Soil Classification Methods

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Soil Classification Methods
The Unified Soil Classification System identifies soil types for construction, gardening and drilling operations. (Jupiterimages/Comstock/Getty Images)

Soil is a single substance or a mixture consisting of gravel, sand, clay, silt or organic matter. Soil scientists use the Unified Soil Classification System to describe and classify soil. The benefit of using the USCS to classify your site soil is that anyone who uses the information for gardening, drilling, or construction on the property will understand the data because USCS is a standardised classification method.

Sieve Test

A sieve test is the first step in soil classification. This test determines the size of soil grains. Set up stacking sieves using as many sieves as you like, but include a number four and a number 200 sieve. Stack the sieves with the largest openings, number four, on the top and the smallest openings, number 200, on the bottom. Pour the soil into the top sieve and shake the sieves allowing material to pass through the mesh. Soils that do not pass through the number four sieve are gravel. Soils that do not pass through the number 200 sieve are sands. Clay and silt soil will pass through the number 200 sieve.


Sands and gravels are coarse-grained soil particles, while clay and silt are fine-grained soil particles. Review the sieve test results to determine if the soil is a single coarse-grained substance, called "clean," or a mixture of two or more soil particle sizes. If none of the soil passes through the number four sieve, classify the soil as clean gravel. If most of the soil is retained in the number 200 sieve, but some passes through, the result is sand with fines, or a mixture of sand, silt and clay. Follow the USCS chart to identify the group symbol for the soil type. You can purchase the USCS standard and identification chart from The standard is ASTM D 2487.

Grade Coarse-Grained Particles

Classify coarse-grained gravels and sands as either well graded or poorly graded. A well-graded gravel or sand has the same grain size throughout the sample. A poorly graded gravel or sand has a variety of grain sizes throughout the sample.

Identify Silts, Clays and Organics

Silt, clay and organic materials are fine-grained particles. Silt particles are slightly larger than clay and will feel grainy between your fingers. Observe if silt or clay has organic material intermixed. The final classification is typically an organic or inorganic silt or clay. If you are relying on the data for construction, submit a sample to a geotechnical laboratory to determine the liquid limit of the soil. Use the USCS chart to identify the group symbol for the soil type.

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