PLASTICITY INDEX
The Plasticity Index (PI) is a measure of the plasticity of a soil.
Plasticity is the quality of being easily shaped or moulded.
PI is the moisture content range where the soil exhibits plastic properties.
Example:
In the example above, the soil is solid or semi-solid from 0% moisture (oven dry) up to 14% moisture content. The soil starts behaving plastic at 14% moisture. This is the Plastic Limit.
As water is added, the soil continues to act plastic up until 32% moisture content. At this point, the soil starts to flow as a liquid (25 blows in the Casagrande apparatus). This is the Liquid Limit.
The Plasticity Index is the range of moisture between the Plastic Limit (PL) and the Liquid Limit (LL)
LL-PL = PI
32-14 = 18
PI = 18
(This material has a moisture range of 18% where it exhibits plastic properties)
The higher the PI of a soil, the more reactive it is.
Soils with a high PI tend to be clay,
Those with a lower PI tend to be silt, and
Those with a PI near zero tend to have little or no silt or clay (fines) present.