Because the plasticity index comprehensively reflects various important factors affecting the characteristics of cohesive soil to a certain extent. The greater the plasticity index, the finer the soil particles, the larger the specific surface area, the higher the content of clay particles or hydrophilic minerals (such as montmorillonite), and the larger the water content range of soil in plastic state. In other words, plasticity index can comprehensively reflect the influence of mineral composition and particle size of soil. Therefore, cohesive soils are often classified according to plasticity index in engineering. Silty soil is soil with plasticity index less than or equal to 10, and particle content greater than 0.075 does not exceed 50% of the total mass; Cohesive soil refers to soil with plasticity index greater than 10, particle content greater than 0.075 and more than 50% of the total mass, in which:

Ip> 17 clay

Ip> 10 silty clay

Ip< 10 or Ip= 10 sludge.

The ratio of the difference between liquid limit and natural water content of soil and plasticity index is called natural consistency. The characteristics of adsorption and water binding capacity of reactive soil include liquid limit, plastic limit and plastic index. Among these three indexes, liquid limit and plasticity index are more closely related to the engineering properties of soil and have stronger regularity. Therefore, plastic index or liquid limit plus plastic limit index is often used as the classification index of fine-grained soil at home and abroad.