Plastic Limit Test

Plastic Limit Test


For determination of the plastic limit of soil.

Reference Standard

IS : 2720(Part 5)-1985- Methods of test for soils : Determination of liquid and plastic limit.

Equipment & Apparatus

  • Oven
  • Balance (0.01 g accuracy)
  • Sieve [425 micron]
  • Flat glass surface for rolling

Preparation sample

After receiving the soil sample it is dried in air or in oven (maintained at a temperature of 600C). If clods are there in soil sample then it is broken with the help of wooden mallet. The soil passing 425 micron sieve is used in this test.


  1. A soil sample of 20 gm. passing 425 micron IS sieve is to be taken.
  2. It is to be mixed with distilled water thoroughly in the evaporating dish till the soil mass becomes plastic enough to be easily moulded with fingers.
  3. It is to be allowed to season for sufficient time, to allow water to permeate throughout the soil mass.
  4. 10 gms. of the above plastic mass is to be taken and is to be rolled between fingers and glass plate with just sufficient pressure to roll the mass into a thread of uniform diameter throughout its length. The rate of rolling shall be between 60 and 90 stokes per minute.
  5. The rolling is to be continued till the thread becomes 3 mm. in diameter.
  6. The soil is then kneaded together to a uniform mass and rolled again.
  7. The process is to be continued until the thread crumbled with the diameter of 3 mm.
  8. The pieces of the crumbled thread are to be collected in a air tight container for moisture content determination.


The Plastic limit is to be determined for at least three portions of soil passing 425 micron IS sieve. The average of the results calculated to the nearest whole   number is to be reported as the plastic limit of the soil.

Safety & Precautions

  • Soil used for plastic limit determination should not be oven dried prior to testing.
  • After mixing the water to the soil sample , sufficient time should be given to permeate the water throughout out the soil mass
  • Wet soil taken in the container for moisture content determination should not be left open in the air, the container with soil sample should either be placed in desiccators or immediately be weighed.


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