How to find the Plastic limit of soil, Properties of soil

SCOPE: 

To determine the plastic limit of soil.

 The plastic limit of a soil is the moisture content, expressed as a percentage of the weight of the oven-dry soil, at the boundary between the plastic and semisolid states of consistency.

 IS CODE: 

 IS: 2720 (Part 5) – 1985. 

 APPARATUS: 1.Porcelain evaporating disc 

 2.Spatula 

 3.Balance 

 4.Oven

 5.Ground glass plate

  6.Rod - 3mm dia. and about 10cm long. 

 SHORT PROCEDURE: 

 1. Take out 20g of air-dried soil passing through 425 µm IS Sieve. Mix the soil with distilled water in an evaporating dish. 

 2. Take about 8g of the soil and roll it with fingers on a glass plate. The rate of rolling should be between 80 to 90 strokes per minute to form a 3mm Dia. 

 3. If the dia of the threads can be reduced to less than 3mm, without any cracks appearing, it means that the water content is more than its plastic limit. Knead the soil to reduce the water content and roll it into a thread again 

 4. Repeat the process of alternate rolling and kneading until the thread crumbles 

 5. Collect and keep the pieces of crumbled soil thread in the oven to determine the moisture content. 

 6. Repeat the process at least twice more with fresh samples of plastic soil each time. 

 7. The plastic limit should be determined for at least three portions of the soil passing through 425µm IS Sieve. 

 Note, 

 The plastic limit shall be determined for at least three portion of the soil passing 425 microns IS sieve. The average of the result calculated to the nearest whole number shall be reported as the plastic limit of the soil.