Soil Mechanics

Dr. Tushara ChamindaDepartment of Civil & Environmental Engineering,Faculty of Engineering, Uni. of Ruhuna

Classification and

characteristics of soils

Sieve and hydrometer analysis theory and tests, Particle-size distribution curve, Atterberg limits and their determinations, Classification of soils for engineering purposes, Standard classification systems

Classification and

characteristics of soils

Karl von Terzaghi (1883 -1963):

An Austrian civil engineer and

geologist, called the father of soil

mechanics.

He started modern soil mechanics

with his theories of consolidation,

lateral earth pressures, bearing

capacity, and stability

Physical properties of soil Soil colour

Use to determine nature of soil properties

Soil texture

Proportion of sand, silt and clay sized fractions

As particle become smaller they have different properties (e.g. influence of surface areas on water holding capacity, cation exchange capacity and rate of weathering)

Soil structure

Shape size and distinctiveness of soil aggregates

Blocky, spheriodal, platy and prismatic

The finer textured the stronger the structure

Determines soil porosity - effects water and air movement

Index Properties of Soil

The laboratory tests, which provide information on physical

properties of soil, are known as classification tests and

numerical results of such tests are known as index

properties.

Soil materials having similar index properties are likely to exhibit similar

engineering behavior.

The index properties are of a great value to the civil engineer;

provide means in the correlation of construction experience form a basis for information of the correctness of the field identification of

a given material

If the material is improperly identified, the index properties indicate theerrors and lead to correct classification

Index properties may be divided into two general types:

1. Soil Grain Properties

2. Soil Aggregate Properties

Soil Grain Properties:

Properties of the individual particles of which the soil is

composed of and are independent in the manner of soil

formation. These properties can be determined from

distributed samples. Soil grain properties are commonly

used for soil identification and classification.

Soil Aggregate Properties:

Depend on the structure and the arrangement of the

particles in the soil mass. The soil aggregate properties

have a greater influence on the engineering behavior of soil

mass. The engineering behavior of a soil mass depends on

its strength, compressibility and permeability characteristics.

It is a sort of labeling with different labels. It is more convenient

to study the behavior of groups than that of individual soils

As there is a wide variety of soils covering earth, it is desirable to

systematize or classify the soils into broad groups of similar

behavior.

Soil classification for engineering purpose is necessary to

describe the many type of soil that exist in nature.

Soil classification

Soil classification divides soils into groups and subgroups based

on common engineering properties such as grain-size

distribution, liquid limit and plastic limit.

Soil classification permits us to solve many problems related to

soils and guide the test programs if the difficulty and importance

of the problem dictates further investigations.

Soil classification .

Purpose of soil classification

1. Provides a concise and systematic method for designating

various types of soil

2. Enables useful engineering conclusions to be made about

soil properties

3. Provides a common language which organizes the

engineering knowledge of soil, and is a means of

communication

10

Classifying soils into groups with similar behavior, in terms

of simple indices, can provide geotechnical engineers a

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

Communicate

between

engineers

Soils are basically divided into two broad categories

called cohesion-less soils and cohesive soils. The

differences depend on whether the individual particles

are held together only as a result of;

Gravity or external loads (cohesion-less soil)

Inter-particle bonds (cohesive soil)

Major Soil Groups

Granular ( or cohesionless) Soils:

Include gravel, sand, non-plastic silt and combination of them.Sources of shear strength are the sliding friction andmechanical interlocking between the particles (compressivecontact forces). Soil particles do not tend to stick together. Theyare highly permeable.

Cohesive Soils:

Include clay or other soils with significant clay contents. Inter-particle bonding (which is independent of the normal load)provides the major part of the shear strength. Soil particles tendto stick together. They are less permeable

Major Soil Groups

Major Soil Groups Clay:magnified about 1,600 times

Sand grain: magnified about 40 times

Source: The Nature and Properties of Soils, 8th edition,

Macmillan Publishing Co

Granular Soils

High shear strength - Large bearing capacity

Small lateral pressure; High permeability (easily drained)

Good backfill materials for retaining walls

Relatively small settlements

Good embankment material

Good foundation materials for supporting roads and structures

Engineering properties of granular soils are affected by

Grain sizes

Shapes

Grain-size distribution

Compactness

Cohesive Soils

Sticky, plastic, and compressible

Expand when wetted; Shrink when dried

Creep (deform plastically) over time under constant load(when the shear stress is approaching its shear strength)

Develop large lateral pressure

Not good for retaining wall backfills

Low permeability or Impervious

Good core materials for earthen dams and dikes

Lower shear strength

Generally undesirable engineering properties

1) Particle size characteristics

The particle size analysis of a soil sample involves determining

the percentage by mass of particles within the different size

ranges.

