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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."

Contents

Symbol chartEdit

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-2487Edit

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
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

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

[1]

See alsoEdit

ReferencesEdit

Specific
  1. ^ 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.