38623 Soil Classification

Soils and their Classification

Soils - What are they?

Particulate materials

- Sedimentary origins (usually)- Residual

Wide range of particle sizes

- larger particles: quartz, feldspar- very small particles: clay minerals

Voids between particles

Aragonite-rich soil x 2000

Need for Simple Classification

Usually soil on site has to be used.– Soils differ from other engineering materials in that one

has little control over their properties

Extent and properties of the soil have to be determined

Cheap and simple tests are required to give an indication of engineering properties, e.g. stiffness, strength, for preliminary design

The classification must use core samples obtained from the ground. This information is often supplemented by in-situ tests such as cone penetration tests.

Classification based on Particle Size Particle size is used because it is related to mineralogy

– e.g. very small particles usually contain clay minerals

Broad Classification

– Coarse grained soils» sands, gravels - visible to naked eye

Classification based on Particle Size Particle size is used because it is related to mineralogy

– e.g. very small particles usually contain clay minerals

Broad Classification

– Coarse grained soils» sands, gravels - visible to naked eye

– Fine grained soils» silts, clays, organic soils

Procedure for grain size determination Sieving - used for particles > 75 m

Hydrometer test - used for smaller particles– Analysis based on Stoke’s Law, velocity proportional to diameter



Figure 1 Schematic diagram of hydrometer test



Figure 1 Schematic diagram of hydrometer test

Grading curves

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

Grading curves

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

U Uniform

Grading curves

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

U Uniform

P Poorly graded

Grading curves

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

U Uniform

P Poorly graded

C Well graded with some clay

Grading curves

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

U Uniform

P Poorly graded

C Well graded with some clay

F Well graded with an excess of fines

Simple Classification In general soils contain a wide range of particle sizes

Some means of describing the characteristics of soils with different proportions of sand/silt/clay is required.



1009080706050403020100

100

90

80

70

60

50

40

30

20

10

0100

90

80

70

60

50

40

30

20

10

0

SiltSizes(%)

SandSiltySand SandySilt

Clay-Sand Clay-Silt

SandyClay SiltyClay

Clay

LOWERMISSISSIPPIVALLEYDIVISION,U.S.ENGINEERDEPT.



Note the importance of clay - Finest 20% control behaviour1009080706050403020100

100

90

80

70

60

50

40

30

20

10

0100

90

80

70

60

50

40

30

20

10

0

SiltSizes(%)

SandSiltySand SandySilt

Clay-Sand Clay-Silt

SandyClay SiltyClay

Clay

LOWERMISSISSIPPIVALLEYDIVISION,U.S.ENGINEERDEPT.

Example: equal amounts sand/silt/clay



Atterberg Limits Particle size is not that useful for fine grained soils


Figure 4 Moisture content versus volume relation during drying

010203040

0 20 40 60Moisture Content (%)

Volu

me

LLSL PL


Figure 4 Moisture content versus volume relation during drying

SL - Shrinkage Limit PL - Plastic Limit LL - Liquid limit

010203040

0 20 40 60Moisture Content (%)

Volu

me

LLSL PL

Atterberg Limits

SL - Shrinkage LimitPL - Plastic LimitLL - Liquid limit

Atterberg Limits


Plasticity Index = LL - PL = PI or Ip

Atterberg Limits


Plasticity Index = LL - PL = PI or Ip

Liquidity Index = (m - PL)/Ip = LI

Classification Systems Used to determine the suitability of different soils

Used to develop correlations with useful soil properties

Special Purpose (Local) Systems– e.g. PRA system of AAHSO

» 1. Well graded sand or gravel: may include fines» 2. Sands and Gravels with excess fines» 3. Fine sands» 4. Low compressibility silts» 5. High compressibility silts» 6. Low to medium compressibility clays» 7. High compressibility clays» 8. Peat and organic soils

Unified Soil Classification Each soil is given a 2 letter classification (e.g. SW). The

following procedure is used.



