Clay Activity (Skempton)
Computes clay activity as defined by Skempton, A = PI ÷ (% of particles finer than 2 μm), the ratio of the soil's plasticity index to the truly clay-sized fraction. It separates the clay mineral's effect from the mere amount of fines: two soils with the same PI behave very differently if one owes its plasticity to a little highly active clay and the other to a lot of inert clay. The usual classification is A < 0.75 inactive (kaolinite), 0.75 to 1.25 normal (illite) and A > 1.25 active (montmorillonite), the range where the expansive soils that warp pavements and shallow foundations are found. Enter the plasticity index and the clay fraction.
Result
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Skempton Activity: telling active clay from inert fines
Two soils can reach the laboratory with the same plasticity index and behave in opposite ways on site. One owes its plasticity to 15 percent montmorillonite, the other to 60 percent kaolinite. The first swells, lifts slabs and warps pavement; the second is merely tedious to compact. A plasticity index on its own separates neither case, which is why a geotechnical engineer divides it by the genuinely clay-sized fraction before deciding whether to swap the material, treat it with lime or deepen the footing.
Skempton defined activity as A = PI ÷ (% of particles finer than 2 μm). The plasticity index comes from Atterberg limits — liquid limit minus plastic limit, ASTM D4318 — while the clay fraction comes from a hydrometer or sedimentation test. Reading the bands: under 0.75 the soil counts as inactive and kaolinitic, with values around 0.4; from 0.75 to 1.25 it ranks as normal, illite territory; above 1.25 it turns active, with calcium montmorillonite landing between 1.5 and 3 and the sodium form running past 5. Expansive soils live in that upper band.
The ratio inherits every uncertainty of the two tests feeding it. The percentage finer than 2 μm depends on the dispersing agent, the settling time and the temperature of the sedimentation test, and it varies between laboratories more than anyone would like. There is also the question of reference base: the plasticity index is measured on material passing the 0.425 mm sieve, so quoting a clay percentage over the whole sample, gravel included, inflates activity. Activity suggests a likely mineral; only swell testing or X-ray diffraction confirms it.
Frequently asked questions
What kind of soil gives A = 0.800 from PI 32 and 40 percent clay?
Is the clay percentage taken over the whole sample or over the fines?
Does the tool accept a plasticity index of zero?
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.