For the construction of highways, airports, and other structures, it is often necessary to compact soil to improve its strength. Proctor (1933) developed a laboratory compaction test procedure to determine the maximum dry unit weight of compaction of soils which can be used for specification of field compaction.

Compaction is the process of densification of soil mass by reducing the air voids. The purpose of laboratory compaction test is to determine the proper amount of water at which the weight of the soil grains in a unit volume of the compacted soil is maximum and this value is thus called the Optimum Moisture Content (OMC). In the laboratory, the different values of moisture contents and the resulting dry densities obtained after compaction are plotted, the former as abscissa and the latter as ordinate. The points thus obtained are joined together in a curve. The maximum dry density and the corresponding OMC are noted from the curve.

Wet density of the compacted soil is calculated as: γ_t = (w₁ - w₂) / V

where w₁ = Weight of mould with moist compacted soil, w₂ = Weight of empty mould, V = Volume of mould

The dry density of the soil shall be calculated as: γ_d = Wet density of compacted soil (γ_t) / (1 + Water content of soil (w)).