Plastic Limit of Soil


The minimum water content at which soil is in the plastic stage is called plastic limit water content. If a soil sample has water content equals to its plastic limit then it will crumble when rolled into a 3 mm diameter thread. The method to find the plastic limit of soil is based on IS 2720 Part 5.

Procedure:


  1. Take soil sample of 20 gram from the thoroughly mixed portion of material passing through 425-micron sieve.
  2. Mix the soil sample with distilled water thoroughly in an evaporating dish on the glass plate (20 X 15 cm) till the soil mass become plastic enough to be easily moulded with fingers.
  3. Form a ball with about 8 gm of this plastic soil mass.
  4. Roll this ball between fingers and the glass plate with sufficient pressure to roll the mass into a thread of uniform diameter throughout its length.
    Rolling shall be done till the threads of 3 mm diameter.
  5. Form the ball from these 3 mm diameter threads.
  6. Repeat the process of alternate kneading and rolling until the thread crumbles and it can no longer be rolled into a thread.
  7. Collect the crumbled pieces of soil in an airtight container for its water content determination.

The plastic limit shall be determined for at least 3 portions of the soil passing 425 microns IS sieve and the average of these 3 will be the plastic limit of the soil.

Plasticity Index


Plasticity index is the difference of liquid limit and plastic limit.

${I}_{P}={W}_{L}-{W}_{P}$


This property is due to the presence of clay minerals.

Atterberg Limits for soils
Soil Type ${W}_{L}$ ${W}_{P}$ ${I}_{P}$
 Sand  non-plastic
 Silt $ 30-40$ $ 20-25$ $ 10-15$
 Clay $ 40-50$ $ 25-50$ $ 15-100$

Consistency of Soil
${I}_{P}$ Consistency
$ 0$  Non Plastic
$ <7$  Low Plastic
$ 7-17$  Medium Plastic
$ >17$  Highly PLastic


Consistency Index or Relative Consistency


${{I}_{C}}=\frac{{{W}_{L}}-w}{{{I}_{P}}}$


Liquidity Index


${{I}_{L}}=\frac{w-{{W}_{P}}}{{{I}_{P}}}$



Consistency of Soil on the basis of ${I}_{C}$ and ${I}_{L}$
${I}_{C}$ ${I}_{L}$ Consistency
$ >1$ $ <0$  Very Stiff
$ 1-0.75$ $ 0-0.25$  Stiff
 $0.75-0.5$ $ 0.25-0.5$  Medium Stiff
$ 0.5-0.25$  $0.5-0.75$  Soft
$ 0.25-0$ $ 0.75-1$  Very Soft
$ <0$ $ >1$  Liquid State








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