A quick clay is characterized by such a large reduction of its shear strength when remoulded that it becomes a liquid. In practice a clay is called quick when its sensitivity (St), i.e., the ratio of the shear strength in the undisturbed and remoulded states, exceeds a certain value, usually 30-50.
Quick-clay deposits are frequently found in Scandinavia and North America. In some cases, sensitivity values of more than 500 (cone test) have been determined. A correspondingly large reduction of the shear strength by remoulding is of great practical importance. Thus, in certain parts of Scandinavia such as the Grta River Valley in Sweden, extremely quick clays make necessary special precautions in road and house building. For instance the very small lateral support of the surrounding clay mass on driven piles must be taken into consideration in extreme cases.
The effect of quick clay in a slide area is especially important. Thus, while a slide in a normally sensitive clay usually involves only relatively small movements of the unstable soil mass, a slide in quick clay may cause a flow over a large area. Well-known examples of large slides in quick-clay areas are the Vaerdalen (Norway, 1893) and the Anchorage (Alaska, 1964) slides.
After the occurrence of the large slides at Surte (1950) and G/Sta (1957), research work was initiated in Sweden to find out the physico-chemical background of the formation of quick clay. Also in the U.S.A. and Canada the quickclay problem has been studied during the last decade.
In Norway, where slides in quick clays have occurred frequently, research on quick-clay formation was started earlier. The initial research work, the results of which were published by Ivan Th. Rosenqvist in 1946, showed that leaching of originally sea-water deposited clays brings about increased repulsive inter-particle forces, which means that the clay turns into the liquid-like state when the clay structure is broken down by remoulding.
Systematic, recent investigations in several countries have shown that a quick condition can be effected by introducing into a normally sensitive clay,