Soil is a three-phase material that consists of solid particles and voids which are filled with water and air. The specific gravity (GS) of a soil refers to the ratio of the solid particles’ unit weight to the unit weight of water. GS should not be confused with the soil density since it is a dimensionless unit and expresses the ratio of two particular densities.
GS is a significant parameter of soil mechanics since it can be associated with the soil’s mineral composition and weathering. It is also used to derive several important soil parameters such as the porosity, the dry and saturated density and the degree of saturation.
To derive the Specific Gravity of a soil, the following equipment is needed:
The Specific Gravity is computed as the ratio of the weight in air of a given volume of soil particles at a stated temperature to the weight in air of an equal volume of distilled water at the same temperature. The procedure that is followed towards that goal is the following:
To better understand how the Specific Gravity is calculated, the aforementioned measured quantities are presented in Figure 1.
Based on the temperature of the water that was recorded in the last step, the density of the distilled water ρw is derived through specific tables (i.e., ρw=998.23 kg/m3 at T=20°C)
Therefore, the Specific Gravity GS is calculated as:
A correction is utilized to adjust the results at a reference temperature T=20°C:
where K is the temperature correction factor.
The Specific Gravity of soils is usually between 2.65-2.80 with finer soils having higher values than coarser ones.
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