1 6. Soil Classification (Das, chapter 5) Sections: All except 5.1.

1

6. Soil Classification(Das, chapter 5)

Sections: All except 5.1

2

Purpose Classifying soils into groups or sub-groups with similar

engineering behavior. Classification systems were developed in terms of simple

indices (GSD and plasticity). These classifications can provide geotechnical engineers

with general guidance about engineering properties of the soils through the accumulated experience.

Simple indices

GSD, LL, PI

Classification system

(Language)

Estimate engineering properties

Achieve engineering

purposes

Use the accumulate

d experience

Communicate between engineers

3

Classification Systems

Two commonly classification system used are:

1. Unified Soil Classification System (USCS) (preferred by geotechnical engineers).

2. American Association of State Highway and Transportation Officials (AASHTO) System (preferred by Transportation engineers).

4

1. Unified Soil Classification System (USCS)

i. Definition of Grain Size

Boulders Cobbles Gravel Sand

Silt and

Clay

Coarse Fine Coarse FineMedium

300 mm 75 mm

19 mm

No.4

4.75 mmNo.10

2.0 mm

No.40

0.425 mm

No.200

0.075 mm

No specific grain size-use

Atterberg limits

5


Coarse-grained soils Fine-grained soils Silt (M) Clay (C)

•Grain size distribution •Use Plasticity chart

•LL, PL

% Passing sieve No. 200 (0.075 mm)

Required tests: Sieve analysis

Atterberg limit

> 50%< 50%

6

Used for Fine grained soils to determine whether silt (M) or clay (C)

Below A-line is silt – use symbol MAbove A-line is clay – use symbol C


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

0

10

20

30

40

50

60Pl

astici

tyin

dex

CH

OH

or

MH

CLOL

MLor

CL

ML

Comparing soils at equal liquid limit

Toughness and dry strength increase

with increasing plasticity index

Plasticity chartfor laboratory classification of fine grained soils

LL > 50 High plasticity LL< 50 low plasticity

7



•Grain size distribution •Use Plasticity chart

ML, MH, CL, CH


Required tests: Sieve analysis

Atterberg limit

> 50%< 50%

8



•Use Plasticity chart

•LL, PL


> 50%< 50%

% Coarse soil (Co) = 100 - % Passing # 200

% Gravel (G) = 100 - % Passing # 4

G > 1/2 Co G < 1/2 Co

Gravel (G) Sand (S)

% Passing sieve No. 200

< 5%

5% -12 %

> 12%

GW, GP, SW or SP Use Cu, Cc W : well graded P: poorly graded

GW-GM, GW-GC, GP-GM, GP-GC, SW-SM, SW-SC, SP-SM, SP-SC

GM, GC, SM, SC Use plasticity charts

9


To determine if well graded (W) or poorly graded (P), calculate Cu and Cc

CD

D Dc 302

60 10( )

CD

Du 60

10

Coefficient of uniformity

Coefficient of gradation

10


CD

D Dc 302

60 10( )

CD

Du 60

10Coefficient of uniformity

Coefficient of gradation

Conditions for Well-graded soils

For gravels Cu > 4 and Cc is between 1 and 3

For Sand W if Cu > 6 and Cc is between 1 and 3

11


12


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

0

10

20

30

40

50

60

Plas

tici

tyin

dex

CH

OH

or

MH

CLOL

MLor

CL

ML





CL-ML

13

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 nofines

Poorly graded gravels, gravel-sand mixtures, little or nofines

Silty 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 sizesmissing

Non-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 strengthcrushing

character-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(reaction

to 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 clays

Organic silts and organic silt-clays of low plasticity

inorganic silts, micaceous ordictomaceous fine sandy orsilty soils, elastic silts

