What is the plasticity index defined as?

The plasticity index is a crucial parameter in soil classification that quantifies the plasticity of a soil. It is defined as the difference between the moisture content at the liquid limit and the moisture content at the plastic limit. This index measures the range of moisture content over which the soil behaves in a plastic manner, indicating how much water can be added to a soil before it transitions from a plastic state to a liquid state.

The plasticity index provides insight into the behavior of soils under varying moisture conditions. A higher plasticity index signifies a greater range of water content over which the soil remains plastic, indicating that the soil may exhibit more significant changes in consistency and strength with changes in moisture.

In contrast, the other options focus on specific moisture contents or averages that do not encapsulate the behavior indicated by the plasticity index. The moisture content at the plastic limit signifies the threshold where the soil transitions from a plastic to a semi-solid state, while the moisture content at the liquid limit represents the stage where the soil transitions to a liquid. The average of these two values does not reflect the true measure of how the soil reacts plastically between these states. Therefore, understanding the plasticity index provides valuable information for predicting soil behavior in various engineering applications.