2) Liquid limit and plasticity index (Atterberg Limits)

Plasticity is an important characteristic in the case of fine soils,

the term plasticity describing the ability of a soil to undergo

unrecoverable deformation without cracking or crumbling.

Most commonly used soil classification tests;

Most soil classifications employ very simple index-type tests to obtain the

characteristics of the soil needed to place it in a given group. The most

commonly used characteristics are particle size and plasticity. The particle

size distribution and the Atterberg limits are useful index tests inherently

involves disturbance of the soil, they may not give a good indication of the

behavior of the in situ, undisturbed soil.

Particle size characteristics

18

0.002 4.750.075

Grain size (mm)

Boulder

Clay Silt Sand Gravel Cobble

Fine grain

soils

Coarse grain

soils

Granular soils or

Cohesionless soils

Cohesive

soils

Characterization of Soils Based on Particle Size

Unified Soil Classification System (USCS)

Comparison of four systems for describing soils based on

particle size

British Standards (BS)

Unified Soil Classification System (USCS),

American Association of State Highway and Transportation Officials (AASHTO)

American Society for Testing and Materials (ASTM) (a modification of the USCS system)

21

Because of the particulate nature of soil, it isnatural to consider the size of particles for

classification.

This test is performed to determine the percentageof different grain sizes contained within a soil.

The mechanical or sieve analysis is performed todetermine the distribution of the coarser, larger-

sized particles, and the hydrometer method is used

to determine the distribution of the finer particles.

Characterization of Soils Based on Particle

Size

Significance of Particle Size Distribution

(Granular Size Distribution)

The distribution of different grain sizes affects theengineering properties of soil

Grain size analysis provides the grain size distribution, and itis required in classifying the soil.

This test is conducted to know the relative proportions of different

grain sizes. The particle size distribution is an important factor

influencing the geotechnical characteristics of a coarse grain soil

Applications of particle size analysis:

Selection of fill material for embankments and dams (i.e. well graded

for better strength and high compaction)

Selection of aggregate materials (i.e. sand for concrete). In

exploration for sand and gravel, particle size analysis is the main

criteria.

Selection of materials for road sub-bases

Drainage fillers (grading of material for filler requirements)

Ground water drainage (largely depend on the portion of fine grained

soil)

Grouting and chemical injection

24

Drainage filter:

25

Selection of materials for road, dams, embankments :

26

Grouting and chemical injection

27

Grain Size Distribution (GSD)

1) In coarse grain soils ... By sieve analysis

Determination of GSD:

2) In fine grain soils ... By hydrometer analysis

Hydrometer Analysis

soil/water suspension

hydrometer

stack of sieves

Sieve Analysis

Very often, soils contain both coarse andfine grains and it is necessary to do both

sieve and hydrometer analyses to obtain

the complete grain size distribution data

Sieve analysis is carried out first, and onthe soil fraction passing 75 m sieve, a

hydrometer analysis is carried out

Grain Size Distribution (GSD)

29

Sieve Analysis

Grain Size Analysis

Sieve Analysis

Sieve analysis is carried out by using a set of standard sieves

A nest of sieves is prepared by stacking sieves one abovethe other with the largest opening at the top followed by

sieves of successively smaller openings and a catch pan at

the bottom.

Sieves are made by weaving two sets of wires at right anglesto one another

The square holes thus formed between the wires provide thelimit which determines the size of the particles retained on a

particular sieve

The sieve sizes are given in terms of the number of openingsper inch

The number of openings per inch varies according to differentstandards.

Sieve Analysis

33

Sieve Designation

# 10 sieve

Smaller sieves are numbered according to the number of

openings per inch

1-inch

10 openings per

inch

Two scales that are used to classify particle sizes are the US

Sieve Series and Tyler Standard Sieve Series

35

(1) Write down the weight of each sieve as well as the bottom pan

to be used in the analysis.

(2) Record the weight of the given dry soil sample.

(3) Make sure that all the sieves are clean, and assemble them in

the ascending order of sieve numbers (#200 sieve at bottom).

Place the pan below #200 sieve. Carefully pour the soil sample

into the top sieve and place the cap over it.

