Unified Soil Classification System 

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

Symbol chart

Major divisions Group symbol Group name
Coarse grained soils
more than 50% retained on 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
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 plasticity, fat clay
organic OH organic clay, organic silt
Highly organic soils Pt peat

See also

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.