Plasticity Index of Soil (PI)

It is the numerical difference between the liquid limit and the plastic limit of a soil and indicates the magnitude of the range of the moisture contents over which the soil is in a plastic condition as defined by the tests. The liquid and plastic limits are both dependent on the amount and type of clay in a soil, but the plasticity index is generally only dependent on the amount of clay present. It indicates the fineness of the soil and its characteristics as regards plasticity and cohesiveness, i.e., its power to change shape without altering its volume. The information regarding the type of clay in the soil may be obtained by considering the PI in relation to the LL. A high value of PI indicates excess of clay or colloids. When the PL is equal to or greater than the LL, the PI is zero.


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