Unified Soil Classification System

The Unified Soil Classification System (or USCS) is used in engineering, geology and soil science disciplines to describe the texture and grain size of a soil. The classification system can be applied to any unconsolidated material, and is usually represented by a two-letter symbol.

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 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 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.