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:

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[1])
W  | well-graded (many different particle sizes[1])
H  | high liquid limit[2]
L  | low liquid limit[2]

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 (0.075 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

[edit]

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

[2][3]

## See also

[edit]

* AASHTO Soil Classification System
* AASHTO
* ASTM International
* USDA Soil Taxonomy (Soil classification for agricultural purposes)

## References

[edit]

* Classification of Soils for Engineering Purposes: Annual Book of ASTM Standards, D 2487-83, vol. 04, American Society for Testing and Materials, 1985, pp. 395–408
* Evett, Jack and Cheng Liu (2007), Soils and Foundations (7 ed.), Prentice Hall, pp. 9–29, ISBN 978-0132221382

Specific

1. ^ a b Day, Robert W. (2013). Geotechnical engineer's portable handbook : with the 2012 international building code (2nd ed.). New York: McGraw-Hill. ISBN 978-0-07-178971-4. OCLC 837179787.
2. ^ a b c D 2487 – 06: Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System) (PDF) (Technical report). ASTM International. 2006.
3. ^ 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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Soil science

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Main fields|

* Pedology
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Soil topics|

* Soil
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Soil type|

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Soil classification

World Reference Base
for Soil Resources (1998–)|

* Acrisols
* Alisols
* Andosols
* Anthrosols
* Arenosols
* Calcisols
* Cambisols
* Chernozem
* Cryosols
* Durisols
* Ferralsols
* Fluvisols
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* Gypsisols
* Histosol
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* Podzols
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* Solonetz
* Stagnosol
* Technosols
* Umbrisols
* Vertisols

USDA soil taxonomy|

* Alfisols
* Andisols
* Aridisols
* Entisols
* Gelisols
* Histosols
* Inceptisols
* Mollisols
* Oxisols
* Spodosols
* Ultisols
* Vertisols

Other systems|

* FAO soil classification (1974–1998)
* Unified Soil Classification System
* AASHTO Soil Classification System
* Référentiel pédologique (French classification system)
* Canadian system of soil classification
* Australian Soil Classification
* Polish Soil Classification
* 1938 USDA soil taxonomy
* List of U.S. state soils
* List of vineyard soil types
* PG-3 (Spanish classification system)

Non-systematic soil types|

* Sand
* Silt
* Clay
* Loam
* Topsoil
* Subsoil
* Soil crust
* Claypan
* Hardpan
* Gypcrust
* Caliche
* Parent material
* Pedosphere
* Laimosphere
* Rhizosphere
* Bulk soil
* Alkali soil
* Bay mud
* Blue goo
* Brickearth
* Brown earth
* Calcareous grassland
* Dark earth
* Dry quicksand
* Duplex soil
* Eluvium
* Expansive clay
* Fill dirt
* Fuller's earth
* Hydrophobic soil
* Loess
* Lunar soil
* Martian soil
* Mud
* Muskeg
* Paleosol
* Peat
* Prime farmland
* Quicksand
* Serpentine soil
* Spodic soil
* Stagnogley
* Subaqueous soil
* Takir
* Terra preta
* Terra rossa
* Tropical peat
* Yedoma

Types of soil

Applications|

* Soil conservation
* Soil management
* Soil guideline value
* Soil survey
* Soil test
* Soil governance
* Soil value
* Soil salinity control
* Erosion control
* Agroecology
* Liming (soil)

Related fields|

* Geology
* Geochemistry
* Petrology
* Geomorphology
* Geotechnical engineering
* Hydrology
* Hydrogeology
* Biogeography
* Earth materials
* Archaeology
* Agricultural science
* Agrology

Societies, Initiatives|

* Australian Society of Soil Science Incorporated
* Canadian Society of Soil Science
* Central Soil Salinity Research Institute (India)
* German Soil Science Society
* Indian Institute of Soil Science
* International Union of Soil Sciences
* International Year of Soil
* National Society of Consulting Soil Scientists (US)
* OPAL Soil Centre (UK)
* Soil Science Society of Poland
* Soil and Water Conservation Society (US)
* Soil Science Society of America
* World Congress of Soil Science

Scientific journals|

* Acta Agriculturae Scandinavica B
* Journal of Soil and Water Conservation
* Plant and Soil
* Pochvovedenie
* Soil Research
* Soil Science Society of America Journal

See also|

* Land use
* Land conversion
* Land management
* Vegetation
* Infiltration (hydrology)
* Groundwater
* Crust (geology)
* Impervious surface/Surface runoff
* Petrichor

* Wikipedia:WikiProject Soil
*   Category soil
*   Category soil science
* List of soil scientists

*[Cu]: Coefficient of uniformity
*[Cc]: Coefficient of curvature
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