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 and so is much less than its strength upon initial loading. That is caused by a 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.
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:
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.