Theory

Plastic limit is water content corresponding is an arbitrary limit between plastic and semi solid state of consistency of soil. It is defined as the minimum water content at which soil will just begin to crumble when rolled into a thread approximately 3mm in diameter.

Application

It is resistance to the flow of soil due to more percentage of water in soil by obtaining plastic limit. We can determine suitability of soil in various earthen construction such as dam, retaining wall etc.

Items required

1.   Mechanical liquid, limit device, consistency of brass cup and cutting mounted on base.

2.    Grooving tools.

3.    Flexible spatula with blade about 8cm long and 2cm width.

4.    Balance.

5.     Dessicator.


Procedure

1.  Take about 20gm of air tight soil from the thoroughly mixed proportion of material passing through 4.25mm I.S. sieve mix it on the marble plate with sufficient distill water to make it plastic enough to be shaped into a rolled . the plastic soil mass for sometimes to mature in same plate place in plastic soil mass may be left to stand for 24hrs to allow water to permit through out the soil mass.

2.   Take about 8gms of plastic soil make a ball of it rolled it on the marble plate with a hand with just sufficient pressure to roll the mass into thread of uniform diameter of a thread as decreased to 3mm. The specimen is kneaded together a rolled out again continuous the process until the thread just crumbled at 3mm diameter.

3. Collected the crumbled soil thread in the air tight container, keep it for water content determination.

4.    Also determine natural water content of soil sample obtain from the field.


Diagram



Precaution

1.   The figure pressure should not be applied in the excess while rolling.

2. The thread should not be rolled further one they had started crumbling.

3.  The role of rolling the ball should be not increased 80 to 90 strokes/min.


Observation table


No. of blows
33
19
22
Mass of container + wet soil
30.29
31.39
31.87
Mass of container + dry soil
29.75
28.75
29.27
Mass of water
0.54
2.64
26
Mass of container
24.01
22.79
23.42
Mass of dry soil
25.74
5.96
5.85
Water contents
9.40
44.29
44.1


Calculation

Natural water content of field soil (w) = 32.8 %

1.      Plastic limit (wp) = 32.6 (since w2 = 48.9 %)

2.    Plasticity index (IP) = wL – wp = 48.5 – 32.6 = 15.9

3.    Liquidity index IL = w – wp / IP = 32.8 – 32.6 / 15.9 = 0.012

4.    Consistency index IC = wL – w / IP = 48.5 – 32.8 / 15.9 = 0.987

5.     Flow index IP = 24.5

6.    Toughness index = IP / IF = 24.5 / 15.9 = 15.4


Result

Following are values of different terms.

1.      Natural water content of field soil = 32.8 %

2.    Plastic limit = 32.6

3.    Plastic index = 15.9

4.    Consistency index = 0.987

5.     Liquidity index = 0..012

6.    Flow index = 0.012


7.     Toughness index = 1.54