OBJECTIVE

Determination of specific gravity of soil solids by pycnometer method.

REFERENCE STANDARD

IS : 2720 (Part 4) – 1985 – Method of test for soil (Part 4-Grain size analysis)

EQUIPMENT & APPARATUS

The major equipment used in this test is Pycnometer. It is a glass jar of 1 litre capacity that is fitted at its top by a conical cap made of brass. It has a screw type cover as shown.

• Pycnometer
• Sieve (4.75 mm)
• Vaccum Pump
• Oven
• Weighing balance
• Glass rod

PREPARATION SAMPLE

After receiving the soil sample it is dried in oven at a temperature of 105⁰C to 115⁰C for a period of 16 to 24 hours.

PROCEDURE

1. Dry the pycnometer completely and weigh it with its cap (W₁)Take its mass (M₁) to the nearest of 0.1 g.
2. Take about 300 g of oven dried soil passing through 4.75mm sieve into the pycnometer and weigh it again (W₂).
3. Unscrew the cap and add sufficient amount of de-aired water to the Pycnometer so as to cover the soil. Screw on the cap.
4. Shake the pycnometer thoroughly and connect it to the vaccum pump to remove entrapped air for about 10 to 20 minutes.
5. After the air has been removed, fill the pycnometer with water and weigh it (W₃).
6. Clean the pycnometer by washing thoroughly.
7. Fill the cleaned pycnometer completely with water upto its top with cap screw on.
8. Weigh the pycnometer after drying it on the outside thoroughly(W₄).

CALCULATION

The Specific gravity of soil solids (Gs) is calculated using the following equation.

Where

W₁=weight of empty pycnometer

W₂=Weight of pycnometer + oven dry soil

W₃=Weight of pycnometer + oven dry soil + water

W₄=Weight of pycnometer + water full.

Results

Specific gravity of soil at ___⁰C = _____.

Specific gravity of solids is defined as weight of solids in a given volume, to the weight of equivalent volume of water.

Unless specified, the specific gravity values of the soil will be reported at 27-degree Celsius.

The specific gravity of soil particles will come within the range of 2.65 and 2.85. If the soils consist of porous and organic materials, a specific gravity value of less than 2 will be obtained. A specific gravity value greater than 3 will be shown by soils that have heavy substances.

SAFETY & PRECAUTIONS

• Completely dry the Soil grains whose specific gravity is to be determined.
• If on drying soil lumps are formed, they should be broken to its original size.
• Inaccuracies in weighing and failure to completely eliminate the entrapped air are the main sources of error. Both should be avoided.
• While cleaning the glass jar, please be careful as there may be glass grains projecting out and it may tear the skin.
• Make sure, you handle the glass jar and conical cap without falling on your legs or floor. Hence, handle the equipment with care.