(4) Place the sieve stack in the mechanical shaker and shake for

10 minutes.

(5) Remove the stack from the shaker and carefully weigh and

record the weight of each sieve with its retained soil. In

addition, remember to weigh and record the weight of the

bottom pan with its retained fine soil.

Sieving procedure

38

Sieve

Shaker

39

(1)Obtain the mass of soil retained on each sieve by

subtracting the weight of the empty sieve from the mass of

the sieve + retained soil, and record this mass as the

weight retained on the data sheet. The sum of these

retained masses should be approximately equals the initial

mass of the soil sample. A loss of more than two percent is

unsatisfactory.

(2)Calculate the percent retained on each sieve by dividing the

weight retained on each sieve by the total sample mass.

(3)Calculate the percent passing (or percent finer) by starting

with 100 percent and subtracting the percent retained on

each sieve as a cumulative procedure.

Data Analysis:

Example : Total weight of sample = 600g

In science and engineering, a semi-log graph or semi-logplot is a way of visualizing data that are changing with an

exponential relationship.

One axis is plotted on a logarithmic scale.

This kind of plot is useful when one of the variables beingplotted covers a large range of values and the other has

only a restricted range

The advantage being that it can bring out features in thedata that would not easily be seen if both variables had

been plotted linearly

Grain size distribution Curve

43

Grain size distribution Curve

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

44

Grading curves

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

W Well graded

45

Grading curves

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

U Uniform

W Well graded

46

Grading curves

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

P Gap graded

U Uniform

W Well graded

Poorly

Graded

Well-graded

Uniform-graded

Gap-graded

Poorly

Graded

Well Graded Uniformly Graded Gap Graded

49

0.0001 0.001 0.01 0.1 1 10 100

0

20

40

60

80

100

Particle size (mm)

% F

iner

x% of the soil has particles smaller than Dx

D10

10 %

80 %

D80

Dx is the diameter corresponding to x% finer

in the particle-size distribution

Particle-Size Distribution Curve

1. Effective Size (D10)

2. Uniformity Coefficient (Cu)

3. Coefficient of Curvature (Cc)

10

60u

D

DC

1060

2

30c

DD

DC

10% Finer Hydraulic Conductivity

Particle-Size Distribution Curve

Criteria for Well-Graded Soil (USCS)

sands)(for

6Cand3C1

gravels)(for

4Cand3C1

uc

uc

52

What is the Cu for a soil with only one grain size?

1D

DC

uniformityoftCoefficien

10

60u

D

% F

iner

Grain size distribution, mm

Aggregations of particles not thoroughly broken.

Overloading sieves

Sieves shaken for too short a period or with

inadequate motions

Broken or deformed sieve screens.

Loss of material during the analysis process

Possible Errors in Sieve Analysis

Hydrometer analysis

(Sedimentation)

Particle Size Distribution - Sedimentation

For particles 75m (silt and clay fractions)sedimentation methods based on Stokes law are used

to deduce particle size distribution.

Soil particles settle in aqueous solution attainingterminal velocities proportional to their mass and size.

The amount of suspended soil after a given settlingtime is used to determine particle size fractions.

The amount of soil in suspension is determined byeither extracting a sample by the pipette method or

from a direct hydrometer reading.

What is a Hydrometer?

Device used to determine directly thespecific gravity of a liquid

Consists of a thin glass tube closed atboth ends

Large bulb contains lead shot to causethe instrument to float upright in liquid.

Scale is calibrated to indicate thespecific gravity of the liquid.

Required Test Equipment

Measure Sample

Collect 50 g of fine soil, passing through No. 200 (0.075mm)

from mechanical sieving procedure

Dispersion agent

Add sample to 125 ml of 40g/L Sodium Hexometaphosphateas deflocculating solution. (sodium oxalate or sodium silicate

may also used)

Allow to soak for 12~16 hours

Finer grains of soil carry charges on their surface and hence have a

tendency to form flocs. Thus if the floc formation is not prevented the grain

diameter obtained would be the diameter of flocs and not of the individual

grain. Hence in sedimentation analysis, deflocculating agents are to be

added.

Mix sample with spatula to dislodge settled particles

Pour sample into mixing cup

Use distilled water to rinse beaker

Add distilled water to the soil in mixer cup to make it about two-thirds full.

Mix using mixer for 2~3 minutes

Sample Preparation

Pour the mix into the standard sedimentation 1000mLcylinder.

