Reappraisal of the Activity of Clays. Activity Chart

The proposed activity chart, which aims to classify soils (<425 μm) using the Atterberg limits, differs from the plasticity chart recently proposed by the author because the inorganic soils with platey clay minerals are classified (in addition to clay percentage) on the basis of their activity rather than on the degree of plasticity as a function of the liquid limit value, currently used by the standards. This is because soils that have the same liquid limit value may have very different characteristics because of both the amount and type of clay minerals contained in the soils. On the activity chart, the silt and the clay zones are subdivided (by two straight lines) in three new groups (low activity, medium activity and high activity) making it possible to classify inorganic soils with platey clay minerals (or their fraction) <425 μm on the basis of the amount and type of clay minerals they contain. The activity chart adapted to predict residual shear behaviour of cohesive soils is also shown. Finally, the distinction between coarse-grained soils and fine-grained soils based on their percentage of clay (<2 μm), might be useful in characterizing and predicting soils' engineering behaviour.