Plastic Limit of Soil

The plastic limit signifies the percentage of moisture at which the soil changes, with decreasing wetness, from a plastic to a semi-solid state, or with increasing wetness, from the semisolid to the plastic state. It is the lower limit of the plastic state. It is the moisture content at which a thread of soil can be rolled without breaking until it is only 3 mm in diameter, when it just begins to crumble under pressure exerted by the hand. A small increase in moisture above the PL will destroy cohesion and shear strength of the soil.

Sands, gravel and peat do not possess plasticity and have no plastic limit and cannot be rolled into threads at any moisture content. Clays and colloids possess a high degree of plasticity and silts have only occasionally a PL. Clays have an average PL of 45, colloids 46, and silts 20. Both liquid limits and plastic limits are dependent upon the amount and type of clay present in a soil. A soil with high clay content usually has high liquid and plastic limits and a less cohesive soil gives low figures. The liquid and plastic limit tests attempt to fix the moisture contents at which a clay soil passes from the solid to the plastic state and from the plastic to the liquid state.