How to Estimate the USCS Soil Classification Based on the USDA Textural Triangle
- 1). Pinpoint an intersection near the center of the appropriate textural class for your soil on the USDA textural triangle. Follow the horizontal line to the left to find the percentage of clay, the line that slopes upward to the right for the percentage of silt, and the line that slopes downward to the right for the percentage of sand.
- 2). Classify the soil as "sand" if it contains more than 50 percent sand according to the textural triangle. Further classify it as "clean sand" if it falls within the "sand" or "loamy sand" areas of the textural triangle. The textural triangle does not provide enough information to classify clean sand as well graded or poorly graded.
- 3). Place the soil in the "sand with fines" category if it has more than 50 percent sand but is not "clean sand." Categorize it as "silty sand" or SM if it is in the "sandy loam" area of the textural triangle; categorize it as "clayey sand" or SC if it is in the "sandy clay" or "sandy clay loam" areas.
- 4). Classify the soil as "inorganic silt or clay" if it contains less than 50 percent sand. You cannot determine the liquid limit of the soil based on the textural triangle.
- 5). Categorize the soil as "silt" or "silt of high plasticity" if it is in the "silt," "silt loam," "loam" or "silty clay loam" areas of the textural triangle. Classify the soil as "clay" or "clay of high plasticity" if it is in the "clay," "silty clay" or "clay loam" areas.
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