PLASTICITY INDEX
The Plasticity Index (PI) is a measure of the plasticity of a soil.
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Plasticity is the quality of being easily shaped or moulded.
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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.
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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.
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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)
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The higher the PI of a soil, the more reactive it is.
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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.
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