Clay soil classification

Clay soil is classified as one of the major soil types based on its particle size and composition. It falls under the category of fine-textured soils and is known for its small particle size and high plasticity when wet. The classification of clay soil is typically based on its clay content and the relative proportions of sand, silt, and clay particles.

The Unified Soil Classification System (USCS) and the AASHTO Soil Classification System are two common methods used to classify clay soil.

In the Unified Soil Classification System, clay soil is designated by the letter “C” followed by a specific symbol depending on its properties:

  1. CL: Clay soil with low plasticity and exhibits low strength when dry.
  2. CH: Clay soil with high plasticity and exhibits high strength when dry.
  3. CL-ML: Silty clay with low plasticity.
  4. CH-CH: Silty clay with high plasticity.

In the AASHTO Soil Classification System, clay soil is classified based on its Atterberg limits, which measure the soil’s liquid limit (LL) and plasticity index (PI). The symbol “A” is used to designate fine-grained soils, including clay soils, and is followed by a two-letter group based on the liquid limit and plasticity index values.

For example:

  • A-7: Highly plastic clay with a liquid limit between 50 and 80 and a plasticity index greater than 25.
  • A-6: Non-plastic silt with a liquid limit less than 50 and a plasticity index of 0.

It’s important to note that clay soil can vary in its properties, and its classification can depend on regional factors and the specific testing methods used. Understanding the classification of clay soil helps in determining its engineering properties, suitable agricultural practices, and the appropriate amendments needed to improve its fertility and drainage.


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