What determines the optimum moisture content of soil?

The optimum moisture content of soil is determined through the Proctor test. This test evaluates how soil compacts at different moisture levels by finding the moisture content at which the maximum dry density is achieved. During the Proctor test, soil samples are compacted at various moisture contents, and a curve is created plotting dry density against moisture content. The peak of this curve indicates the optimum moisture content — the specific level at which soil will achieve its greatest density when compacted. This information is critical for activities such as construction and earthworks, as it informs engineers how to best prepare the soil for stability and strength under load.

While other tests, such as the compaction test, explore similar concepts of soil density and behavior under load, they do not specifically determine optimum moisture content. The Atterberg limit test assesses the plasticity characteristics of soil but does not measure moisture content related to compaction. Similarly, the California Bearing Ratio test evaluates the strength of subgrade soil but is not used for determining optimum moisture content.