SPECIFIC GRAVITY DETERMINATION

OBJECTIVE:
To familiarize the students with general method of obtaining the specific gravity of a mass of any type of material composed of small particles (specifically soil).

THEORY:
A value of specific gravity is necessary to compute the void ratio of a soil, it is used in the hydrometer analysis, and it is useful to predict the unit weight of a soil. Occasionally, the specific gravity may be useful in soil mineral classification; e.g., iron minerals have larger value of specific gravity than silicas.
The specific gravity of any substance is defined as the unit weight of the material divided by the unit weight of distilled water at 4 oC. Thu, specific gravity of soil can be found as;

Gs = (ϒsoil) / (ϒ water)

As long as equal volume of water and soil are involved, the above stated form can be simplified as;

Gs = (( Wsoil ) / V) / (( Wwater) / V )

Strictly speaking above mentioned equation is only valid if we do not consider any density change with temperature. However, a slight increase in precision to account for temperature effects on the density of water can be obtained by rewriting above stated equation as;

Gs = ( Wsoil ) / ( Wwater) x α

Where, α is the ratio of the unit weight of water at temperature T of the test and at 4oC. The value of Gs obtained at temperature T (which will be too large if T > 4^oC) is appropriately reduced.

α = ϒT / ϒ4^oC

APPARATUS

• Pycnometer
• Weighing balance (Least count of 0.01 grams)
• Thermometer
• Hot plate or Bunsen burner
• Funnel
• Drying oven
• Paper towel

PROCEDURE:

• Weigh the dry pycnometer to nearest 0.01 gram and record it as W1.
• Take about 100 grams of oven dried soil and put it into the flask. Weigh the flask and dry soil to the nearest 0.01 gram. Record this weight as W2.
• Add water in the pycnometer until it is about two-third full. In order to remove the entrapped air from soil and water, heat the mixture for at least 2 h after the soil-water mixture comes to a full boil. Use only enough heat to keep the slurry boiling. Agitate the slurry as necessary to prevent any soil from sticking to or drying onto the glass above the slurry surface.
• Allow the mixture to cool, and then fill the flask with distilled water to above the calibration mark.
• Place the stopper in the bottle while removing the excess water. Be sure the entire exterior of the flask is dry. Weigh the flask to the nearest 0.01 gram and record this weight as W3.
• Empty the flask, wash it thoroughly and fill it completely with water. Dry the exterior of the flask. Weigh the flask and record it as W4.
• Repeat the procedure three times.
• Record the temperature of soil water mixture.

Gs = (( W2 - W1 )α ) /((W4 - W1) - (W3 - W2))

PRECAUTIONS

• Make sure no air is entrapped within the soil water mixture.
• Weights should be obtained from a properly balanced weighing scale.

OBSERVATIONS AND CALCULATIONS

Test No.	1	2	3
Volume of flask
W1 (grams)
W2 (grams)
W3 (grams)
W4 (grams)
α
Gs

Typical values of correction factor, α;

T (oC)	Correction Factor, α
4	1.0000
15	0.9999
20	0.9982
25	0.9971
30	0.9957
35	0.9941

REFERENCE

ASTM D854 – 02
Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer

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