Fundamental Aspects of the Atterberg Limits

The physical significance of the Atterberg limits is examined and theoretical relationships between the liquid and plastic limits and the clay content of a soil are developed. The relationships are shown to be in good agreement with actual test data for inorganic and organic clays. With the aid of these results, an analysis is made to determine the relationship between liquid limit and plasticity index for organic and inorganic clays. It is shown that inorganic clays will necessarily lie in a small zone of the plasticity chart; organic clays will lie in a somewhat larger but generally lower zone of the chart. The predicted positions of the different types of clay are in excellent agreement with previous experimental determinations. It is shown that at clay contents greater than approximately 40%, the plasticity indices of soils containing a given type of clay will be directly proportional to the clay content although this relationship will not be true at lower clay contents. Thus, definitions of the activity of clays based on simple linear relationships should be used with caution.