– Coarse grained (>50% larger than 75 m)




» Prefix S if > 50% of coarse is Sand» Prefix G if > 50% of coarse is Gravel





» Suffix depends on %fines





» Suffix depends on %fines

» if %fines < 5% suffix is either W or P» if %fines > 12% suffix is either M or C» if 5% < %fines < 12% Dual symbols are used

Unified Soil ClassificationTo determine if W or P, calculate Cu and Cc

CDDu 60

10

CD

D Dc 302

60 10( )

x% of the soil has particles smaller than Dx

Unified Soil ClassificationTo determine W or P, calculate Cu and Cc

CDDu 60

10

CD

D Dc 302

60 10( )

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner



CDDu 60

10

CD

D Dc 302

60 10( )

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner



CDDu 60

10

CD

D Dc 302

60 10( )

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

D90 = 3 mm


Unified Soil Classification

To determine W or P, calculate Cu and Cc

If prefix is G then suffix is W if Cu > 4 and Cc is between 1 and 3

otherwise use P

If prefix is S then suffix is W if Cu > 6 and Cc is between 1 and 3

otherwise use P

CDDu 60

10

CD

D Dc 302

60 10( )

Unified Soil Classification

0 10 20 30 40 50 60 70 80 90 100Liquid limit

0

10

20

30

40

50

60

Plasti

cityi

ndex

CH

OH

or

MH

CLOL

MLor

CL

ML

Comparing soils at equal liquid limitToughness and dry strength increasewith increasing plasticity index

Plasticity chartfor laboratory classification of fine grained soils

Coarse grained soilsTo determine M or C use plasticity chart

Below A-line use suffix M - Silt

Above A-line use suffix C - Clay

Unified Soil Classification– Fine grained soils (> 50% finer than 75 m)– Both letters determined from plasticity chart

0 10 20 30 40 50 60 70 80 90 100Liquid limit

0

10

20

30

40

50

60

Plas

ticity

index

CH

OH

or

MH

CLOL

MLor

CL

ML



Give typical names: indicate ap-proximate percentages of sandand gravel: maximum size:angularity, surface condition,and hardness of the coarsegrains: local or geological nameand other pertinent descriptiveinformation and symbol inparentheses.

For undisturbed soils add infor-mation on stratification, degreeof compactness, cementation,moisture conditions and drain-age characteristics.

Example:

Well graded gravels, gravel-sand mixtures, little or nofinesPoorly graded gravels, gravel-sand mixtures, little or nofinesSilty gravels, poorlygraded gravel-sand-silt mixtures

Clayey gravels, poorly gradedgravel-sand-clay mixtures

Well graded sands, gravellysands, little or no fines

Poorly graded sands, gravellysands, little or no fines

Silty sands, poorly gradedsand-silt mixtures

Clayey sands, poorly gradedsand-clay mixtures

GW

GP

GM

GC

SW

SP

SM

SC

Wide range of grain size and substantialamounts of all intermediate particlesizesPredominantly one size or a range ofsizes with some intermediate sizesmissingNon-plastic fines (for identificationprocedures see ML below)

Plastic fines (for identification pro-cedures see CL below)

Wide range in grain sizes and sub-stantial amounts of all intermediateparticle sizes

Predominantely one size or a range ofsizes with some intermediate sizes missing

Non-plastic fines (for identification pro-cedures, see ML below)

Plastic fines (for identification pro-cedures, see CL below)

ML

CL,CI

OL

MH

CH

OH

Pt

Dry strengthcrushingcharacter-

istics

None toslight

Medium tohigh

Slight tomedium

Slight tomedium

High to veryhigh

Medium tohigh

Readily identified by colour, odourspongy feel and frequently by fibroustexture

Dilatency(reactionto shaking)

Quick toslow

None to veryslow

Slow

Slow tonone

None

None to veryhigh

Toughness(consistencynear plastic

limit)

None

Medium

Slight

Slight tomedium

High

Slight tomedium

Inorganic silts and very fine sands,rock flour, silty or clayeyfine sands with slight plasticityInorganic clays of low to mediumplasticity, gravelly clays, sandyclays, silty clays, lean claysOrganic silts and organic silt-clays of low plasticityinorganic silts, micaceous ordictomaceous fine sandy orsilty soils, elastic siltsInorganic clays of highplasticity, fat clays

Organic clays of medium tohigh plasticity

Peat and other highly organic soils

Give typical name; indicate degreeand character of plasticity,amount and maximum size ofcoarse grains: colour in wet con-dition, odour if any, local orgeological name, and other pert-inent descriptive information, andsymbol in parentheses

For undisturbed soils add infor-mation on structure, stratif-ication, consistency and undis-turbed and remoulded states,moisture and drainage conditions

ExampleClayey silt, brown: slightly plastic:small percentage of fine sand:numerous vertical root holes: firmand dry in places; loess; (ML)

Field identification procedures(Excluding particles larger than 75mm and basing fractions on

estimated weights)

