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.