The plasticity index (PI) is a measure of the plasticity of a soil. The plasticity index is the size of the range of water contents where the soil exhibits plastic properties. The PI is the difference between the liquid limit and the plastic limit (PI = LL-PL). Soils with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 tend to have little or no silt or clay.

PI is used to relate to that how expansive the clays were. A PI lower than 20 to 24 was generally a safe area...but higher than that and we would then have to respond to swelling clay conditions.

It is a measure of the cohesive qualities of the binder resulting from the clay content. Also, it givessome indication of the amount of swelling and shrinkage that will result in the wetting and drying of that fraction tested. If some soils do not have sufficient mechanical interlock they require amounts of cohesive materials to give a satisfactory performance. A deficiency of clay binder may cause raveling of gravel wearing courses during dry weather and excessive permeability

regards

A soil may or may not deform under stress would be related to whether it is cohesive or non-cohesive, which again would be depending upon water's amount in soil which controls deformation behaviour of a soil i.e. Soil Consistency. As the water content increases, it may cause a non-cohesive soil to deform as viscous fluid. These changes in any soil are termed as Atterbergs Limits i.e. Plastic Limit (PL) and Liquid Limit (LL).

PL and LL are to be estimated in Laboratory, while the PI can be computed from estimated PL and LL values. PI, is nothing but the difference of LL and PL (i.e. PI = LL - PL), and thus represents the range of water content in the soil under consideration.

Regards

AK

I agreed with @Atul Kohli. Furthermore, ‘Plasticity’ of a soil is defined as that property which allows it to be deformed, without rupture and without elastic rebound, and without a noticeable change in volume. Also, a soil is said to be in a plastic state when the water content is such that it can change its shape without producing surface cracks. Plasticity is probably the most conspicuous property of clay.

When a structure is proposed to construct over a clay stratum, this is a key indicator in judging its behavior under loading.

Plasticity Index is simply the difference between liquid limit and plastic limit.

It is used in civil engineering for classification of soils .Plasticity chart is developed based on this index which is used for classification of soils.

Mr. Kumar and Kohli have given information about Plasticity Index.

High plasticity index soils if present in the foundation of civil engineering structures may cause sliding of the structures. Presence of such type of soils can also cause collapse of the tunnels, sliding of slopes and landslides. You can refer following literature for detail information which is available on Researchgate:

Book Geotechnical Evaluation of Major Dams in Lower Narmada Valle...

With best wishes

It is an index or a measure that gives a clue about the amount of water for a certain volume of soil ( fine grained/cohesive of course) to be mixed such that the soil gains a consistency good enough to roll it into threads without crumbling.

Plasticity Index represents range of water contents where the soil shows plastic properties. Liquid limit- Plastic limit= Plasticity Index

PI= LL-PL

Now when PI = 0 , LL= PL The soil therefore could be non plastic silt.

As per Burmister Classification, when PI =0, the soil is non plastic.

On the other hand, a high PI indicates excess of clay or colloids. The significance can be summarized as under:

1. It indicates fineness of soil. For instance plasticity Index increases when particle size decreases.

2. It helps in soil classification PI in relation to Liquid limit

3. It has nexus between permeability and compressibility . For the same PI when liquid limit increases, permeability and compressibility increases and the toughness and dry strength decreases.

Thus the significance of PI in Civil Engineering is immense.This is very widely used in Reinforced Earth wall Construction in bridge construction

The plasticity index (PI) is a measure of the plasticity of a soil. The plasticity index is the size of the range of water contents where the soil exhibits plastic properties. The PI is the difference between the liquid limit and the plastic limit (PI = LL-PL). Soils with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 tend to have little or no silt or clay.

PI is used to relate to that how expansive the clays were. A PI lower than 20 to 24 was generally a safe area...but higher than that and we would then have to respond to swelling clay conditions.

It is a measure of the cohesive qualities of the binder resulting from the clay content. Also, it givessome indication of the amount of swelling and shrinkage that will result in the wetting and drying of that fraction tested. If some soils do not have sufficient mechanical interlock they require amounts of cohesive materials to give a satisfactory performance. A deficiency of clay binder may cause raveling of gravel wearing courses during dry weather and excessive permeability

regards

I agree with Hassan Nima, the plasticity index is the size of the range of water contents where the soil exhibits plastic properties.

yes