Groupsymbols

1Typical names Information required for

describing soilsLaboratory classification

criteria

C = Greater than 4DD----60

10U

C = Between 1 and 3(D )D x D----------------------30

10c

2

60

Not meeting all gradation requirements for GW

Atterberg limits below"A" line or PI less than 4

Atterberg limits above "A"line with PI greater than 7

Above "A" line withPI between 4 and 7are borderline casesrequiring use of dualsymbols

Not meeting all gradation requirements for SW

C = Greater than 6DD---- 60

10U

C = Between 1 and 3(D )D x D----------------------30

10c

2

60

Atterberg limits below"A" line or PI less than 4

Atterberg limits above "A"line with PI greater than 7

Above "A" line withPI between 4 and 7are borderline casesrequiring use of dualsymbolsD

eter

min

epe

rcen

tage

sof

grav

elan

dsa

ndfr

omgr

ain

size

curv

e

Use

grai

nsi

zecu

rve

inid

entif

ying

the

frac

tions

asgi

ven

unde

rfie

ldid

entif

icat

ion

Dep

endi

ngon

perc

enta

ges

offi

nes

(fra

ctio

nsm

alle

rtha

n.0

75m

msi

eve

size

)coa

rse

grai

ned

soils

are

clas

sifie

das

follo

ws

Les

sth

an5%

Mor

eth

an12

%5%

to12

%

GW

,GP,

SW,S

PG

M,G

C,S

M,S

CB

orde

line

case

requ

irin

gus

eof

dual

sym

bols

The

.075

mm

siev

esi

zeis

abou

tthe

smal

lest

parti

cle

visi

ble

toth

ena

ked

eye

Fine

grainedsoils

Morethan

halfof

materialissm

allerthan

.075

mmsievesize

Coarsegrainedsoils

Morethan

halfof

materialislarger

than

.075

mmsievesize

Silts

andclays

liquidlim

itgreaterthan

50

Silts

andclays

liquidlim

itless

than

50

Sands

Morethan

halfof

coarse

fractio

nissm

allerthan

2.36

mm

Gravels

Morethan

halfof

coarse

fractio

nislarger

than

2.36

mm

Sandswith

fines

(app

reciable

amount

offin

es)

Clean

sands

(littleor

nofin

es)

Gravelswith

fines

(apreciable

amount

offin

es)

Clean

gravels

(littleor

nofin

es)

Identification procedure on fraction smaller than .425mmsieve size

Highly organic soils

Unified soil classification (including identification and description)

Silty sand, gravelly; about 20%hard angular gravel particles12.5mm maximum size; roundedand subangular sand grainscoarse to fine, about 15% non-plastic lines with low drystrength; well compacted andmoist in places; alluvial sand;(SM)

0 10 20 30 40 50 60 70 80 90 100Liquid limit

0

10

20

30

40

50

60

Plas

ticity

inde

x

CH

OH

or

MHOL

MLor

CL



CI

CL-MLCL-ML

Example

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

Example

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

• %fines (% finer than 75 m) = 11% - Dual symbols required

Example

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

• %fines (% finer than 75 m) = 11% - Dual symbols required

• D10 = 0.06 mm, D30 = 0.25 mm, D60 = 0.75 mm

Example

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

Particle size fractions: Gravel 17%

Sand 73%

Silt and Clay 10%

Of the coarse fraction about 80% is sand, hence Prefix is S

Cu = 12.5, Cc = 1.38

Suffix1 = W

From Atterberg Tests

LL = 32, PL = 26

Ip = 32 - 26 = 6

Example

0 10 20 30 40 50 60 70 80 90 100Liquid limit

0

10

20

30

40

50

60Pl

astic

ityind

ex

CH

OH

or

MH

CLOL

MLor

CL

ML




Cu = 12.5, Cc = 1.38

Suffix1 = W


LL = 32, PL = 26

Ip = 32 - 26 = 6

From Plasticity Chart point lies below A-line

Suffix2 = M


Cu = 12.5, Cc = 1.38

Suffix1 = W


LL = 32, PL = 26

Ip = 32 - 26 = 6


Suffix2 = M

Dual Symbols are SW-SM


Cu = 12.5, Cc = 1.38

Suffix1 = W


LL = 32, PL = 26

Ip = 32 - 26 = 6


Suffix2 = M

Dual Symbols are SW-SM

To complete the classification the Symbols should be accompanied by a description

Documents

38623 Soil Classification