Question

A fine-grained soil has a Liquid Limit of 60% and a Plastic Limit of 25%. What is its classification based on the Unified Soil Classification System&#x27;s plasticity chart alone?

Accepted Answer

The classification of a fine-grained soil based on the Unified Soil Classification System&#x27;s (USCS) plasticity chart involves determining its Plasticity Index (PI) and Liquid Limit (LL) and then locating this point on the chart relative to key boundary lines. The Liquid Limit (LL) is the water content, expressed as a percentage, at which a soil transitions from a plastic state to a liquid state. For this soil, LL is given as 60%. The Plastic Limit (PL) is the water content, also expressed as a percentage, at which a soil begins to crumble when rolled into a 3 mm diameter thread, marking the lower boundary of its plastic state. For this soil, PL is 25%. The Plasticity Index (PI) is calculated as the difference between the Liquid Limit and the Plastic Limit (PI = LL - PL). It quantifies the range of water content over which the soil exhibits plastic behavior. For the given values, PI = 60% - 25% = 35%. With LL = 60% and PI = 35%, these values are plotted on the USCS Plasticity Chart, which has the Liquid Limit on the horizontal axis and the Plasticity Index on the vertical axis. The classification is determined by the soil&#x27;s position relative to the A-line and the LL = 50% line. The A-line is defined by the equation PI = 0.73 (LL - 20). This line serves to distinguish between clays (plotting above the A-line) and silts or organic soils (plotting below the A-line). For an LL of 60%, the A-line PI value is calculated as 0.73 (60 - 20) = 0.73 40 = 29.2%. Since the soil&#x27;s calculated PI (35%) is greater than the A-line PI (29.2%), the soil plots above the A-line, indicating it is a clay. The LL = 50% line is a vertical boundary on the chart. Soils with a Liquid Limit less than 50% are considered to have low plasticity, while soils with a Liquid Limit equal to or greater than 50% are considered to have high plasticity. With an LL of 60%, which is greater than 50%, this soil exhibits high plasticity. Combining these findings, the soil plots above the A-line (clay) and has a Liquid Limit greater than 50% (high plasticity). Therefore, based on the USCS plasticity chart alone, the soil is classified as CH, which represents a high plasticity clay. The U-line, defined by PI = 0.90 (LL - 8), serves as an approximate upper limit for natural soil plasticity. For an LL of 60%, the U-line PI is 0.90 (60 - 8) = 0.90 52 = 46.8%. As the soil&#x27;s PI (35%) is below the U-line (46.8%), the plasticity values are within the expected range for soils.

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A fine-grained soil has a Liquid Limit of 60% and a Plastic Limit of 25%. What is its classification based on the Unified Soil Classification System's plasticity chart alone?