Inorganic 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

enti

fyin

gth

efr

actio

nsas

give

nun

der

fiel

did

entif

icat

ion

Dep

endi

ngon

perc

enta

ges

offi

nes

(fra

ctio

nsm

alle

rth

an.0

75m

msi

eve

size

)co

arse

grai

ned

soils

are

clas

sifi

edas

foll

ows

Les

sth

an5%

Mor

eth

an12

%5%

to12

%

GW

,GP,

SW

,SP

GM

,GC

,SM

,SC

Bor

deli

neca

sere

quir

ing

use

ofdu

alsy

mbo

ls

The

.075

mm

siev

esi

zeis

abou

tthe

smal

lest

part

icle

visi

ble

toth

ena

ked

eye

Fin

egr

aine

dso

ils

Mor

eth

anha

lfof

mat

eria

lis

smal

ler

than

.075

mm

siev

esi

ze

Coa

rse

grai

ned

soils

Mor

eth

anha

lfof

mat

eria

lis

larg

erth

an.0

75m

msi

eve

size

Silt

san

dcl

ays

liqui

dlim

itgr

eate

rth

an50

Silt

san

dcl

ays

liqui

dlim

itle

ssth

an50

Sand

sM

ore

than

half

ofco

arse

frac

tion

issm

alle

rth

an2.

36m

m

Gra

vels

Mor

eth

anha

lfof

coar

sefr

actio

nis

larg

erth

an2.

36m

m

Sand

sw

ithfin

es(a

ppre

ciab

leam

ount

offin

es)

Cle

ansa

nds

(lit

tleor

nofin

es)

Gra

vels

wit

hfin

es(a

prec

iabl

eam

ount

offin

es)

Cle

angr

avel

s(l

ittle

orno

fines

)

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

Pla

stic

ity

inde

x

CH

OH

or

MHOL

MLor

CL





CI

CL-MLCL-ML

14

Organic Soils• Highly organic soils- Peat (Group symbol PT)

A sample composed primarily of vegetable tissue in various stages of decomposition and has a fibrous to amorphous texture, a dark-brown to black color, and an organic odor should be designated as a highly organic soil and shall be classified as peat, PT.

• Organic clay or silt (group symbol OL or OH): “The soil’s liquid limit (LL) after oven drying is less than

75 % of its liquid limit before oven drying.” If the above statement is true, then the first symbol is O.

The second symbol is obtained by locating the values of PI and LL (not oven dried) in the plasticity chart.

15

Borderline Cases (Dual Symbols)Coarse-grained soils with 5% - 12% fines.

About 7 % fines can change the hydraulic conductivity of the coarse-grained media by orders of magnitude.

The first symbol indicates whether the coarse fraction is well or poorly graded. The second symbol describe the contained fines. For example: SP-SM, poorly graded sand with silt.

Fine-grained soils with limits within the shaded zone. (PI between 4 and 7 and LL between about 12 and 25).

It is hard to distinguish between the silty and more claylike materials.

CL-ML: Silty clay, SC-SM: Silty, clayed sand.

Soil contain similar fines and coarse-grained fractions. possible dual symbols GM-ML

16

Example 1

0.0001 0.001 0.01 0.1 1 10 1000

20

40

60

80

100

Particle size (mm)

% F

iner

17

Example 1

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

18

Example 1

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

19

Example 1

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%

20

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 1

21

Example 1

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

0

10

20

30

40

50

60

Plas

tici

tyin

dex

CH

OH

or

MH

CLOL

MLor

CL

ML





22


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

Example 1

23


Cu = 12.5, Cc = 1.38

Suffix1 = W


LL = 32, PL = 26

Ip = 32 - 26 = 6


Suffix2 = M

Dual Symbols are SW-SM

Example 1

24


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

Example 1

25

Example 2

Classify the following soils Using Unified Classification System.

