Soil can be classified into four different categories based on its water content. These are solid, semi-solid, plastic, and liquid. The engineering properties changes in each state. Boundaries of these states can be defined as Liquid Limit, Plastic Limit, and Shrinkage Limit. They are called Atterberg Limits collectively.
The importance of atterberg limits and indices are in the following:
- Plasticity index of soil depends mainly on the amount of clay present in a soil sample. So, a change in plasticity index indicates a change in clay volume in soil. Therefore, you can say plasticity index of soil is a measure of clay amount in soil. Moreover, The study of plasticity index, in combination with liquid limit, gives information about the type of clay. It is directly proportional to the percentage of clay-size fraction present in the soil. Besides, both the liquid and plastic limits depend on the type and amount of clay in the soil.
- Plasticity index is a measure of the fineness of the particles.
- As the particle size decreases, both the liquid and plastic limits increase, but the liquid limit increases at a greater rate. Therefore, the plasticity index increases at a rapid rate with the decrease of particle size.
- Sandy soils do not have plasticity and are classified as nonplastic (NP).
- If organic matter is added to the soil, plastic limit increases without any significant increase in liquid limit. Generally, organic content-rich soil has a low plasticity index.
- Liquid limit of soil indicates the compressibility behavior of soil. The compressibility of the soil generally increases with an increase in liquid limit.
- Flow index is the rate at which a soil mass loses its shear strength with an increase in water content.
- Another shear strength measuring index is toughness index. The higher value of the toughness index indicates a greater percentage of colloidal clay.
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