Proposal for a new plasticity chart

This study investigates the consistency limits of the pure clay minerals kaolinite and montmorillonite, and their respective mixtures with fine silica sand in various proportions. Plotting the plasticity index data as a function of the liquid limit allows the zones where mixtures with the same clay contents fall to be defined. In particular, the line corresponding to 50% clay (designated as the 0·5C-line) makes it possible to distinguish the points that lie below the line, namely clay, from the points lying above the line in the silt zone. The clay zone includes inorganic soils with clay contents ≥50%, whereas the silt zone includes inorganic soils composed of silt and/or sand (2–425 μm) in percentages >50%. A new plasticity chart, which aims to classify soils (<425 μm) using the Atterberg limits, is defined herein. It differs from Casagrande's plasticity chart, especially in terms of its silt and clay zones, whose positions are reversed compared with Casagrande's chart. This can be explained by the fact that, contrary to what is commonly believed, in inorganic soils—liquid limits being equal—the plasticity index increases as the clay content decreases. Save only a few exceptions, the examples of inorganic soils plotted on the new plasticity chart lie above the 0·5C-line in the silt zone or below the line in the clay zone when their clay contents are less or greater than 50% respectively. On Casagrande's plasticity chart, the same soils lie above the A-line in the clay zone, regardless of their clay contents (9·7–100%). Only the kaolinite samples (relatively pure) lie below the A-line in the silt zone.