Unified Soil Classification System

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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 lettersSecond letter
Ppoorly graded (uniform particle sizes)
Wwell-graded (diversified particle sizes)
Hhigh plasticity
Llow plasticity

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[edit]

Major divisionsGroup symbolGroup name
Coarse grained soils
more than 50% retained on or above No.200 (0.075 mm) sieve
> 50% of coarse fraction retained on No. 4 (4.75 mm) sieve
clean gravel <5% smaller than #200 SieveGWwell-graded gravel, fine to coarse gravel
GPpoorly graded gravel
gravel with >12% finesGMsilty gravel
GCclayey gravel
≥ 50% of coarse fraction passes No.4 sieve
clean sandSWwell-graded sand, fine to coarse sand
SPpoorly graded sand
sand with >12% finesSMsilty sand
SCclayey sand
Fine grained soils
50% or more passing the No.200 sieve
silt and clay
liquid limit < 50
CLclay of low plasticity, lean clay
organicOLorganic silt, organic clay
silt and clay
liquid limit ≥ 50
inorganicMHsilt of high plasticity, elastic silt
CHclay of high plasticity, fat clay
organicOHorganic clay, organic silt
Highly organic soilsPtpeat

See also[edit]


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