What specific engineering characteristic does the Plasticity Index primarily define for fine-grained soils?

The Plasticity Index (PI) primarily defines the range of water content over which a fine-grained soil exhibits plastic behavior, often referred to as the soil's degree of plasticity. Fine-grained soils are those composed predominantly of silt and clay particles, whose engineering properties are significantly influenced by their interaction with water. Plastic behavior is the ability of a soil to undergo continuous deformation without fracturing or crumbling, maintaining its molded shape under applied stress....