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Early in the development of soil mechanics, a need arose for a system for classifying soils according to their physical characteristics. To meet such a need, Atterberg, in 1911, established upper and lower limits of plasticity for soils and developed test procedures to determine those values. For the upper limit, Atterberg selected a soil-water mixture which had very little strength and flowed like a liquid; hence his designation “liquid limit.” His test was designed to measure the water content at which a soil-water mixture would flow together in a groove of specified dimensions under a specific number of applied impacts. The lower limit of plasticity, which he called the “plastic limit,” denotes the water content at which a soil-water mixture ceases to be plastic and crumbles or breaks when rolled out into threads. The numerical difference between the liquid and plastic limit values, called the “plasticity index,” was found by Atterberg to be a satisfactory measure of the degree of plasticity of a soil. Because of their simplicity and ease of performance, the Atterberg limits tests have become a valuable and widely used aid for quickly predicting soil behavior.
Mitchell, James E.
Chief, U. S. Army Engineer Waterways Experiment Station, Vicksburg, Miss.