STANDARDS
- IS: 2720 (Part-5) 1985.
OBJECTIVE
- Plastic limit is defined as minimum water content at which thread of the soil starts crumbling when it reaches at 3 mm state during rolling.
APPARATUS
- Porcelain evaporating dish about 12cm in diameter.
- Flat glass plate 10mm thick and about 45cm square or longer.
- Spatula flexible with the blade about 8cm long and 2cm in wide.
- Ground glass plate 20 x 15 cm.
- Airtight containers.
- Balance of capacity 500 grams and sensitivity 0. 01 gram.
- Thermostatically controlled oven with capacity up to 250 °C.
- Rod 3 mm in diameter and about 10 cm long.
PROCEDURE
- Take representative soil sample of approximately 20g from the portion of the material passing 425 micron IS sieve and mix thoroughly with distilled water in an evaporating dish fill the soil mass becomes plastic enough to be easily molded with fingers.
- Form a ball with about 8 grams of this soil mass and roll between the fingers and the 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 80 and 90 strokes/minute counting the stroke as one complete motion of the hand forward and back to the starting position again.
- Continue the rolling fill the thread crumbles exactly at 3 mm diameter.
- If the soil thread doesn’t crumble exactly at 3 mm knead the soil together to a uniform mass and roll it again.
- Continue this process of alternate rolling and kneading until the thread crumbles under the pressure exactly at 3 mm diameter.
- Collect the pieces of crumbled soil thread in an airtight container and determine its moisture content.
- Determine the plastic limit for at least two observations of the soil passing 425 micron IS sieve.
REPORT
- Report the individual and the mean of the results as the plastic limit of the soil to the nearest second decimal.
PRECAUTIONS
- At no time shall an attempt be made to produce failure at exactly 3mm diameter by allowing the thread to reach 3 mm then reducing the rate of rolling or pressure or both and continuing the rolling with out further deformation until the thread falls a part.
PLASTICITY INDEX
DEFINITION
- The plasticity Index is defined as the numerical difference between its Liquid Limit and Plastic Limit of soil.
REPORT
- Plasticity Index = Liquid Limit – Plastic Limit.
PRECAUTIONS
- In the case of sandy soils plastic limit should be determined first.
- When plastic limit cannot be determined the Plasticity Index should be reported as NP (Non-Plastic).
- When the plastic limit is equal to or greater than liquid limit, the plasticity index shall be reported as zero.
DETERMINATION OF LIQUID LIMIT AND PLASTIC LIMIT
IS:2720 (Part 5)1970
| Description |
Liquid Limit |
Plastic Limit |
||||
| Observation No. |
1 |
2 | 3 | 4 | 1 |
2 |
| No. of Blows | ||||||
| Container No. | ||||||
| Weight of container(W1) | ||||||
| Weight of container +wet soil(W2) | ||||||
| Weight of container +wet soil(W3) | ||||||
| Weight of Water(W2-W3) | ||||||
| Weight of dried soil(W3-W1) | ||||||
| Water Content=
( (W2-W3)/(W3-W1))x100 |
||||||
| Result from Graph | |
| Liquid Limit(%) | |
| Plastic Limit(%) | |
| Plasticity Index(%) | |
Specified Limits
| Activity | L.L.(Max) | P.I.(Max.) |
| EMB./SGD | 70 | 45 |
| GSB | 25 | 6 |
| WMM | – | 6 |