What signifies high plasticity in cohesive soils according to the Unified Soil Classification System?

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In the Unified Soil Classification System (USCS), high plasticity in cohesive soils is indicated by the letter "H." This classification is used to denote soils with a high plasticity index, which suggests that the soil has a significant ability to deform without breaking when moisture content changes. Soils classified as high plasticity typically include clays that can retain moisture and become very malleable, making them capable of significant volume change with variations in moisture. This characteristic is important for engineers and geotechnical professionals, as these soils can affect the stability and behavior of structures placed on or in them.

The other classifications, such as "M" for medium plasticity, "L" for low plasticity, and "C" which is not a classification for plasticity but rather indicates a cohesive soil, do not represent high plasticity. Understanding these classifications helps in assessing soil behavior and making informed decisions in construction and earthworks.