Correct option is A
Toughness Index (IT)
It is the ratio of the plasticity index (IP) of the soil to the flow index (IF) of the soil.
It varies between 0 to 3.
This gives us an idea of the shear strength of soil at its plastic limit.
When the toughness index is less than 1, the soil is said to be friable, which means it can be easily crushed at the plastic limit.
Plasticity Index (IP)
The plasticity index (IP) is the range of water content over which the soil remains in the plastic state.
Plasticity Index = Liquid Limit – Plastic Limit.
It is an important index property of fine-grained soil, especially clayey soils. The plasticity index is maximum for clayey soil.
