What is the primary basis for the United Soil Classification System (USCS)?

The United Soil Classification System (USCS) primarily categorizes soils based on their particle size distribution, liquid limit (LL), and plasticity index (PI). This system is essential for understanding the engineering properties of soils, which ultimately influence their behavior under various conditions, such as loading and environmental changes.

The grain size distribution provides insight into the proportions of different sized particles within the soil, which affects the soil's strength and stability. The liquid limit is the water content at which soil changes from a plastic state to a liquid state, while the plasticity index measures the range of moisture content over which the soil remains plastic. Together, these parameters help classify soils into different categories, such as granular or cohesive, which can guide engineers in selecting appropriate materials and construction methods based on the expected performance of these soils in engineering applications.

In contrast, while moisture content and clay mineralogy do influence soil characteristics, they are not the primary basis of the USCS. Similarly, shear strength parameters and compaction efforts are crucial for understanding soil behavior in specific conditions, but they are secondary considerations in the classification process established by the USCS. Thus, the most relevant factors for the primary classification in the USCS are indeed grain size distribution, liquid limit, and plastic