In the case of Atterberg limits of a soil, the shrinkage index of the soil will be equal to:

Shrinkage Index of Soil:

The shrinkage index (SI) of a soil is a measure of the range of moisture content within which the soil remains in a semi-solid state. It is defined as the difference between the plastic limit (PL) and the shrinkage limit (SL) of the soil. This concept is part of the Atterberg limits, which are used to describe the critical water contents of fine-grained soils.

• Plastic Limit (PL):
  • The plastic limit is the moisture content at which the soil changes from a plastic state to a semi-solid state. It is the lowest moisture content at which the soil can be molded without breaking.
• Shrinkage Limit (SL):
  • The shrinkage limit is the moisture content at which the soil changes from a semi-solid state to a solid state. It is the highest moisture content at which further loss of moisture does not cause a decrease in the volume of the soil.
• Shrinkage Index (SI):
  • The shrinkage index is calculated as the difference between the plastic limit and the shrinkage limit:
  • SI = PL - SL
  • This index provides useful information about the volume change characteristics of the soil as it dries out. Soils with a high shrinkage index have a significant volume reduction when they dry from the plastic limit to the shrinkage limit, indicating a higher potential for shrinkage and cracking.

Thus, the shrinkage index of a soil is a crucial parameter in understanding the behavior of fine-grained soils, particularly in relation to their volume change characteristics under varying moisture content.