Make sure that all of the soil solids are washed out of themixer cup.

Fill the graduated cylinder with distilled water to bring thewater level to 1000 cc mark

The suspension in the cylinder is then shaken forapproximately 1 min by placing the palm of the hand overthe open end and turning the cylinder upside down andback

Sample Preparation

Set the jar on the bench and record the time. This is time (t=0) on

your data sheet.

Take a hydrometer reading and temperature reading at prescribed

intervals (, , 1, 2, 5, 10, 20 minutes, 1hr, 2hr, 4hr, 8hr, 16hr, 24 hr

etc, approximately doubling the previous time interval).

Take frequent temperature measurements of suspension. Hydrometer

readings should be taken in the distilled water with same amount of

dispersion agent at regular time intervals.

After each reading the hydrometer is put into the transparent cylinder

Starting the Test

Stokes Law

Gravitational Force

Buoyancy Force

(weight of displaced liquid)

Drag Force (exerted by

the surrounding liquid)

g)3r4(F 3sg

grF fb )34(3

Vr6Fd l....density liquid [kg/m

3]

s ...density solid [kg/m3]

r.....radius sphere [m]

g ....acceleration of gravity [m/s2]

V....settling velocity [m/s]

dynamic viscosity [kg/m s]

Three forces acting on a spherical particle.

Gravitaional

Bouyancy

Drag

Buoyancy and drag forces act against the gravitational force.

dbgi FFF0F

The three forces acting upon the settling particle quickly equilibrate and the particle

reaches a constant settling velocity

We can solve the force balance equation to obtain the settling velocity

Since the velocity (V) equals length per time we can calculate the time particles of a

certain size need to settle through a distance L,:

t is the time required for particles of a certain size to settle

below a certain depth L.

Stokes Law :

dbgi FFF0F

186

3

4

3

40

233 gdVVrg

rg

r fsfs

gd

Lt

gd

t

LV

fs

fs

2

2

)(

18

18

All particles are

in suspension

Only Silt and Clay

particles are

in suspension

Only Clay

particles are

in suspension

StartAfter Hours

After Minutes

Hydrometer Analysis

V1 V2

Example of Hydrometer

Calibration Curve

L

L

67

Corrections should be done for effect of Meniscus

and Dispersion agents

Meniscus correction :

Assumptions used for Hydrometer test

Particles are large enough to be unaffected by thethermal (Brownian) motion of the fluid molecules

All particles are rigid, spherical, and smooth

All particles have the same density

The suspension is dilute enough that particles donot interfere with each other

Fluid flow around the particles is laminar. Thatmeans no particle exceeds the critical velocity for

the onset of turbulence

69

Oven drying may cause permanent changes in the particle sizes.

Unequal density of soil particles

Affect from Brownian motion

Soil particles are not truly spherical

Unsatisfactory type or quantity of dispersing agent (may form flocs) .

Insufficient shaking or agitating of suspension in cylinder at start of test.

Too much soil in suspension.

Disturbance of suspension while inserting or removing hydrometer.

Stem of hydrometer not clear.

Dirt or grease on the stem may prevent full development of the meniscus.

Nonsymmetrical heating of suspension.

Excessive variation in temperature of suspension during test.

Loss of material after test.

Possible Errors in Hydrometer Analysis

70

Modern methods for particle size analyses

-Optical Microscopy

-Sieving with digital image processing

- Transmission/Scanning Electron Microscopy

- X-ray attenuation

- Particle counting (Coulter method)

- Light Scattering and Laser Diffraction Methods

71

Soil Consistency

72

Soil Consistency

Soil consistence provides a means of describing the

degree and kind of cohesion and adhesion between the soil

particles as related to the resistance of the soil to deform

or rupture

Soil Behave Like:

SOILD at very low moisture content

LIQUID at very high moisture content

73

Consistency is a term used to indicate the degree of firmness ofcohesive soils. The consistency of natural cohesive soil deposits is

expressed qualitatively by such terms as very soft, soft, stiff, very

stiff and hard

The physical properties of clays greatly differ at different watercontents. A soil which is very soft at a higher percentage of water

content becomes very hard with a decrease in water content

However, it has been found that at the same water content, twosamples of clay of different origins may possess different

consistency. One clay may be relatively soft while the other may be

hard. Further, a decrease in water content may have little effect on

one sample of clay but may transform the other sample from almost

a liquid to a very firm condition

Water content alone, therefore, is not an adequate index ofconsistency for engineering and many other purposes.

