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Unified Soil Classification System

{{Short description|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

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|

{|class="wikitable"

Letter

! Definition

G

| gravel

S

| sand

M

| silt

C

| clay

O

| organic

PT

| peat

|

class="wikitable"
Letter

! Definition

P

| poorly graded (many particle of about the same size{{Cite book|last=Day, Robert W.|url=https://www.worldcat.org/oclc/837179787|title=Geotechnical engineer's portable handbook : with the 2012 international building code|date=2013|publisher=McGraw-Hill|isbn=978-0-07-178971-4|edition=2nd|location=New York|oclc=837179787}})

W

| well-graded (many different particle sizes)

H

| high liquid limit{{cite tech report|year=2006|title=D 2487 – 06: Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)|publisher=ASTM International|url=https://lauwtjunnji.weebly.com/uploads/1/0/1/7/10171621/astm_d-2487_classification_of_soils_for_engineering_purposes_unified_soil_classification_system.pdf}}

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

class="wikitable"
colspan="3" | Major divisions

! Group symbol

! Group name

rowspan="8" | Coarse grained soils
more than 50% retained on or above No.200 (0.075 mm) sieve

| rowspan="4" | gravel
> 50% of coarse fraction retained on No.4 (4.75 mm) sieve

| rowspan="2" | clean gravel <5% smaller than No.200 Sieve

! GW

| well-graded gravel, fine to coarse gravel

GP

| poorly graded gravel

rowspan="2" | gravel with >12% fines

! GM

| silty gravel

GC

| clayey gravel

rowspan="4" | sand
≥ 50% of coarse fraction passes No.4 (4.75 mm) sieve

| rowspan="2" | clean sand

! SW

| well-graded sand, fine to coarse sand

SP

| poorly graded sand

rowspan="2" | sand with >12% fines

! SM

| silty sand

SC

| clayey sand

rowspan="6" | Fine grained soils
50% or more passing the No.200 (0.075 mm) sieve

| rowspan="3" | silt and clay
liquid limit < 50

| rowspan="2" | inorganic

! ML

| silt

CL

| lean clay

organic

! OL

| organic silt, organic clay

rowspan="3" | silt and clay
liquid limit ≥ 50

| rowspan="2" | inorganic

! MH

| elastic silt

CH

| fat clay

organic

! OH

| organic clay, organic silt

colspan="3" | Highly organic soils

! PT

| peat

= ASTM D-2487 =

class="wikitable" style="text-align:center"

! rowspan="2" colspan="4" | Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests

! colspan="2" | Soil Classification

Group Symbol

| Group Name

rowspan="8" | COARSE-GRAINED SOILS

More than 50% retained on No.200 Sieve

| rowspan="4" | Gravels

More than 50% of coarse fraction on No. 4 Sieve

| rowspan="2" | Clean Gravels

Less than 5% fines

| {{abbr|Cu|Coefficient of uniformity}} ≥ 4 and 1 ≤ {{abbr|Cc|Coefficient of curvature}} ≤ 3

| GW

| Well-graded gravel

Cu < 4 and/or Cc < 1 or Cc > 3

| GP

| Poorly graded gravel

rowspan="2" | 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

rowspan="4" | Sands

50% or more of coarse fraction passes No.4 sieve

| rowspan="2" | 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

rowspan="2" | 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

rowspan="8" | FINE-GRAINED SOILS

50% or more passes the no. 200 Sieve

| rowspan="4" | Silts and Clays

Liquid limit less than 50

| rowspan="2" | Inorganic

| PI > 7 and plots on or above "A" line

| CL

| Lean clay

PI < 4 and plots below "A" line

| ML

| Silt

rowspan="2" | Organic

| Liquid limit—oven dried < 0.75

| OL

| Organic clay

Liquid limit—not dried

| OL

| Organic silt

rowspan="4" | Silts and Clays

Liquid limit 50 or more

| rowspan="2" | Inorganic

| PI plots on or above "A" line

| CH

| Fat clay

PI plots below "A" line

| MH

| Elastic silt

rowspan="2" | Organic

| Liquid limit—oven dried < 0.75

| OH

| Organic clay

Liquid limit - not dried

| OH

| Organic silt

colspan="4" | HIGHLY ORGANIC SOILS

| PT

| Peat

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.

See also

References

  • {{Citation|publisher=American Society for Testing and Materials |year=1985 |series=D 2487-83 |title=Classification of Soils for Engineering Purposes: Annual Book of ASTM Standards |volume=04 |issue=8 |pages=395–408 |url=http://www.astm.org/Standards/D2487.htm}}
  • {{Citation|author=Evett, Jack and Cheng Liu |publisher=Prentice Hall |year=2007 |title=Soils and Foundations |edition=7 |pages= 9–29 |isbn=978-0132221382}}

;Specific

{{soil science topics}}

Category:Soil classification