TO DETERMINE THE PLASTIC LIMIT OF A SOIL SPECIMEN

Theory:

The plastic limit of a soil is the water content of the soil below which it ceases to be plastic. It begins to crumble when rolled into threads of 3mm diameter.

Equipment:

1. Porcelain evaporating dish about 120mm diameter or a glass plate 450mm square and 10mm thick.

2. Ground glass plate about 200mm x 150mm

3. Metallic rod 3mm dia and 100mm long

4. Oven

5. Spatula or plate knife

6. Moisture content can

Procedure:

1. Take about 30g of air dried soil from a thoroughly mixed sample of the soil passing 425 sieve.

2. Mix the soil with distilled water in an evaporating dish or on a glass plate o make it plastic enough to shape into a small ball.

3. Leave the plastic soil mass for some time for maturing. For some fat clay, this period may be even upto 24 hours.

4. Take about 8g of the plastic soil, and roll it with fingers on a glass plate. The rate of rolling should be about 80 to 90 strokes per minute to form a thread of 3mm diameter counting one stroke when the hand moves forward and backward to the starting point.

5. If the diameter of the thread becomes less than 3mm without cracks, it shows that the water content is more than plastic limit. Knead the soil to reduce the water content and roll it again into thread. Repeat the process of alternate rolling and kneading until the tread crumbles and the soil can no longer be rolled into thread.

Note: If the crumbling occurs when the thread has a diameter slightly greater than 3mm it may be taken as plastic limit, provided the soil had been rolled into a thread of 3mm diameter immediately before kneading. Do not attempt to produce failure exactly at 3mm diameter.

6. Collect the pieces of the crumbled soil thread in a moisture content container.

7. Repeat the procedure at least twice more with a fresh samples of plastic soil each time.

Fig: Determination of Plastic Limit

DATA SHEET FOR PLASTIC LIMIT TEST

 Sl. No. Observations and Calculations Determination No. 1 2 3 Observation 1 Moisture content container number 2 Mass of empty container () 3 Mass of container + wet soil () 4 Mass of container + dry soil () Calculations 5 Mass of water = 6 Mass of dry  soil = 7 Water content = w = [(5) / (6)] x 100

Result:

Plastic limit of the soil = _____%