How can Black Cotton Soil be Classified?

In the previous post we have discussed about lime treatment for black cotton soil and the related chemical reactions to change their molecular structure. To perform a treatment to black cotton soil, it is important to classify the soil. Depending on the classification, the treatment process is chosen and in case of same treatment process geotechnical engineers can determine the degree of treatment. Black cotton soil can be classified, depending on plasticity index and expansion index.

We know black cotton soil produce problems in road pavement, airfield, retaining structure, canal, and underground plumbing and sewerage premises and as a whole all types of foundation are affected by it. In case of building structures it not only disturbs in design phase but also produce mishaps to construction and even in some maintenance work. 

In 1995 wray reported that-

Plasticity index less than 20 is considered less expansive.

Plasticity index >20 to <40 is considered expansive.

Plasticity index >40 to 60 is considered highly expansive

Plasticity index 60>is considered very expansive.


Many engineers use expansion potential of black cotton soil i.e. expansion index (EI)

Expansion index of ≤50 is considered less expansive.

Expansion index ≥91 is considered as high expansive.


Expansive soils of any kind obviously have a minimum and maximum moisture content limit. Whatever happened, either swelling or shrinkage, depend on this critical limit. So knowing the actual type of black cotton soil is important to have a proper foundation design prediction. An elaborate discussion and tables will be provided in next post.