26



•Use Plasticity chart

•LL, PL


> 50%< 50%

% Coarse soil (Co) = 100 - % Passing # 200

% Gravel (G) = 100 - % Passing # 4

G > 1/2 Co G < 1/2 Co

Gravel (G) Sand (S)

% Passing sieve No. 200

< 5%

5% -12 %

> 12%

GW, GP, SW or SP Use Cu, Cc W : well graded P: poorly graded

GW-GM, GW-GC, GP-GM, GP-GC, SW-SM, SW-SC, SP-SM, SP-SC

GM, GC, SM, SC Use plasticity charts

27

Example 2

28

29

30

Example 3

31

32

Summary of the USCS

0 5 12 50 100

% of fines

fine grain soils

CoGr

e.g., GPCoGr - CoF

e.g., GP-GC

Co: Coarse

G = Gravel

S = Sands

F: Fines

M = Silts

C = Clays

CoF

e.g., SM

FP

e.g., CH

Gr: Gradation

W = well graded

P = poorly graded

P: Plasticity

H = LL > 50

L = LL < 50

coarse grain soils[>50% larger than 0.075 mm] [>50% smaller than 0.075 mm]

33

SymbolsSoil symbols:G: GravelS: SandM: SiltC: ClayO: OrganicPt: Peat

Liquid limit symbols:H: High LL (LL>50)L: Low LL (LL<50)

Gradation symbols:W: Well-gradedP : Poorly-graded

Example: SW, Well-graded sand

SC, Clayey sand

SM, Silty sand,

MH, Elastic silt )sandsfor(

6Cand3C1

)gravelsfor(

4Cand3C1

soilgradedWell

uc

uc

34

2. American Association of State Highway and Transportation Officials system (AASHTO)

Origin of AASHTO: (For road construction)

This system was originally developed by Hogentogler and Terzaghi in 1929 as the Public Roads Classification System.

Afterwards, there are several revisions. The present AASHTO (1978) system is primarily based on the version in 1945. (Holtz and Kovacs, 1981)

35

2. AASHTO

i. Definition of Grain Size

Boulders CobblesGravel

Sand

Silt and

Clay

300 mm 75 mm

19 mm

No.4

4.75 mmNo.10

2.0 mm

No.40

0.425 mm

No.200

0.075 mm

No specific grain size-use

Atterberg limits

36

8 major groups: A1~ A7 (with several subgroups) and organic soils A8 The required tests are sieve analysis and Atterberg limits. The group index, an empirical formula, is used to further evaluate soils within a

group (subgroups).

The original purpose of this classification system is used for road construction (subgrade rating).

A4 ~ A7A1 ~ A3

Granular Materials

35% pass No. 200 sieve

Silt-clay Materials

36% pass No. 200 sieve

Using LL and PI separates silty materials from clayey materials

Using LL and PI separates silty materials from clayey materials (only for A2 group)

2. AASHTOii. General guidance

37

2. AASHTOiii. Classification

Classification starts from left to right

38

2. AASHTOiii. Classification

Note:

The first group from the left to fit the test data is the correct AASHTO classification.

39

Group Index

)10PI)(15F(01.0

)40LL(005.02.0)35F(GI

200

200

)10PI)(15F(01.0GI 200

For Groups A-2-6 and A-2-7

The first term is determined by the LL

The second term is determined by the PI

In general, the rating for a pavement subgrade is inversely proportional to the group index, GI.

use the second term only

F200: percentage passing through the No.200 sieve

• Used to evaluate the quality of a soil as a highway subgrade material.

• This index is written in parentheses after the group or subgroup designation [e.g. A-4(3)].

(1)

40

Some rules of Group Index GI

1. If Eq. (1) yields a negative value for GI, it is taken as 0.

2. The group index is rounded off to the nearest whole number (for example, GI 3.4 is rounded off to 3; GI 3.5 is rounded off to 4).

3. There is no upper limit for the group index.

4. The group index of soils belonging to groups A-1-a, A-1-b, A-2-4, A-2-5, and A-3 is always 0.

41

Example 1

42

Example 1 [Soil B]

43

Example 2

Classify the following soil Using AASHTO System.

Given: % passing No. 10 = 100; % passing No. 40 = 80; '% passing No.200 = 58 LL = 30; PI = 10.

44

Example 2

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1 6. Soil Classification (Das, chapter 5) Sections: All except 5.1.

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