lecture 4 classification of soil

INTERNATIONAL UNIVERSITY FOR SCIENCE & TECHNOLOGY

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CIVIL ENGINEERING AND

ENVIRONMENTAL DEPARTMENT

303322 - Soil Mechanics

Classification of Soils

Dr. Abdulmannan Orabi

Lecture

2

Lecture

4

Dr. Abdulmannan Orabi IUST 2

Das, B., M. (2014), “ Principles of geotechnical Engineering ” Eighth Edition, CENGAGE Learning, ISBN-13: 978-0-495-41130-7.

Knappett, J. A. and Craig R. F. (2012), “ Craig’s Soil Mechanics” Eighth Edition, Spon Press, ISBN: 978-0-415-56125-9.

References

A soil classification system-It is the arrangement of different soils with similar properties into groups & subgroups based on their application or to their probable engineering behavior.

Introduction

A classification for engineering purposes should be based mainly on mechanical properties: permeability, stiffness, strength.


Most of the soils classification systems that have been developed for engineering purposes are based on simple index properties such as particle size distribution & plasticity.

Introduction

The aim of a classification system is to establish a set of conditions which will allow useful comparisons to be made between different soils.


Classification System

SOIL CLASSIFICATION BASICS

Commonly based on grain size and soil consistency. Several classification systems exist:1. Unified Soil Classification System (USCS) (ASTM D2487-11).

2. American Association of State Highway and Transportation Officials (AASHTO) (ASTM D3282-09).


Unified Soil Classification System (USCS)

According to the USCS, the soils are divided into two major categories:

1. Coarse-grained soil that are gravelly and sandy in nature with less than 50 % passing through the No 200 sieve ( that is F200 < 50%).

2. Fine-grained soils with 50% or more passing through the No. 200 sieve ( that is F 200 > 50%).


•The soil is classified into 15 groups.

• Each group is designated a symbol consisting of two capital letters.

•The first letter is based on main soil type.

•The second letter is based on gradation and plasticity .




The following Symbols are used in this system:

1. Coarse-grained

P – Poorly graded

W– Well graded

M – Silty

C – Clayey

G – Gravel

S – Sand

2. Fine-grained soil

M – Silt, C – Clay, O – Organic

L – Low Plastic H – High Plastic

First Letter

Second Letter


Group Symbol Typical Name

Sw or Gw Well graded soil (sand or gravel )

SP or GP Poorly graded soil ( sand or gravel )

GM Silty gravel or Silty gravel with sand

GC Clayey gravel or Clayey gravel with sand

SM Silty sand or Silty sand with gravel

SC Clayey sand or Clayey sand with gravel


1. Coarse-grained



Group Symbol Typical Name

CL Inorganic clays of low plasticity.

ML Inorganic silts with slight plasticity.

OL Organic soil of low plasticity.

CH Inorganic clays of high plasticity.

MH Inorganic silts with high plasticity.

OH Organic soil of high plasticity.

Pt Peat.

2. Fine-grained soil



The classification is based on material passing a 75mm sieve:

Coa

rse

–g

rain

ed S

oil

Fin

e –

gra

ined

S

oil

R 200 > 50

F200 > 50

Gravel

Sand

Silt & Clay

Sieve No. 4

Sieve No. 200

F200

R200

F4

R4

4.75 mm

0.075 mm



� F 200 = percent passing No. 200 sieve ( % of fines )

� R4 = percent retained above No.4 sieve. ( % of Gravel )

� R200 = percent retained above No.200 sieve.

� F4 = percent passing No. 4 sieve


Worked Example

F200 = 14%

R4 = 18 %

R200 = 86%

F4 = 82%

Gravel = 18 % Sand = 68 %Silt and Clay= 14%

Gravel

Sand

Silt & Clay

Sieve No. 4

Sieve No. 200

F200

R200

F4

R4

4.75 mm

0.075 mm



If R4/ R200 > 0.5, the soil is a gravel ( G )

1. Coarse-grained soil

Symbol Criteria

GW F200<5 % Cu > 4 , 1< Cc < 3

GP F200<5 % Not meeting the GW criteria of Cu & Cc

GM F200>12 % PI < 4, or plots below A-Line ( Plasticity Chart )

GC F200>12 % PI > 7, and plots on or above A-Line

GW-GM 5<F200<12 Satisfies Cu & Cc crit. of GW, meets PI for GM

GW-GC 5<F200<12 Satisfies Cu & Cc crit. of GW, meets PI for GC

GP-GM 5<F200<12 Doesn't satisfy Cu & Cc crit, of GP, meets PI,GM

GP-GC 5<F200<12 Doesn't satisfy Cu & Cc crit, of GP, meets PI,GM

GM-GC F200>12 % PI plots in the hatched area ( Plasticity Chart )



