Density of Soils
The density or true weight of a soil is equal to the specific gravity of the solid materials x 1000 (weight or density of water per Cu. m). A soil consists of solids, pores or voids and the moisture. The overall weight of the mass (including solid particles and the effect of voids whether filled with air or water) per unit volume, i.e., total weight of soil ÷ total volume of soil, is termed Bulk Density. Bulk density varies with the type of the soil, moisture content and its compaction. The weight of the dry solid matters contained in a unit volume of soil, i.e., weight of soil particles ÷ total volume of soil, (determined after the water has been dried without bulk volume change) is termed Dry Density.
The usual method of measuring compaction in the field is to determine the dry density of the soil in site. The maximum dry density of a soils is obtained by a specified amount of compaction at the optimum moisture content by the Proctor Compaction Test. For each compaction method, there is an optimum moisture content at which a given soil can be compacted to greatest density, and different soils have different maximum densities and optimum moisture contents. Dry density varies from about 2.00 grams/cu. cm for coarse grained well graded gravels and sands to about 1.45 grams/cu. cm for heavy clays, the corresponding moisture contents being about 4 per cent for the gravel and 26 per cent for the clay. The density of the solids alone is sometimes termed absolute density.