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:

First and/or second letters	Second letter
Letter Definition G gravel S sand M silt C clay O organic PT peat	Letter Definition P poorly graded (many particle of about the same size) W well-graded (many different particle sizes) H high liquid limit L low liquid limit
Letter	Definition
G	gravel
S	sand
M	silt
C	clay
O	organic
PT	peat
Letter	Definition
P	poorly graded (many particle of about the same size)
W	well-graded (many different particle sizes)
H	high liquid limit
L	low liquid limit

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 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 (0.075 mm) sieve	silt and clay liquid limit < 50	inorganic	ML	silt
CL	lean clay
organic	OL	organic silt, organic clay
silt and clay liquid limit ≥ 50	inorganic	MH	elastic silt
CH	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/or Cc < 1 or Cc > 3	GP	Poorly graded gravel
Gravels 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/or Cc < 1 or 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 Sieve	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

Unified Soil Classification System

Symbol chart

ASTM D-2487

See also