Soil Activity (Skempton)
The swell/mineralogy index the plasticity index alone hides -- what atterberg-indices names as its own gap. Skempton's activity A = PI / (percent clay finer than 2 microns) collapses plasticity and clay content into one number that tracks the clay MINERAL: A < 0.75 inactive (kaolinite ~0.4, low swell), 0.75-1.25 normal (illite ~0.9), A > 1.25 active (Na-montmorillonite runs several times higher, expansive). PI 30 on 25% clay is A 1.20 (normal); the SAME PI 30 on only 15% clay is A 2.0 (active) -- a soil with a modest PI can still be strongly expansive if its clay fraction is small, which is why activity, not PI, flags the swelling clay. The clay fraction is the hydrometer's percent finer than 2 microns (ASTM D7928 / D422). A mineralogy and swell screen, not a heave calculation; the soil test data and the geotechnical engineer govern.
Formula and source
A = PI / (percent clay finer than 2 microns); A < 0.75 inactive, 0.75-1.25 normal, A > 1.25 active.
Skempton (1953) soil activity, the ratio of plasticity index to clay fraction, and its inactive/normal/active bands, a standard geotechnical result, by name.
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