Casagrande Plasticity Chart — Atterberg Limits Plotter
Plot liquid limit against plasticity index on the Casagrande chart, with the A-line and U-line drawn, and read the USCS group symbol straight off. Enter LL and PL — the plasticity index is worked out for you.
What the Casagrande plasticity chart shows
The plasticity chart plots liquid limit (LL) on the horizontal axis against plasticity index (PI = LL − PL) on the vertical. Each fine-grained soil becomes one point, and where that point falls decides how the soil is classified and, broadly, how it will behave.
The Atterberg limits behind it are water contents at which a remoulded soil changes state. Above the liquid limit it flows as a viscous liquid; below the plastic limit it crumbles rather than deforming. The gap between them — the plasticity index — is the range of water contents over which the soil is plastic and workable, and it is a direct proxy for how much active clay mineral is present.
Data
Paste straight from Excel or Google Sheets — include the header row and the columns are matched by name, in any order.
| # | Sample(opt) | LL% | PL%(opt) | PI%(opt) | w%(opt) | Group(opt) | |
|---|---|---|---|---|---|---|---|
| 1 | |||||||
| 2 | |||||||
| 3 | |||||||
| 4 |
0 rows of data
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The A-line, the U-line and the USCS symbols
The A-line, PI = 0.73(LL − 20), is the empirical boundary Arthur Casagrande drew between clays above it and silts (and organic soils) below it. It is not a law of nature; it is a line through a large body of test data that has held up for eighty years. The U-line, PI = 0.9(LL − 8), is the practical upper bound — a point plotting above it almost always means a testing error, and it should send you back to the lab rather than into a report.
The classification follows from four quadrants split at LL = 50. Above the A-line: CL below LL 50 (lean clay), CH above it (fat clay). Below the A-line: ML and OL below LL 50 (silt, organic silt), MH and OH above (elastic silt, organic clay). The small hatched zone between PI 4 and 7 above the A-line is CL-ML, a deliberate dual symbol for soils that are genuinely borderline. Distinguishing the organic O-soils from their inorganic twins is not done on this chart at all — it needs the ratio of oven-dried to as-received liquid limit, below 0.75 meaning organic.
Read the chart as a description of behaviour, not just a label. Points high above the A-line at high LL are the montmorillonitic clays that shrink and swell, take forever to consolidate and make poor subgrade. Points just below the A-line at moderate LL are silts and rock flour: low plasticity, but frost-susceptible and prone to losing strength when disturbed.
Who uses one
Geotechnical engineers classifying soils for foundation and earthworks design, highway engineers assessing subgrade suitability and frost susceptibility, and civil engineering students running the Casagrande cup and thread-rolling tests in a soil-mechanics lab. Environmental engineers specifying clay liners, and anyone screening a site for shrink–swell risk.
Common questions
- Do I enter PI, or LL and PL?
- Enter liquid limit and plastic limit and the plasticity index is computed as LL − PL. If your lab sheet reports PI instead of PL, put it in the optional PI column and it is used as given.
- What if a point plots above the U-line?
- Treat it as a suspect result. The U-line is an empirical upper bound on real soils, so a point above it normally means an error in the liquid or plastic limit determination rather than an unusual soil.
- Does this chart cover non-plastic soils?
- No. A soil whose plastic limit cannot be determined is reported as non-plastic and has no point on the chart — it is classified from its grading instead, on the grain size distribution curve.
- Is this the USCS chart or the BS 5930 one?
- The USCS / ASTM D2487 chart, with the A-line at PI = 0.73(LL − 20) and the split at LL = 50. The British system uses the same A-line but subdivides the liquid limit into five bands (L, I, H, V, E), so quote the scheme alongside the symbol in any report.