Unified Soil Classification System

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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[edit]

Major divisions Group symbol Group name
Coarse grained soils
more than 50% retained on or above No.200 (0.074 mm) sieve
> 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
≥ 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

See also[edit]