Symbol Criteria

SW F200<5 % Cu > 6 , 1< Cc < 3

SP F200<5 % Not meeting the GW criteria of Cu & Cc

SM F200>12 % PI < 4, or plots below A-Line ( Plasticity Chart )

SC F200>12 % PI > 7, and plots on or above A-Line

SW-SM 5<F200<12 Satisfies Cu & Cc crit. of SW, meets PI for SM

SW-SC 5<F200<12 Satisfies Cu & Cc crit. of SW, meets PI for SC

SP-SM 5<F200<12 Doesn't satisfy Cu & Cc crit, of SP, meets PI,SM

SP-SC 5<F200<12 Doesn't satisfy Cu & Cc crit, of SP, meets PI,SM

SM-SC F200>12 % PI plots in the hatched area ( Plasticity Chart )

1. Coarse-grained soil

If R4/ R200 > 0.5, the soil is a gravel ( G )



2. Fine – Grained Soil

Silts and Clays: LL < 50 %

ML PI < 4 or plots below A- Line CL PI > 7 and plots on or above A –Line CL- ML PI plots in the hatched area

OL PI plots in the OL area



2. Fine – Grained Soil

Silts and Clays: LL > 50 %

MH PI plots above A- Line

CH PI plots on or above A –Line OH plots in the OH area

Pt High organic matter (Peat)



0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90 100

CH or OH

LL

PI

MH

or

OH

CL or

ML or

OL

CL - ML

Liquid Limit ( % )

Pla

sticity In

de

x

OL

Plasticity Chart



Plasticity Chart

� Note:

If the results of your soil tests fall above the U – line on the plasticity chart, you should be suspicious of your results and repeat your tests.


Example 1


Using the grain-size distribution curve shown below, determine the uniformity coefficient Cu and classify the soil

Particle diameter (mm)

Per

cen

t fi

ner

#10 #200 #60

10

20

30

40

100

0 0.01 1 10

50

60

70

80

90

D30 D10D60



Solution� From the grain distribution curve, D60 = 0.60 mm, D30 =

0.20 mm and D10 = 0.09 mm,

F200= 0, R4= 3 therefore the coefficients are,

A well-graded soil has a uniformity coefficient greater than 4 for gravels and 6 for sands.

Therefore, this soil is classified as uniform sand. (According USCS, the soil is SP ).

�� = ��

��

=0.6

0.09= 6.67 �� =

��

�� ∗ ��

= 0.74



Example 2

Laboratory tests were performed on a light-brown sandy soil, which visually has several pieces of gravel larger than 6 mm, indicated that PL = 22.6 % and LL = 33.2 %. Classify this soil using USCS.

Sieve No. % Passing

4 98.0

40 36.5

200 28.5



Solution

� F200 = 28.5% < 50% , the soil is coarse grained. R4 = 2%, R4/R200 < 0.5, the soil is sand

� F200 = 28.5% > 12% The soil group is SM or SC

� PI = 33.2 – 22.6 = 10.6%

� Atterberg limits above A-Line

� Therefore, this soil can be classified as SC


AASHTO Classification System

The classification system is based on the following criteria

1. Grain size

a. Gravel: fraction passing the 75 mm sieve and retained on the No. 10 ( 2mm ) sieve

b. Sand: fraction passing the No. 10 sieve and retained on the No. 200 U.S. sieve.

c. Silt and clay: fraction passing No.200sieve.



2. Plasticity : the term silty is applied when the fine frictions of the soil have a plasticity index of 10 or less.

The term clayey is applied when the fine frictions have a plasticity index of 11or more.

3. If cobbles and boulders are encountered, they are excluded form the portion of the soil sample which classification is made.


According to the present form of this system, soils can be classified according to eight major groups, A-1 through A-8, based on their grain size distribution, liquid limit, and plasticity indices.



Soils listed in groups A-1, A-2, and A-3 are coarse-grained materials, and those in groups A-4, A-5, A-6, and A-7 are fine-grained materials.

Peat, muck, and other highly organic soils are classified under A-8. They are identified by visual inspection.




According to the AASHTO, the soils are divided into two major categories:

1. Coarse-grained soil that are gravelly and sandy in nature with less than 35 % passing through the No 200 sieve ( that is F200 < 35%).

