Atterberg Plasticity Indices and A-Line Classification
The consistency numbers a geotech report leads with, and an earthwork spec screens fill against.
Example
You enter
- Liquid limit LL (%) 45
- Plastic limit PL (%) 22
- In-situ water content (%, optional for LI) 30
You get
- Plasticity index (A-line PI) 23
- Aline 18.25
- Liquidity index 0.348
Checked against published worked examples from: University of Tennessee at Chattanooga (Warrington).
Details, formula, and sources
ScopeThe 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 or tablePI = LL - PL; LI = (w - PL)/PI; A-line PI = 0.73(LL - 20); on or above A-line -> clay (CL/CH), below -> silt (ML/MH), split at LL = 50; below LL 50, PI 4-7 on or above the line -> CL-ML and PI < 4 -> ML (ASTM D2487).
Source and editionThe Atterberg-limit indices (plasticity index, liquidity index) and the USCS A-line classification, with the ASTM D4318 limit tests, by name.
Free accessThe Atterberg-limit definitions and the USCS A-line are public geotechnical results; ASTM D4318 defines the limit tests.
What governsEstimate. AHJ and licensed professional govern.
API field namesll, pl, w_pct, pi, aline, li
- Plasticity index PI = LL - PL; a soil with PL >= LL is nonplasticASTM D4318
- A-line PI = 0.73(LL - 20); above -> clay (CL/CH), below -> silt (ML/MH)USCS (ASTM D2487)
- Plasticity split LL = 50 separates low (L) from high (H) plasticityUSCS chart