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
Letter Definition
G gravel
S sand
M silt
C clay
O organic
Letter Definition
P poorly graded (uniform particle sizes)
W well-graded (diversified particle sizes)
H high plasticity
L low 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

Major divisions Group symbol Group name
Coarse grained soils
more than 50% retained on or above No.200 (0.075 mm) sieve		 gravel
> 50% of coarse fraction retained on No. 4 (4.75 mm) sieve		 clean gravel <5% smaller than #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 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 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 

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