The plastic limit, also known as the lower plastic limit, is the water content at which a soil changes from the plastic state to a semisolid state. A plastic limit set is an apparatus that is used to determine the plastic limit of soil.

The change from a semisolid state to a plastic state of soil is a gradual process. According to a standard empirical definition, the plastic limit is the water content at which a soil passing a 0.425mm sieve can be rolled into threads one-eighth inch in diameter without the soil breaking into pieces.

Generally, the terms plastic limit and liquid limit are used together for soil identification and classification.

Features of Plastic Limit Set

The entire set consists of the following :

• One glass plate having ground ends and one side frosted
• One flexible spatula
• One stainless steel or brass rod comparator
• One plastic wash bottle
• A set of moisture containers
• One porcelain basin
• One graduated cylinder
• One mixing dish

Procedure of determining Plastic Limit

• First of all a soil paste is prepared and a part of it is placed on the glass mixing plate.
• The soil is allowed to dry to such an extent that it becomes plastic and can be shaped into a ball.
• The ball is then moulded by pressing gently between fingers and palms so that it dries up and cracks appear on the soil.
• The soil is then divided into two sub-samples and separate determination process is carried out for both the samples. Moisture is evenly spread on both soil samples by pressing them between fingers. A thread is formed of about 6mm diameter between the first finger and thumb of each hand.
• The thread is rolled between the fingers, from the finger-tip to the second joint, of one hand and the surface of the glass rolling plate.

The process is repeated until the thread shears both longitudinally and transversely when it has been rolled to about 3mm diameter, as gauged by a rod.

Application

Plastic limit set finds application the following areas :

• Agriculture
• Construction (Bridges, Dams, Buildings)
• Mining