Plastic limit test is one of the laboratory tests used internationally to differentiate or classify soils into groups. As the starting procedures of liquid limit test and plastic limit test are common, the both tests are executed at the same time in most of the purposes. The Swedish Scientist, A. Atterberg defined the plastic limit as, boundary of soil between the plastic and solid state.
The boundary of soil between the plastic and solid state means, the limit at which the soil starts to behave as a plastic. From this it’s clear that the water content or moisture of the soil determines that boundary level. When the water content or moisture content of soil increases beyond a limit then the soil starts to behave as liquid. As the same when the water content or moisture content of soil decreases beyond a limit then the soil start to behave as solid. Here, in the above statements the “a limit” can refer as the plastic limit of that soil. So the plastic limit of a soil is the statement of water or moisture content as a percentage of its dried weight.

Plastic limit test procedures

Apparatus needed to carry out the test are, laboratory porcelain dish, spatula, soil sample, balance to an accuracy of 0.01g, wash bottle with distilled water, drying oven, glass plate and similar weight moisture cans. The following procedures are followed step by step.

  •  Before start on with soil sample, measure the weights of moisture cans (for easy identification name the cans as A, B, C or 1, 2, 3) and record the weights with respective to their names on a sheet.
  • Take the soil sample which passed the no. 40 sieve pan and add water from wash bottle little by little while mixing the sample with spatula.
  • Form a ball from the watered soil sample using your palms and the ball should be non sticky to your palms.
  • Then form a uniform thread from the obtained soil ball by rolling it on a glass plate using your palms or fingers. In this step you have to provide enough pressure by exerting 90 strokes per minutes – here, a stroke means forward and backward movement of your hand from the starting position.
  • The thread should be roll until it achieves 3mm or 1/8 inches of diameter. Then the thread should be break into pieces and repeat the same procedures of above two steps for those broken pieces.
  • The procedure can be repeated until the rolling thread crumbles.
  • Then measure the weight of the crumbled soil which is collected in the moisture cans (A, B, C or 1, 2, 3).
  • Finally keep the moisture can (remove the lids) in oven for not less than sixteen hours to dry them well.
  • Obtain the dry weights of all soil samples separately by measuring them after heating to a temperature of 115 degree Celsius.
  • Then follow the following procedure to calculate the moisture content of each can (A, B, C or 1, 2, 3)
plastic limit test equation

Finally, calculate the plastic limit by calculating the average water or moisture content of samples.


Example calculation

Sample no 1 2
Mass of empty, clean can + lid (grams) 7.78 15.16
Mass of can, lid, and moist soil (grams) 16.39 21.23
Mass of can, lid, and dry soil (grams) 15.28 20.43
Mass of soil solids (grams) 7.5 5.27
Mass of water  or moisture(grams) 1.11 0.8
Water content, w% 14.8 15.1

Therefore, here the plastic limit is, (14.8 + 15.1) / 2 = 14.95.