The value of the toughness index of most soil lies between the:

Explanation:

Toughness Index (\(I_T\))

• It is the ratio of the plasticity index (\(I_P\)) of the soil to the flow index (\(I_F\)) 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.

Additional InformationPlasticity Index (\(I_P\))

• The plasticity index (\(I_P\)) 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.

Flow Index (\(I_F\))

• It is defined as the rate at which a soil mass loses its shear strength with an increase in water content.
• It is obtained by the slope of the curve drawn for Water content v/s Number of blows
• \(I_F = { w_1-w_2\over log{(N_2/N_1)}}\) ; where w=water content, N= No. of blows