Atterberg Limits and the Plasticity Chart: Why Does Clay Change with Water?

1. Four states of consistency

The behaviour of a fine-grained soil — clay and silt — depends strongly on its water content. In 1911 the Swedish chemist Albert Atterberg described this behaviour with four states of consistency: solid, semi-solid, plastic and liquid. The boundaries between them, expressed as water contents, are known today as the "Atterberg limits". Arthur Casagrande standardised the tests in the 1930s and published the classification chart based on these limits in 1948.

2. Liquid limit and plastic limit

• Liquid limit (LL): the water content at which the soil passes from the plastic to the liquid state. In the Casagrande apparatus it is the water content at which a groove cut in the soil closes after 25 blows. The fall-cone method is also used.
• Plastic limit (PL): the water content at which the soil passes from the semi-solid to the plastic state. The soil is rolled by hand into a thread about 3 mm in diameter; the water content at which the thread just begins to crumble at that diameter is the plastic limit.

Test details are given in ASTM D4318 and TS 1900-1.

3. Plasticity index and liquidity index

The plasticity index (PI) is the width of the water-content range over which the soil behaves plastically; it says a lot about the amount and type of clay minerals. The liquidity index (LI) shows where the natural water content w lies in that range: LI = 0 means the soil is at its plastic limit, LI = 1 at its liquid limit. For a clay with w = 38%, LL = 45% and PL = 22%, LI = (38 − 22) / (45 − 22) = 0.70: the soil is soft, close to its liquid limit.

4. The plasticity chart

Casagrande's plasticity chart shows the liquid limit on the horizontal axis and the plasticity index on the vertical. Two basic lines divide it:

Soils above the A-line are classified as clays (C), those below as silts (M); LL < 50 means low plasticity (L), LL ≥ 50 high plasticity (H). This gives four basic classes: CL, CH, ML and MH. The upper limit of the chart is the U-line, PI = 0.9 · (LL − 8): natural soils rarely plot above it, and if one does, the test data should be checked. The chart is drawn in Figure 30 on the site.

5. Five examples

The five soils below are classified with the plasticity-chart rules of ASTM D2487:

The last row is a special zone: if PI is between 4 and 7 and above the A-line, the soil is shown as CL–ML, a borderline class between clay and silt. The "or OL / OH" for ML and MH arises because organic soils plot in the same zone; to separate them, the liquid limit of an oven-dried sample is compared with that of the undried sample (ASTM D2487 treats a ratio below 0.75 as organic).

6. Uncertainty, limits and cautions

• Measurement uncertainty can change the class. The plastic limit test depends heavily on the operator; different people finding results a few points apart on the same sample is normal. For a soil near the A-line this changes the class. For LL = 45%, the A-line is at PI = 0.73 × 25 = 18.25. If PL = 27% is measured, PI = 18 and the soil is ML (just below the line); if PL = 26%, PI = 19 and it is CL. One point is the boundary between silt and clay. Results near the line should therefore not be taken as definitive on the basis of a single test; the test should be repeated or the result reported as "borderline".
• The chart is for fine-grained soils only. Whether a soil counts as fine-grained is first decided by sieve analysis; in coarse-grained soils the plasticity of the fine fraction is a secondary classification criterion.
• Atterberg limits are not strength. The bearing capacity or settlement of a clay requires separate shear-strength and compressibility tests. The limits give a quick and cheap first picture of the soil's general character.
• Seismically: plasticity is one of the criteria used to judge the liquefaction susceptibility of fine-grained soils. This is covered in the site's TBDY liquefaction tool.