Quick clay

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Quick clay, also known as Leda Clay in Canada, is a unique form of highly sensitive marine clay, with the tendency to change from a relatively stiff condition to a liquid mass when it is disturbed. Undisturbed quick clay resembles a water-saturated gel. When a block of clay is held in the hand and struck, however, it instantly turns into a flowing ooze, a process known as spontaneous liquefaction. Quick clay behaves this way because, although it is solid, it has a very high water content, up to 80%. The clay retains a solid structure despite the high water content, because surface tension holds water-coated flakes of clay together in a delicate structure. When the structure is broken by a shock, it reverts to a fluid state. The fluid state can also be regarded as onset by dilution of the original salt water content by fresh water, as this affects the chemical bonds between clay particles.

Quick clay is only found in the northern countries such as Russia, Canada, Alaska, Norway, Sweden, and Finland, which were glaciated during the Pleistocene epoch.

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. [1]

[edit] Formation of quick clay

At the height of the past glaciation (about 10,000 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. [2]

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.

For various reasons, the rainwater in these northern countries was quite aggressive to these clays, either 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).

With the addition of human habitation and their structures the scene is set for catastrophe. The most disastrous such landslide occurred in 1908 at Notre-Dame-de-la-Salette, Quebec, with the loss of 33 lives. Another famous flow of quick clay at Rissa, Norway in 1978 caused about 33 hectares (83 acres) of farmland to liquefy and flow into Lake Botnen 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.

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.