Plasticity Index (PI = LL − PL)

Albert Atterberg was a Swedish agricultural chemist trying to grade ceramic clays when, around 1911, he defined the moisture contents at which a soil passes from brittle to plastic to liquid. Arthur Casagrande turned them into repeatable tests at Harvard in the 1930s — the grooved brass cup dropped 25 times for the liquid limit, the 3 mm thread rolled on a glass plate for the plastic limit — and the plasticity index, the gap between them, became the backbone of the Unified Soil Classification System. A clay with LL = 52% and PL = 24% has PI = 28, which on Casagrande's A-line chart plots as a high-plasticity clay (CH).

The trap is treating PI as a strength. It is not; it is a measure of how much water a soil can absorb while still behaving like putty, and therefore of how much trouble it will cause. High PI means high shrink-swell, and it is why Texas and Colorado foundations crack on expansive clays with PI above 35 while a PI of 6 barely registers. The second trap: a soil that cannot be rolled into a thread at all is non-plastic, and its PI is reported as NP, not as zero — the arithmetic difference and the physical absence of plasticity are different findings.