Under the Unified Soil Classification System, what specific laboratory test directly distinguishes a silt from a clay with similar grain sizes?

Under the Unified Soil Classification System (USCS), the specific laboratory tests that directly distinguish a silt from a clay with similar grain sizes are the Atterberg Limits. The Atterberg Limits are a series of standardized tests that determine the water content at which fine-grained soils transition between different states of consistency: liquid, plastic, and semi-solid. The two most critical Atterberg Limits for this distinction are the Liquid Limit (LL) and the Plastic Limit (PL). The Liquid Limit (LL) is the minimum water content at which a soil behaves as a liquid and just begins to flow. The Plastic Limit (PL) is the minimum water content at which a soil can be rolled into a 3 mm diameter thread without crumbling, marking the transition from a plastic to a semi-solid state. From these two values, the Plasticity Index (PI) is calculated as the numerical difference between the Liquid Limit and the Plastic Limit (PI = LL - PL). The Plasticity Index represents the range of water content over which the soil exhibits plastic behavior. The direct distinction between silt and clay is then made by plotting the determined Liquid Limit and Plasticity Index values on the Casagrande Plasticity Chart. This chart is a graphical tool specifically designed for classifying fine-grained soils based on their plasticity characteristics. A key feature of the Casagrande Plasticity Chart is the A-line, an empirical boundary defined by the equation PI = 0.73 (LL - 20). Soils plotting above the A-line are classified as clays, indicating higher plasticity and cohesive properties due to their mineralogy and particle interactions. Soils plotting below the A-line are classified as silts, which exhibit lower plasticity and are generally less cohesive. This method effectively distinguishes silts from clays even when they share similar particle sizes because it relies on their fundamental differences in engineering behavior and interaction with water, rather than solely on grain size.