in these northern countries was quite aggressive to these clays, perhaps because it was softer (containing less calcium), or the higher silt content allowed more rainwater and snowmelt to penetrate. The final result was that the ionic 'glue' of the clay was weakened, to give a weak, loose soil skeleton, enclosing significant amounts of water (high sensitivity with high moisture content).
Quick clay deposits are rarely located directly at the ground surface, but are typically covered by a normal layer of topsoil. While this topsoil can absorb most normal stresses, such as normal rainfall or a modest earth tremor, a shock that exceeds the capacity of the topsoil layer — such as a larger earthquake, or an abnormal rainfall which leaves the topsoil fully saturated so that additional water has nowhere to permeate except into the clay — can disturb the clay and initiate the process of liquefaction.
Because the clay layer is typically covered with topsoil, a location which is vulnerable to a quick clay landslide is usually identifiable only by soil testing, and is rarely obvious to a casual observer. Thus human settlements and transportation links have often been built on or near clay deposits, resulting in a number of notable catastrophes:
- In 1702, a landslide destroyed almost all traces of the medieval town Sarpsborg. 15 people and 200 animals were killed.
- On 19 May 1893, a landslide in Verdal, Norway, killed 116 people and destroyed 105 farms. It left a crater several kilometers in diameter.
- The most disastrous such landslide to affect North America occurred in 1908, when a slide into the frozen Du Lièvre River propelled a wave of ice-filled water into Notre-Dame-de-la-Salette, Quebec causing the loss of 33 lives and the destruction of 12 homes.
- In 1955, a landslide affected part of the downtown of Nicolet, Quebec, causing $10 million in damages.
- On May 4, 1971, 31 lives were lost when 40 homes were swallowed in a retrogressive flowslide in Saint-Jean-Vianney, Quebec, resulting in the relocation of the entire town when the government declared the area uninhabitable due to the presence of Leda clay.
- The experience of Saint-Jean-Vianney contributed to the abandonment of the town of Lemieux, Ontario in 1991, after a 1989 study showed it was also located on the same type of clay along the South Nation River. In 1993, those findings were borne out when the town's abandoned main street was swallowed by a massive 17 hectare landslide.
- Another famous flow of quick clay at Rissa, Norway, in 1978 caused about 33 hectares (82 acres) of farmland to liquefy and flow into the lake Botn over a few hours, with the loss of one life. The Rissa slide was well recorded by local citizens and a documentary film was made about it in 1981.
- On May 11, 2010, quick clay took the lives of a family living in Saint-Jude, Quebec, when the land their house was built on suddenly tumbled down toward the Salvail River. The landslide was so sudden that the family members died where they sat; they had been watching an ice hockey game on television. The slide took out a portion of rural road which took a year to reinstate.
- On 2 February 2015, a landslide collapsed a pillar on the Skjeggestad Bridge. The landslide was caused by nearby earthworks.
These landslides are retrogressive, 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.
In modern times, areas known to have quick clay deposits are commonly tested in advance of any major human development. It is not always possible to entirely avoid building on a quick clay site, although modern engineering techniques have found technical precautions which can be taken to mitigate the risk of disaster. For example, when Ontario's Highway 416 had to pass through a quick clay deposit near Nepean, lighter fill materials such as polystyrene were used for the road bed, vertical wick drains were inserted along the route and groundwater cutoff walls were built under the highway to limit water infiltration into the clay.