What does the plasticity index measure?

The plasticity index is a key indicator in soil testing that measures the range of moisture content over which a soil behaves as a plastic material. It quantifies the difference between the liquid limit and the plastic limit of soil, which are two critical moisture content points that define a soil's plastic behavior. The liquid limit is the moisture content at which soil changes from a plastic state to a liquid state, while the plastic limit is the moisture content at which soil transitions from a semi-solid state to a plastic state.

By calculating the plasticity index as the difference between these two limits, engineers and geologists can assess the clay content of the soil and its potential behavior under varying moisture conditions. This information is vital for understanding how the soil will behave in construction applications, particularly in terms of stability and compressibility. Therefore, the correct choice emphasizes the importance of understanding the plasticity index in evaluating the engineering properties of soil.