What characteristic is primarily used to measure the amount of soil compaction?

The primary characteristic used to measure the amount of soil compaction is the density of the soil. Soil density refers to the mass of soil per unit volume and is crucial in determining how well the soil particles are packed together. When soil is compacted, its density increases, which generally leads to improved load-bearing capacity and reduced settlement under loads.

This measurement is vital in construction and excavation projects because higher density typically corresponds with greater stability and strength of the soil, ensuring that structures built on it will be secure and durable. Other factors like moisture content, particle size, and plasticity index do influence the compaction process and the resulting density, but they are not direct measures of the compacted state of soil itself. Moisture content affects the workability and effectiveness of compaction, while particle size influences how tightly soil can be packed. The plasticity index provides insight into the behavior of soil when it changes states but does not measure compaction directly. Hence, density stands out as the primary indicator of soil compaction.