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What is Significance of Plasticity Index of Soil?

Plasticity index

This is equal to difference between liquid limit and plastic limit of soil.

Plasticity index is also denoted by PI. This is the range of the water content within which soil achieves its plastic state. This is a measure of plasticity of soil of our interest.

Wl =liquid limit
Wp=plastic limt
Ip=plasticity index
Equation of plasticity index of soil
This parameter cannot be negative if plastic limit, in some exceptions, is larger than liquid limit, ignoring negative value, it is considered to be zero.

A soil is considered non-plastic when liquid limit or plastic limit cannot be determined and denoted by NP. Clay soils having high plasticity index with high liquid limit, is called highly plastic clays; sometimes called fat clays. Those have low values called lean clays.

Both liquid limit and plastic limit are determined from remolded soils. Thus natural structure of soil is disturbed. If actual behaviors have to be determined, we should test soil in undisturbed condition.

As plasticity index is a product of these limits, it will not provide us complete idea about soils. A rough information about the characteristics of soil at best can be determined.

Plasticity index and liquid limit are considered important in co-relating engineering properties, as they are influenced by identical factors like 

Clay minerals exist in soil

• Ions exist in pore water

• Stress history, i.e. whether normally consolidated or overconsolidated, of soil deposit.



Significance of plasticity index:

• Plasticity index of soil depends chiefly on clay content in soil. So Soils that have high plasticity index are considered to tend to clay.

• With the decrease in particle size, a rapid increase in plasticity index is observed. Thus plasticity index is a measure of fineness of particles.

• Plasticity index in relation with liquid limit, provide us valuable information for soil classification.

• For a same plasticity index, when liquid limit increases permeability and compressibility are found to be increased whereas toughness and dry strength is decreased.

• For a same liquid limit of two samples, when plasticity index is increased, permeability decreases whereas toughness and dry strength are increased. But compressibility is found almost unchanged. 

Relation between plasticity index and engineering properties of soil
• Soils having high plasticity index are considered clay and those having lower value are considered silt. In case of zero value, soil are considered to have little/no clay or silt and called non-plastic soil. 

• A lower plasticity index of two soil is indicative to have high organic matter in soil.

plasticity chart for soil classification
We have all know more or less about plasticity chart this is nothing but a plot with plasticity index against liquid limit. Plasticity chart is extensively used in classifying fine grained soil.


  1. If plasticity index of soil is zero,the soil is??? Sand,ckay,silt or clayey silt

    1. PI zero means non-plastic i.e. tend to have little or no silt. So the soil is clay if PI is zero.

    2. PI of zero would be non plastic, a low PI is silt, and a high PI is clay. A rock would have a PI of zero because it has zero moisture a d can't be rolled.

  2. when i have a Pl of 10 and a Ll of 15, the Pi will be 5 so i read above that when the Pi is low the soil has a high organic matter. my question is about what do we say the PL is either low or high in order to classify the soil

  3. when i have a Pl of 10 and a Ll of 15, the Pi will be 5 so i read above that when the Pi is low the soil has a high organic matter. my question is about what do we say the PL is either low or high in order to classify the soil

  4. what happens to when one mixes soils of different Pi values. Will Pi be of the mixed material take the value of the high Pi soil value or the Pi of the lower soil of the mixture

    1. after mixing the soils of high and low PL values, you start the procedure to determine new PL of the new mixture. the properties of the mixture will change.

    2. when we mix teo diff soil of two diff PI THEN PI OF MIXTURE IS


  5. When there is no plastic limits in soils. The liquid limit is 29. What is the plasticity index. Please

    1. Non-plastic if you can't roll it. A PI of zero is also non-plastic

    2. then in that case liquid limit also must be zero if it cant hold the water

  6. is it good to use soils with every high liquid limit.. ?

  7. Kayak plasticity index ki value negative ho sakti h.

  8. So if I have sandy loam soil with plasticity of .29, how will it affect my foundation? worried i would have sinking/settling

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  10. what will be she PI of stonebase when the plastic limit is zero

  11. Plasticity index is negative possible or not.....???

  12. if the plasticity index is negative, it is considered as zero