Plastic Limit Test (Pl)

 

Plastic limit is defined as the minimum moisture content at which the soil remains in a plastic state. Plasticity index (PI) is defined as the numerical difference between the liquid limit and plastic limit. Plastic index thus indicates the range of moisture content over which the soil is in a plastic condition. Plastic limit is the moisture content at which a soil when rolled in to thread of smallest diameter possible starts crumbling and has a diameter of 3 mm.

Testing procedure

The dry soil that passing 0.425 mm test sieve is mixed with distilled water until it becomes sufficiently plastic to be mould into a ball.

Part of the soil sample is formed into a thread, about 6 mm in diameter between the first finger and thumb of each hand.

The thread is then rolled on a glass plate until its diameter is reduced to about 3 mm.

The thread is then folded and rolled repeatedly until the 3 mm thread begins to crumble.

All threads are collected into the container and placed on the oven for drying.

The dry samples are weighed for the moisture content of soil.

Then percentage of the moisture content is determined from the above reading. Similarly, the moisture content to another sample is determined and calculated the average value of percent of the moisture content, which is defined as plastic limit.

Calculation