Two laboratory numbers decide what a fine-grained soil is called on the borehole log: the liquid limit and the plastic limit. Everything else on the plasticity chart is geometry.
The two axes
The liquid limit (LL) is the water content at which the soil begins to behave as a liquid. The plastic limit (PL) is the water content below which it stops behaving plastically and crumbles. The gap between them is the plasticity index:
PI = LL − PLPI is the width of the water-content range over which the soil is mouldable. A wide range means clay-like behaviour; a narrow one means silt.
Both are measured on the fraction passing the 0.425 mm sieve (ASTM D4318). Gravel and coarse sand are removed first — the chart describes a soil’s fines, not the whole sample.
The lines that divide it
Points above the A-line classify as clays (CL / CH); below it, silts (ML / MH). Hover a point for its values.
The A-line — PI = 0.73 (LL − 20). Above it, clay. Below it, silt or organic soil.
It is empirical: Casagrande drew it through a large body of test data because soils
clustered on either side, not because a theory predicts it.
The LL = 50 line — low plasticity (L) to the left, high plasticity (H) to the right. Combined with the A-line this gives the four familiar symbols:
| Zone | Symbol | Name |
|---|---|---|
| Above A-line, LL < 50 | CL | Lean clay |
| Above A-line, LL ≥ 50 | CH | Fat clay |
| Below A-line, LL < 50 | ML | Silt |
| Below A-line, LL ≥ 50 | MH | Elastic silt |
The U-line — PI = 0.9 (LL − 8). No natural soil should plot above it. When a point
does, the answer is almost always a testing error: a poorly prepared sample, a
miscounted blow in the liquid limit device, or transposed digits. Treat it as a
laboratory check, not a classification boundary.
The CL–ML band — where PI falls between 4 and 7 above the A-line, behaviour is genuinely ambiguous and ASTM D2487 requires the dual symbol CL–ML. Below PI = 4 the soil is ML regardless of where it sits. Sample BH-02 at 4.5 m in the figure above is in this band: reporting it as plain CL would overstate its plasticity.
Organic soils: the check most reports skip
The chart cannot distinguish an organic soil from an inorganic one, because organics plot below the A-line alongside silts. ASTM D2487 resolves it with a second liquid limit test on an oven-dried specimen:
if LL(oven-dried) / LL(not dried) < 0.75 → organic (OL or OH)Without that ratio, an organic clay is quietly logged as ML or MH — and the settlement estimate that follows will be optimistic.
Where classification goes wrong in practice
- Testing the wrong fraction. Limits run on unsieved material give a lower LL and a soil that looks less plastic than it is.
- Reporting PI without PL. PI alone cannot be checked. Log LL and PL, and let PI be derived.
- Forcing a single symbol. CL–ML and other dual symbols exist because the data does not support a single answer; removing them removes information.
- Skipping the organic check on any soil that smells organic, is dark, or comes from a floodplain or marsh deposit.
- Classifying from the chart alone. The plasticity chart applies only when more than 50 % passes the 0.075 mm sieve. Coarser soils are classified by gradation, and the chart only settles the character of their fines.
On the borehole log
The USCS symbol belongs beside the description, not instead of it. CL tells a reader
the sample plotted above the A-line at LL < 50; “stiff grey lean CLAY with fine sand
partings” tells them what the drilling crew actually saw. GeoEQ SPT Logs prints the
group symbol in bold in its own column and keeps the written description separate — the
two are checked against each other, and disagreement is the signal that something needs
a second look.
- USCS
- Atterberg limits
- ASTM D2487
- soil classification
- plasticity chart
References
- 01 ASTM D2487 — Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)
- 02 ASTM D4318 — Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils
- 03 Casagrande, A. (1948) — Classification and identification of soils. Transactions of ASCE, 113(1), 901-930
Frequently asked questions
What is the A-line on the plasticity chart?
The A-line is the empirical boundary PI = 0.73(LL - 20). Fine-grained soils plotting above it are classified as clays (CL or CH) and those below it as silts or organic soils (ML, MH, OL, OH). It was established by Arthur Casagrande from a large set of test data and is embedded in ASTM D2487.
What does it mean if a sample plots above the U-line?
The U-line, PI = 0.9(LL - 8), is the approximate upper limit observed for natural soils. A point above it should be treated as a laboratory or transcription error and the test repeated, rather than accepted as a genuine result.
How is CL-ML different from CL or ML?
CL-ML is a dual symbol for fine-grained soils that plot above the A-line with a plasticity index between 4 and 7 — a band where the distinction between clay and silt behaviour is not reliable. ASTM D2487 requires the dual symbol rather than forcing a choice.
Which fraction of the sample is tested for Atterberg limits?
Only the material passing the 0.425 mm (No. 40) sieve. Gravel and coarse sand are removed before testing, which is why the plasticity chart describes the fines in a soil rather than the soil as a whole.
Cite this article
Malo, R. C. (2026). Reading the plasticity chart: how USCS symbols are decided. GeoEQ. https://geoeq.dev/blog/reading-the-plasticity-chart/
@misc{malo2026reading,
author = {Ripon Chandra Malo},
title = {Reading the plasticity chart: how USCS symbols are decided},
year = {2026},
howpublished = {GeoEQ},
url = {https://geoeq.dev/blog/reading-the-plasticity-chart/},
urldate = {2026-08-10}
} 