ABSTRACT
Plasticity is the property that allows a soil to be deformed without cracking in response to an applied stress. A soil may exhibit plasticity, and hence be remolded, over a range of water contents, first quantified by the Swedish scientist Atterberg (1911, 1912). Above this range, the soil behaves as a liquid, while below it, it behaves as a brittle solid and eventually fractures in response to increasing applied stress. The upper limit of plasticity, known as the liquid limit, is at the water content at which a small slope, forming part of a groove in a sample of the soil, just collapses under the action of a standardized shock force. The corresponding lower limit, the ‘‘plastic limit,’’ is at the water content at which a sample of the soil, when rolled into a thread by the palm of the hand, splits and crumbles when the thread diameter reaches 3 mm. By convention, both water contents are expressed gravimetrically on a percentage basis. The numerical difference between the liquid and plastic limits is defined as the plasticity index. Remarkably, these simple empirical tests have been used, essentially unchanged, for nearly a century by soil engineers and soil scientists (BSI, 1990).
