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 (0.074 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

ASTM D-2487

Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests				Soil Classification
				Group Symbol	Group Name
COARSE-GRAINED SOILS More than 50% retained on No.200 Sieve	Gravels More than 50% of coarse fraction on No. 4 Sieve	Clean Gravels Less than 5% fines	CU >= 4 and 1 <= Cc <= 3	GW	Well-graded gravel
			CU < 4 and 1 > Cc > 3	GP	Poorly graded gravel
		Gavels with Fines More than 12% fines	Fines classify as ML or MH	GM	Silty Gravel
			Fines classify as CL or CH	GC	Clayey gravel
	Sands 50% or more of coarse fraction passes No.4 sieve	Clean Sands Less than 5% fines	CU >= 6 and 1 <= Cc <= 3	SW	Well-graded sand
			CU < 6 and 1 > Cc > 3	SP	Poorly graded sand
		Sands with Fines More than 12% fines	Fines classify as ML or MH	SM	Silty sand
			Fines classify as CL or CH	SC	Clayey sand
FINE-GRAINED SOILS 50% or more passes the no. 200 Seive	Silts and Clays Liquid limit less than 50	inorganic	PI > 7 and plots on or above "A" line	CL	Lean clay
			PI < 4 and plots below "A" line	ML	Silt
		organic	Liquid limit - oven dried < 0.75	OL	Organic clay
			Liquid limit - not dried	OL	Organic silt
	Silts and Clays Liquid limit 50 or more	inorganic	PI plots on or above "A" line	CH	Fat clay
			PI plots below "A" line	MH	Elastic silt
		Organic	Liquid limit - oven dried < 0.75	OH	Organic clay
			Liquid limit - not dried	OH	Organic silt
HIGHLY ORGANIC SOILS				PT	Peat

See also

• AASHTO Soil Classification System
• AASHTO
• ASTM International

References

• Classification of Soils for Engineering Purposes: Annual Book of ASTM Standards, D 2487-83, vol. 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