Plastic Limit Test of Soil | IS: 2720 (Part 5): 1985

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Update: March 06, 2023

PLASTIC LIMIT TEST OF SOIL, IS:  2720 (Part 5): 1985

Scope: To determine the plastic limit of soils.

What is The Plastic Limit of Soil?
Plastic limit test is one of the laboratory tests used internationally to differentiate or classify soils into groups. The plastic limit of soil is the moisture content or water content at which soil begins to behave as a plastic material and below which it stop to behave like a plastic material.  It starts to crumble when rolled in threads of 3 mm diameter. The plastic limit of soil is expressed as a percentage of the weight of the oven-dry soil, at the boundary between the plastic and semisolid states of consistency. i.e. Plastic limit is the water content at the change from a plastic to a semisolid state.

IS Code For Determination of Plastic Limit of Soil:-
  • IS:  2720 (Part 5): 1985

Apparatus Required For The Plastic Limit of Soil Test:-
  • Porcelain Evaporating Dish
  • Surface for Rolling
  • Spatula - flexible, with the blade about 8 cm long and 2 cm wide
  • Balance - sensitive to 0.01 g.
  • Oventhermostatically controlled with interior of non-corroding material to maintain the temperature between 105ºC & 115ºC.
  • Non corroding containers
  • Rod3 mm in diameter and about 10 cm long.

Procedure of The Plastic Limit of Soil Test:-
  1. The soil sample shall be mixed thoroughly with distilled water in an evaporating dish or on the flat glass plate till the soil mass becomes plastic enough to be easily moulded with fingers.
  2. A ball shall be formed with about 8g of this plastic soil mass and rolled between the fingers and the glass plate with just sufficient pressure to roll the mass into a thread of uniform diameter throughout its length.
  3. The rate of rolling shall be between 80 and 90 strokes/min counting a stroke as one complete motion of the hand forward and back to the starting position again.
  4. The rolling shall be done till the threads are of 3 mm diameter.
  5. The soil shall then be needed together to a uniform mass and rolled again.
  6. This process of alternate rolling and kneading shall be continued until the thread crumbles at exactly 3 mm diameter under the pressure required for rolling and the soil can no longer be rolled into a thread.

The pieces of crumbled soil thread shall be collected in an air-tight container and the moisture content determined.

Formula to Calculate Plastic Limit of Soil:-

The moisture content determined is the plastic limit of the soil.

The results of the observation of the plastic limit of soil test shall be recorded suitably. The plastic limit should be reported to the nearest whole number.

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