Unified Soil Classification System
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The Unified Soil Classification System (or USCS) is a soil classification system used in engineering and geology disciplines 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 (with the exception of PT):
| First and/or second letters | Definition | Second letter | Definition |
|---|---|---|---|
| G | gravel | P | poorly graded (well sorted) |
| S | sand | W | well graded (poorly sorted) |
| M | silt | H | high plasticity |
| C | clay | L | low plasticity |
| O | organic | ||
Symbol chart
| Major divisions | Group symbol | Group name | ||
|---|---|---|---|---|
| Coarse grained soils more than 50% retained on No.200 sieve | gravel > 50% of coarse fraction retained on No.4 sieve | clean gravel | GW | well graded gravel, fine to coarse gravel |
| GP | poorly graded gravel | |||
| gravel with >12% fines | GM | silty gravel | ||
| GC | clayey gravel | |||
| sand ≥ 50% of coarse fraction passes No.4 sieve | clean sand | SW | well graded sand, fine to coarse sand | |
| SP | poorly-graded sand | |||
| sand with >12% fines | SM | silty sand | ||
| SC | clayey sand | |||
| Fine grained soils more than 50% passes No.200 sieve | silt and clay liquid limit < 50 | inorganic | ML | silt |
| CL | clay | |||
| organic | OL | organic silt, organic clay | ||
| silt and clay liquid limit ≥ 50 | inorganic | MH | silt of high plasticity, elastic silt | |
| CH | clay of high placticity, fat clay | |||
| organic | OH | organic clay, organic silt | ||
| Highly organic soils | PT | peat | ||
References
American Society for Testing and Materials, 1985, D 2487-83, Classification of Soils for Engineering Purposes: Annual Book of ASTM Standards. Vol. 04.08, pp 395-408.
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