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:
- Take soil sample of 20 gram from the thoroughly mixed portion of material passing through 425-micron sieve.
- 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.
- Form a ball with about 8 gm of this plastic soil mass.
- 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. - Form the ball from these 3 mm diameter threads.
- Repeat the process of alternate kneading and rolling until the thread crumbles and it can no longer be rolled into a thread.
- 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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