The Unified Soil Classification System (USCS): Brief introduction

If you are prepare for the the ASBOG exam, don’t forget to glance over the The Unified Soil Classification System (USCS).

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
Symbol Definition
G gravel
S sand
M silt
C clay
O organic
Second letter
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 with gravel may refer to “poorly graded SAND with silt and gravel.”


Symbol chart

Major divisions Group symbol Group name
Coarse grained soils
more than 50% retained on No. 200 (0.075 mm) sieve
> 50% of coarse fraction retained on No. 4 (4.75 mm) sieve
clean gravel <5% smaller than #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 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
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 plasticity, fat clay
organic OH organic clay, organic silt
Highly organic soils Pt peat

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