What does the plasticity index measure in soil?

The plasticity index is a key parameter in geotechnical engineering that quantifies the plasticity of a soil, specifically focusing on the range of moisture content over which the soil behaves plastically. It is defined as the difference between the liquid limit and the plastic limit of a soil sample. This measurement reflects the soil's ability to change shape without cracking or breaking when subjected to external forces.

In the context of compressibility, the plasticity index helps engineers evaluate how much the soil will deform under load. Soils with a higher plasticity index can exhibit significant volume changes with moisture variation, which may affect their compressibility. Thus, the plasticity index is directly related to how compressible the soil is under various moisture conditions, making it an important metric for assessing soil behavior in foundations, slopes, and other earthworks.

The other options, while related to soil mechanics, do not directly correlate to what the plasticity index measures. Strength pertains to how much load a soil can bear before failing, permeability refers to the soil's ability to transmit water, and viscosity relates to the resistance of the soil to flow. Each of these characteristics can influence the overall engineering properties of soil, but the plasticity index specifically focuses on the material's plastic behavior and