The specific gravity of a solid substance is the ratio of the weight of the solid to the weight of an equal volume of water. In dealing with soils, the specific gravity is necessary for certain tests, such as hydrometer analysis. It is also necessary for computations involving volume and weight relationships. The specific gravity of a soil mass can be expressed in one of three different forms as follows:

SPECIFIC GRAVITY OF SOLIDS (G,) is the ratio of the weight in air of a given volume of soil particles to the weight of an equal volume of distilled water, both at a stated temperature. The specific gravity of solids is only applied to that fraction of a soil that passes a No. 4 sieve.

APPARENT SPECIFIC GRAVITY (Ga) is the ratio of the weight in air of a given volume of the impermeable portion of soil particles to the weight in air of an equal volume of distilled water, both at a stated temperature. The impermeable portion of a porous material, such as most large soil grains, includes the solid material plus im-permeable pores or voids within the particles.

BULK SPECIFIC GRAVITY (G~) is the ratio of the weight in air of a given volume of permeable material (including permeable and im-permeable voids) to the weight of an equal volume of distilled water at a stated temperature.

For specific gravity tests, the soil samples may be either disturbed or undisturbed. Care must be taken, however, to ensure that representative samples are obtained. When the sample contains both large and small particles, the sample should be separated on a No. 4 sieve. Then the specific gravity of the fine fraction is determined separately from the coarse fraction. A composite specific gravity for the entire soil sample is then calculated in the manner to be described later.

For samples smaller than the No. 4 sieve, it is easier to begin the test with an ovendried sample. However, some soils, particularly those with high organic content, should be tested at their natural water content; the ovendried weight determined at the end of the test.