Site investigation · Laboratory: soil classification
Plastic limit & consistency indices calculator
PL from repeat threads, PI, liquidity and consistency index, activity and plasticity chart class (A-line).
Standard: AS 1289.3.2.1 · AS 1289.3.3.1 · AS 1726
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| PL threads: tin, moisture (%) | P1, 21.4 P2, 22.0 P3, 21.7 | |
| Liquid limit | 48 | % |
| Natural moisture content | 30 | % |
| Clay fraction (< 2 µm) | 28 | % |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Plastic limit (mean) | 21.70 | % |
| Plasticity index PI = LL − PL | 26.30 | % |
| Liquidity index (w − PL)/PI | 0.316 | |
| Consistency index (LL − w)/PI | 0.684 | |
| Activity PI / clay % | 0.939 | |
| A-line PI = 0.73(LL − 20) | 20.44 | % |
| Plasticity chart class | CL |
- ✓ PL repeatability: range ≤ 1%
- ✓ Plastic (PI > 0)
Method, formulas and assumptions
- Plastic limit (mean) = AVERAGE([t.w]) = 21.7 %
- Plasticity index PI = LL − PL = 48-21.7 = 26.3 %
- Liquidity index (w − PL)/PI = (30-21.7)/26.3 = 0.31559
- Consistency index (LL − w)/PI = (48-30)/26.3 = 0.68441
- Activity PI / clay % = 26.3/28 = 0.93929
- A-line PI = 0.73(LL − 20) = 0.73*(48-20) = 20.44 %
- Plasticity chart class = IF(26.3>=20.44,IF(48>50,"CH","CL"),IF(48>50,"MH","ML")) = CL
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.