What test is commonly used to establish the moisture-density relationship of soil?

The test commonly used to establish the moisture-density relationship of soil is known as the compaction test, specifically in its most recognized form as the Proctor test. This test is designed to measure how the density of a soil changes with the moisture content. By determining the optimal moisture content at which a soil achieves its maximum dry density, engineers can predict how well the soil will support loads and behave under various conditions.

During the compaction test, soil samples are compacted in a standardized manner, applying specific energy levels, and measuring the resulting density at different moisture contents. This relationship is crucial for construction and earthwork projects as it helps in optimizing soil compaction, which directly affects stability and strength.

The other tests listed, such as the shear strength test, plasticity test, and hydrometer test, although valuable in their respective analyses, serve different purposes. Shear strength tests mainly assess the soil's ability to withstand shear deformation and failure. Plasticity tests evaluate the plastic behavior of soils, including their ability to deform without breaking. Hydrometer tests determine particle size distribution, particularly in fine-grained soils, which does not directly relate to moisture-density relationships.