Lab Manual for Plastic Limit Determination of Soil

 

Lab Manual for Plastic Limit Determination of Soil

Relevant IS Code: IS: 2720 (Part 5) – 1985

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Objective

To determine the Plastic Limit (PL) of soil, which is the water content at which the soil changes from a plastic state to a semi-solid state.

Apparatus Required

1. Porcelain Dish
2. Spatula
3. Glass Plate (about 10 mm thick)
4. Sieve (425 micron)
5. Weighing Balance (sensitive up to 0.01g)
6. Oven
7. Air-tight Containers for water content determination

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Theory

The Plastic Limit (PL) is the lowest water content at which the soil can be rolled into a thread of 3 mm diameter without breaking. It marks the boundary between the plastic and semi-solid states of soil. It is important for classifying fine-grained soils.

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Procedure

1. Soil Preparation

   • Take about 30g of soil that passes through a 425-micron sieve.
   • Mix the soil thoroughly with distilled water until it becomes pliable enough to be rolled into threads.

2. Rolling of Soil Threads

   • Take a portion of the soil sample and roll it between your fingers and the glass plate.
   • Continue rolling until the soil thread reaches a diameter of 3 mm.
   • If the thread breaks before reaching 3 mm, collect it and repeat the process.

3. Drying the Soil

   • When the soil threads start crumbling at a diameter of 3 mm, collect the crumbled pieces.
   • Place the collected soil pieces in a container and determine their water content using the oven-drying method.

4. Water Content Determination

   • Place the soil sample in the oven at 105°C – 110°C for at least 24 hours.
   • Weigh the soil sample before and after drying to calculate the water content.
   • Use the following formula:
   $$\text{Water Content} (w\%) = \frac{W_{wet} - W_{dry}}{W_{dry}} \times 100$$

   Where,

   • $W_{wet}$ = weight of the wet soil
   • $W_{dry}$ = weight of the dry soil

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Observation Table

Sample No.	Wet Weight (W₁)	Dry Weight (W₂)	Water Content (w%)
1
2
3

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Result

The Plastic Limit (PL) of the given soil sample is calculated by averaging the water content of all three samples.

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Precautions

1. Ensure the soil is thoroughly mixed with water to achieve uniform consistency.
2. Roll the soil gently to avoid any excessive pressure which may cause uneven crumbling.
3. The diameter of the soil thread should be accurately measured to ensure it reaches 3 mm.
4. Place the soil samples in an air-tight container immediately after collection to prevent moisture loss.
5. Dry the soil samples completely in the oven to get accurate water content measurements.

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This completes the procedure for determining the Plastic Limit (PL) of soil according to IS: 2720 (Part 5) – 1985.