Atterberg Limits Test

Reference

Two moisture contents that classify every fine grained soil: the water content where it starts to flow, and the one where it stops being mouldable. Between them lives the plasticity index.

PI = LL − PL: the width of the plastic range
StandardASTM D4318
Liquid limit, LLCasagrande cup, the moisture where the groove closes at 25 blows
Plastic limit, PLthe moisture where a rolled 3 mm thread just crumbles
Plasticity index, PILL − PL
The A linePI = 0.73 times LL − 20, clays above, silts below
Low plasticityLL under 50, the CL and ML groups
High plasticityLL over 50, the CH and MH groups
Field flagPI above roughly 20 means moisture sensitive, shrink swell prone ground

How the test works

Run the liquid limit at several moisture contents, counting the blows to close the standard groove, and interpolate the moisture at 25 blows. The plastic limit is stranger and older: roll threads of the soil to 3 mm, fold and reroll, until the thread crumbles right at 3 mm, and measure that moisture. Plot LL against PI on the plasticity chart and the soil lands in its group: above the A line it behaves as clay, below as silt, and the chart position drives the USCS symbol.

Where it goes wrong

Never oven dry a sample before limits testing: drying permanently alters many clays, organics worst of all, and the limits of a dried and rewetted soil are limits of a different material. Test from natural moisture down. The plastic limit is operator sensitive, so consistent technique matters more than speed. And read PI for what it is on site: a high PI subgrade will move with every wet season, shed strength when soaked and crack when it dries, which is information no gradation curve will volunteer.

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