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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