Figure 5.--A, Graph showing relationship between plasticity index and clay fraction for common clay types (from Skempton, 1953). Soil samples AT-1 and AT-3 from the active part of the Slumgullion landslide are plotted on the graph for comparison. Typical activities (A) of the clay minerals and for samples AT-1 and AT-3 are in parentheses. Note that samples AT-1 and AT-3 have activities that are lower than might be expected for smectite-rich (montmorillonite-rich) materials, indicating that the clay size fractions of the materials do not contain significant amounts of highly active clay. B, Classification chart for swelling potential (after Seed and others, 1962). As shown, plotted values for samples AT-1 and AT-3 indicate swelling potentials of approximately 25 percent and 15 percent, respectively.