SOIL MECHANICS : Specific Gravity Of Soil

For determination of specific gravity of soil solids by pycnometer method.

§ Pycnometer

§ Sieve(4.75 mm)

§ Vacuum pump

§ Oven

§ Weighing balance

§ Glass rod

1. Dry the pycnometer and weigh it with its cap(W₁)

2. Take about 200 g to 300 g of oven dried soil passing through 4.75mm sieve into the pycnometer and weigh again(W₂)

3. Add water to cover the soil and screw on the cap.

4. Shake the pycnometer well and connect it to the vacuum 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 up to its top with cap screw on.

8. Weigh the pycnometer after drying it on the outside thoroughly (W₄).

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

Where

W₁=Empty weight of pycnometer

W₂=Weight of pycnometer + oven dry soil

W₃=Weight of pycnometer + oven dry soil + water

W₄=Weight of pycnometer + water full

The result of the specific gravity test is reported to the nearest two digits after decimal.

§ Soil grains whose specific gravity is to be determined should be completely dry.

§ 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.

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