Atterberg Plasticity Indices and A-Line Classification
The consistency numbers a geotech report leads with, and an earthwork spec screens fill against: the plasticity index PI = LL - PL, the liquidity index LI = (w - PL)/PI, and the USCS A-line PI = 0.73(LL - 20) that separates clay from silt. LL 45 / PL 22 gives PI 23 above the A-line 18.3 at LL < 50 -> CL lean clay, LI 0.35 (plastic/workable); a low-PI silt (LL 30, PI 5) plots below the A-line -> ML, the distinction that changes a fill's suitability. A-line chart classification, ASTM D4318. An engineering aid; the soil test data govern.
Formula and source
PI = LL - PL; LI = (w - PL)/PI; A-line PI = 0.73(LL - 20); above A-line -> clay (CL/CH), below -> silt (ML/MH), split at LL = 50.
The Atterberg-limit indices (plasticity index, liquidity index) and the USCS A-line classification, with the ASTM D4318 limit tests, by name.
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