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 (with the exception of Pt):
| First and/or second letters | Second letter | ||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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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."
Symbol chart
| Major divisions | Group symbol | Group name | ||
|---|---|---|---|---|
| Coarse grained soils more than 50% retained on or above No.200 (0.074 mm) sieve |
gravel > 50% of coarse fraction retained on No.4 (4.75 mm) sieve |
clean gravel <5% smaller than No.200 Sieve | 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 (4.75 mm) 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 50% or more passing the No.200 (0.074 mm) sieve |
silt and clay liquid limit < 50 |
inorganic | ML | silt |
| CL | clay of low plasticity, lean 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 plasticity, fat clay | |||
| organic | OH | organic clay, organic silt | ||
| Highly organic soils | Pt | peat | ||
See also
References
- Classification of Soils for Engineering Purposes: Annual Book of ASTM Standards, D 2487-83, 04.08, 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 0132221381