2. Fine-grained soils with 35% or more passing through the No. 200 sieve ( that is F 200 > 35%).



Granular materials ( F200 < 35% )

Group

classification

A – 1

A - 3A – 1 – a A – 1 – b

Sieve analysis (Percentage

passing )

No. 10 50 max

No.40 30 max 50 max 51 min

No.200 15 max 25 max 10 max

Characteristic of fraction passing

No. 40

Liquid Limit

------

Plasticity index

6 max NP

Usual types of materials Stone fragments, gravel, and sand

Fine sand



Granular materials ( F200 < 35% )

Group

classificationA - 2

A – 2 – 4 A – 2 –5 A – 2 - 6 A – 2 - 7

Sieve analysis (Percentage

passing )

No. 10

No.40

No.200 35 max 35 max 35 max 35 max

Characteristic of fraction

passing No. 40

Liquid

Limit

40 max 41 min 40 max 41 min

Plasticity

index

10 max 10 max 11 min 11 min

Usual types of materials Silty or clayey gravel and sand



Plasticity Chart

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90 100

A-4A-2-4

A-7-6

A-2-5A-5

A-2-7

A-7-5A-2-6

A-6

Liquid Limit, LL

Pla

sticity I

ndex,

PI

PI< LL – 30

PI>LL – 30



Group Index ( GI )

To evaluate the quality of a soil as a highway subgrade material, one must also incorporate a number called the group index (GI) with the groups and subgroups of the soil.Group index of a soil depends on amount of

material passing 75 micron IS sieve, liquid limit , and plastic limit.



Group Index ( GI )

1. Used to describe performance of soil when used for pavement construction.

2. Note used to place a soil in a particular group3. It means rating the value of soil as a subgrade

material within its own group 4. Higher the value of GI, poorer is the quality

of material. The quality of performance of a soil as a subgrade material is inversely proportional to GI.



Group Index ( GI )

The group index is given by the equation:

GI = (F200-35)[0.2+0.005(LL-40)]+0.01(F200-15)(PI-10)

When calculating the group index for soils that belong to group A – 2 – 6 and A – 2 – 7 , use the partial group index for PI, or

GI = 0.01(F200 -15) (PI - 10)



A soil having GI of zero is considered as the best.

If the equation gives negative value for GI, consider it zero.

Always round off the GI to nearest whole number.

GI=0 for soils of groups A-1-a, A-1-b ,A-2-4, A-2-5, and A-3.

Group Index ( GI )

There is no upper limit for the group index


The results of the particle-size analysis of a soil are as follows:

Percent passing through the No. 10 sieve = 100



The liquid limit and plasticity index are 30 and 10, respectively.

Classify the soil by the AASHTO system.

Worked Examples

Example 3


F200 = 58% > 35%, the soil is fine- grained ( silt or clay )

Using plasticity chart(slid No. 24), the soil is A-4.

So, the soil will be classified as A-4 (3)

Worked Examples

Solution

GI = (F200-35)[0.2+0.005(LL-40)]+0.01(F200-15)(PI-10)

GI = (58-35)[0.2+0.005(30-40)]+0 .01(58-15)(10-10)

GI = 3.45


Worked Examples

Example 4

� Ninety-five percent of a soil passes through the No. 200 sieve and has a liquid limit of 60%and plasticity index of 40.

Classify the soil by the AASHTO system.

Solution :

F200 = 95% > 35

According to plasticity chart, this soil falls under group A-7 , 40 > 60 – 30, GI = 42

The soil is A-7 – 6 ( 42 )


For an inorganic soil, the following grain-size analysis is given:

For this soil, LL = 23% and PL = 19%. Classify the soil according to:a. the AASHTO soil classification b. the Unified soil classification system

Worked Examples

Example 5

Sieve No. % Passing

4 100

10 90

20 64

40 38

80 18

200 13


Classify the following soils using Unified classification system

Worked Examples

Example 6

SoilSieve analysis, Liquid

Limit

Plasticity

indexNo.4 No.200

A 80 52 30 8

B 79 45 26 4

C 91 80 60 32

D 95 75 41 12

E 82 41 24 2


Worked Examples

Example 6

Classify the following soils using USCS and AASHTO systems and give the GI.

Soil

Sieve analysis,

LL PINo.4

No.

10

No.

20

No.

40

No.

60

No.

100

No.

200

0.01

mm

0.002

mm

A 94 63 21 10 7 5 3 - - - NP

B 98 86 50 28 18 14 10 - - - NP

C 100 100 98 93 88 83 77 65 60 63 25

D 100 100 100 99 95 90 86 42 40 55 28

E 100 100 100 94 82 66 45 26 21 36 22


0

10

20

30

40

50

60

70

80

90

100

0.010.1110

Worked Examples

Curve – soil A Example 6


Classify the following soils using the AASHTO system and give the GI.

Worked Examples

Example 7

Soil

Sieve analysis,

LL PI

No. 10 No. 40 No. 200

A 62 30 8 - NP

B 90 67 35 32 8

C 90 76 34 37 12

D 100 78 8 - NP

E 85 68 45 38 9
