Unified Soil Classification System

The Unified Soil Classification System (USCS) is a soil classification system used in engineering and geology to describe the texture and grain size of a soil. The classification system can be applied to most unconsolidated materials, and is represented by a two-letter symbol. Each letter is described below:

If the soil has 5–12% by weight of fines passing a #200 sieve (5% < P_#200 < 12%), both grain size distribution and plasticity have a significant effect on the engineering properties of the soil, and dual notation may be used for the group symbol. For example, GW-GM corresponds to "well-graded gravel with silt."

If the soil has more than 15% by weight retained on a #4 sieve (R_#4 > 15%), there is a significant amount of gravel, and the suffix "with gravel" may be added to the group name, but the group symbol does not change. For example, SP-SM could refer to "poorly graded SAND with silt" or "poorly graded SAND with silt and gravel."

• AASHTO Soil Classification System
• AASHTO
• ASTM International
• USDA Soil Taxonomy (Soil classification for agricultural purposes)

• Classification of Soils for Engineering Purposes: Annual Book of ASTM Standards, D 2487-83, vol. 04, American Society for Testing and Materials, 1985, pp. 395–408
• Evett, Jack and Cheng Liu (2007), Soils and Foundations (7 ed.), Prentice Hall, pp. 9–29, ISBN 978-0132221382

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