Fine-grained soils are those for which more than 50% of the material has particle sizes less than 0.075 mm. Clay particles have a flaky shape to which water adheres, thus imparting the property of plasticity.
A plasticity chart , based on the values of liquid limit (WL) and plasticity index (IP), is provided in ISSCS to aid classification. The 'A' line in this chart is expressed as IP = 0.73 (WL - 20).
Depending on the point in the chart, fine soils are divided into clays (C), silts (M), or organic soils (O). The organic content is expressed as a percentage of the mass of organic matter in a given mass of soil to the mass of the dry soil solids.Three divisions of plasticity are also defined as follows.
Low plasticity |
WL< 35% |
| Intermediate plasticity |
35% < WL< 50% |
| High plasticity |
WL> 50% |
The 'A' line and vertical lines at WL equal to 35% and 50% separate the soils into various classes.
For example, the combined symbol CH refers to clay of high plasticity.