"Clayey soils" necessarily do not consist of 100% clay size particles. The proportion of clay mineral flakes (< 0.002 mm size) in a fine soil increases its tendency to swell and shrink with changes in water content. This is called the activity of the clayey soil, and it represents the degree of plasticity related to the clay content.
Activity = (PIasticity index) /(% clay particles by weight)
Classification as per activity is:
Activity |
Classification |
< 0.75 |
Inactive |
0.75 - 1.25
|
Normal |
> 1.25 |
Active |
Liquidity Index
In fine soils, especially with clay size content, the existing state is dependent on the current water content (w) with respect to the consistency limits (or Atterberg limits). The liquidity index (LI) provides a quantitative measure of the present state.
Classification as per liquidity index is:
Liquidity index |
Classification |
> 1 |
Liquid |
0.75 - 1.00 |
Very soft |
0.50 - 0.75 |
Soft |
0.25 - 0. 50 |
Medium stiff |
0 - 0.25 |
Stiff |
< 0 |
Semi-solid |
Visual Classification
Soils possess a number of physical characteristics which can be used as aids to identification in the field. A handful of soil rubbed through the fingers can yield the following:
SAND (and coarser) particles are visible to the naked eye.
SILT particles become dusty when dry and are easily brushed off hands.
CLAY particles are sticky when wet and hard when dry, and have to be scraped or washed off hands.