Plastic Limit

  

DETERMINATION OF PLASTIC LIMIT  

 

 

 

STANDARDS

·       IS: 2720 (Part-5) 1985. 

OBJECTIVE

·       Plastic limit is defined as minimum water content at which soil remains in plastic state.

APPARATUS

·       Porcelain evaporating dish about 12cm in diameter.

·       Flat glass plate 10mm thick and about 45cm square or longer.

·       Spatula flexible with the blade about 8cm long and 2cm in wide.

·       Ground glass plate 20 x 15 cm.

·       Airtight containers.

·       Balance of capacity 500grams and sensitivity 0. 01gram.

·       Thermostatically controlled oven with capacity up to 250 ⁰C.

·       Rod 3mm in diameter and about 10cm long. 

PROCEDURE

·       Take representative soil sample of approximately 20g from the portion of the material passing 425 micron IS sieve and mix thoroughly with distilled water in an evaporating dish till the soil mass becomes plastic enough to be easily molded with fingers.

·       In the case of clayey soils, leave the soil mass to stand for 24 hours to ensure uniform distribution of moisture through out the soil.

·       Form a ball with about 8 grams of this soil mass and roll between the fingers and the glass plate as shown in Fig: 2.7.1 with just sufficient pressure to roll the mass into a thread of uniform diameter throughout its length.

·       The rate of rolling shall be between 80 and 90 strokes/minute counting the stroke as one complete motion of the hand forward and back to the starting position again.

·       Continue the rolling till the thread crumbles exactly at 3mm diameter.

 

 

·       If the soil thread doesn’t crumble exactly at 3mm knead the soil together to a uniform mass and roll it again.

·       Continue this process of alternate rolling and kneading until the thread crumbles under the pressure exactly at 3mm diameter.

 

·       Collect the pieces of crumbled soil thread in an airtight container and determine its moisture content.

·       Determine the plastic limit for at least two points of the soil passing 425 micron IS sieve.

REPORT

·       Report the individual and the mean of the results as the plastic limit of the soil to the nearest second decimal.

PRECAUTIONS

·       At no time shall an attempt be made to produce failure at exactly 3mm diameter by allowing the thread to reach 3mm then reducing the rate of rolling or pressure or both and continuing the rolling with out further deformation until the thread falls a part.