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Unified Soil Classification System

System describing soil texture and grain size

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:

Letter	Definition	G	S	M	C	O	PT
gravel
sand
silt
clay
organic
peat

Letter	Definition	P	W	H	L
last=Day, Robert W.	title=Geotechnical engineer's portable handbook : with the 2012 international building code	date=2013	publisher=McGraw-Hill	isbn=978-0-07-178971-4	edition=2nd	location=New York	oclc=837179787}})
well-graded (many different particle sizes)
year=2006	title=D 2487 – 06: Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)	publisher=ASTM International	url=https://lauwtjunnji.weebly.com/uploads/1/0/1/7/10171621/astm_d-2487_classification_of_soils_for_engineering_purposes_unified_soil_classification_system.pdf}}
low liquid limit

If the soil has 5–12% by weight of fines passing a #200 sieve (5% #200

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	well-graded gravel, fine to coarse gravel
poorly graded gravel
gravel with 12% fines	silty gravel
clayey gravel
sand
≥ 50% of coarse fraction passes No.4 (4.75 mm) sieve	clean sand	well-graded sand, fine to coarse sand
poorly graded sand
sand with 12% fines	silty sand
clayey sand
Fine grained soils
50% or more passing the No.200 (0.075 mm) sieve	silt and clay
liquid limit	inorganic	silt
lean clay
organic	organic silt, organic clay
silt and clay
liquid limit ≥ 50	inorganic	elastic silt
fat clay
organic	organic clay, organic silt
Highly organic soils	peat

ASTM D-2487

Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests	Soil Classification
Group Symbol	Group Name
COARSE-GRAINED SOILS
Gravels

Cu 3	GP
Gravels with Fines
Fines classify as ML or MH
Fines classify as CL or CH	GC
Sands
Clean Sands

Cu 3	SP
Sands with Fines
Fines classify as ML or MH
Fines classify as CL or CH	SC
FINE-GRAINED SOILS
Silts and Clays

PI	ML
Organic	Liquid limit—oven dried
Liquid limit—not dried	OL
Silts and Clays
Inorganic
PI plots below "A" line	MH
Organic	Liquid limit—oven dried
Liquid limit - not dried	OH
HIGHLY ORGANIC SOILS	PT

References

;Specific

References

Day, Robert W.. (2013). "Geotechnical engineer's portable handbook : with the 2012 international building code". McGraw-Hill.
(2006). "D 2487 – 06: Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)". [[ASTM International]].
ASTM Standard D2487, 2000, "Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)," ASTM International, West Conshohocken, PA, 2000, DOI: 10.1520/D2487-00, www.astm.org.

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