Determination of Specific Gravity

Specific gravity G is defined as the ratio of the weight of a given volume of soil solids to the weight of a equal volume of distilled water.

It deals with the method of test for determination of specific gravity of soils which finds application in finding out the degree of saturation and unit weight of moist soils. The unit weights are needed in pressure, settlement and stability problems in soil engineering.

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Apparatus

1. Two density bottles (Pycnometer) of approximately 50 ml capacity with stoppers.
2. A water-bath maintained at a constant temperature to within ± 0.20C (If standard density bottles are used, this constant temperature is 27°C.)
3. A vacuum desiccator (a convenient size is one about 200 mm to 250 mm in diameter).
4. A desiccator (a convenient size is one about 200 mm to 250 mm in diameter) containing anhydrous silica gel.
5. A thermostatically controlled drying oven, capable of maintaining a temperature of 105 to 110°C.
6. A balance readable and accurate to 0.001 g.
7. A source of vacuum, such as a good filter pump or a vacuum pump.
8. A spatula (a convenient size is one having a blade 150 mm long and 3 mm wide; the blade has to be small enough to go through the neck of the density bottle), or piece of glass rod about 150 mm long and 3 mm diameter.
9. A wash bottle, preferably made of plastics, containing air-free distilled water.
10. A sample divider of the multiple slot type (riffle box) with 7 mm width of opening
11. A length of rubber tubing to fit the vacuum pump and the desiccator.

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Procedures

1. The complete density bottle with stopper shall be dried at 105 to 110°C, cooled in the desiccator and weighed to the nearest 0.001 g (m1).
2. The 50 g sample obtained as described in the procedure for the preparation of disturbed samples for testing shall, if necessary, be ground to pass a 2-mm IS test sieve.
3. A 5 to 10 g subsample shall be obtained by riffling, and oven-dried at 105 to 1100 C.
4. This sample shall be transferred to the density bottle direct from the desiccator in which it has been cooled.
5. The bottle and contents together with the stopper shall be weighed to the nearest 0.001 g (m2).
6. The stoppered bottle shall then be taken out of the bath, wiped dry and the whole weighed to the nearest 0.001 g (m3).
7. The bottle shall then be taken out of the bath, wiped dry and the whole weighed to the nearest 0.001 g (m4)
8. Two determinations of the specific gravity of the same soils sample shall be made.

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The specific gravity of the soil particles G shall be measured at room temperature. If water has been used as the air-free liquid, then the following equation shall be used

The specific gravity of soil shall be calculated as follows:

$G = \frac{Density of water at {27}^{.}C}{Weight of soil of equal volume}$

$G=\frac{(M_2-M_1)}{(M_4-M_1)- (M_3-M_2)}$

Where,

• G is Specific Gravity
• M₁ is mass of density bottle in gram
• M₂ is mass of bottle and dry soil in gram
• M₃ is mass of bottle, soil and liquid in gram
• M₃ is mass of bottle when full of liquid only in gram

Note: The specific gravity of the soil particles lie with in the range of 2.65 to 2.85. Soils containing organic matter and porous particles may have specific gravity values below 2.0. Soils having heavy substances may have values above 3.0.