Specific Gravity of Soils

Specific gravity G (or Gs) is defined as the ratio of the weight of a given volume of soil solids at a given temperature to the weight of an equal volume of distilled water at that temperature, both weights being taken in air. In other words, it is the ratio of the unit weight of soil solids to that of water.

Equation 1: G = γ_s/γ_w

γ_w is the unit weight of water and is 9.81 kN/m³ or 62.4 lb/ft³. The Geotechnical Standard specifies 20°C as the standard temperature for reporting the specific gravity.

Some qualifying words like: true, absolute, apparent, bulk or mass, etc., are sometimes added to the term ‘specific gravity’. These qualifying words modify the sense of specific gravity as to whether it refers to soil particles or to soil mass. The soil solids have permeable and impermeable voids inside them, the permeable voids being capable of getting filled with water. If all the internal voids of soil particles (permeable and impermeable) are excluded for determining the true volume of solids, the specific gravity obtained is called absolute or true specific gravity. The apparent or mass or bulk specific gravity G_m denotes the specific gravity of soil mass and is given by

Equation 2: G_m = γ/γ_w

Unless otherwise specified, we shall denote the Specific Gravity G (or G_s) (defined by Eq. 1) as the specific gravity of soil solids. Table 1 gives the values of specific gravity of some important soil constituents.

Table 1. Specific gravity of soil constituents

Most soils have a rather narrow range of G_s values: 2.65 to 2.70. This implies that solid particle is about 2.65 to 2.70 times heavier than the weight of water for the same volume. If a specific gravity test was not performed during the initial evaluation of geotechnical engineering problems, assuming G_s as a value between 2.65 or 2.70 would not produce a major error in the results.