Soil Consistency :

74

Strength decreases as water content increases

Soils swell-up when water content increases

Fine-grained soils at very high water content possess

properties similar to liquids

As the water content is reduced, the volume of the soil

decreases and the soils become plastic

If the water content is further reduced, the soil becomes

semi-solid when the volume does not change

Water Content Significantly affects properties of Silty and

Clayey soils. Plasticity property describes the response of a

soil to change in moisture content.

Plasticity property :

75

At a very low moisture content, soil behaves more like

a solid. When the moisture content is very high, the

soil and water may flow like a liquid. Hence, on an

arbitrary basis, depending on the moisture content,

the behavior of soil can be divided into 4 basic states:

solid, semisolid, plastic, and liquid.

76

77

Plasticity

Soils can be in any one of 4 physical states

(based on water content)

Water Content

Plastic LiquidSemi-

solidSolid

Vo

lum

e

Atterberg Limits

Albert Atterberg,

a Swedish scientist,

considered the

consistency of soils in

1911, and proposed a

series of tests for defining

the properties of cohesive

soils.

78

Introduction of Atterberg Limits to

the field of geotechnical

engineering was due to Karl

Terzaghi, who came to realize its

importance at a relatively early

stage of his research. Terzaghisassistant, Arthur Casagrande,

standardized the tests in his paper

in 1932 and the procedures have

been followed worldwide ever

since.

79

80

Atterberg LimitsThe presence of water in fine-grained soils can significantly affect

associated engineering behavior, so we need a reference index to

clarify the effects

Soil Consistency - Atterberg Limits

1. Solid

2. Semi-Solid

3. Plastic

4. Liquid

Shrinkage Limit (SL)

Plastic Limit (PL)

Liquid Limit (LL)

Plasticity Index

(PI) = PL - LL

Mo

istu

re C

on

ten

t (w

)

+

_Depending on Moisture Content soil can be divided into:

81

82

Liquid

LimitPlastic

Limit

Shrinkage

Limit

Water Content

Plastic LiquidSemi-

solidSolid

SL PL LL

Plasticity Index

PI

Volume

Atterberg limits are the points at which the soil

changes phase. Atterberg limits also depend on

the type of predominant mineral in the soil

83

The liquid limit: is the moisture content that defines where the

soil changes from a plastic to a viscous fluid state and begins

to flow.

The plastic limit: is the moisture content that defines where the

soil changes from a semi-solid to a plastic (flexible) state.

Shrinkage Limit (SL) is defined as the moisture content at

which no further volume change occurs with further reduction in

moisture content

Atterberg limits are the points at which the

soil changes phase

Plasticity Index (PI) is the difference between the liquid limit

and plastic limit of a soil

PI = LL PL

Source: Budhu, 201084

85

Liquid Limit-LL

Cone Penetrometer Method

(BS 1377)

This method is developed by the Transport and Road Research Laboratory, UK.

Casagrande Method(ASTM D4318)

Professor Casagrande standardized the test and developed the liquid limit device.

86

Casagrande

Method for LL

(ASTM D4318)

LL is defined as the moisture content (%) required to

close a 2-mm wide groove in a soil pat a distance of

12.7 mm along the bottom of the groove after 25 blows

87

The liquid limit is determined from an apparatus that consists of a

semispherical brass cup that is repeatedly dropped on to a hard rubber base

from a height of 10 mm by a cam operated mechanism

A dry powder of the soil (100g, passing through No:40) is mixed with distilled

water into a paste and placed in the cup to a thickness of about 12.7 mm (1/2

inch). The soil surface is smoothed and a groove is cut into the soil using a

standard grooving tool

The crank operating the cam is turned at a rate of 2 revolutions per second,

and the number of blows required to close the groove over a length of 12.7 mm

(1/2 inch) is counted and recorded.

A specimen of soil within the closed portion is extracted for determination of

the water content.

The liquid limit is defined as the water content at which the groove cut into the

soil will close over a distance of 12.7 mm (1/2 inch) following 25 blows.

This is difficult to achieve in a single test. Four or more tests at different water

contents are usually required for terminal blows (number of blows to close the

groove over a distance of 12.7 mm) ranging from 10 to 50.88

CASAGRANDE METHOD

89

Liquid Limit - Measurement

Liquid Limit (LL) at N = 25

Flow Index = If = Slope of the flow curve (flow curve in the plot of water content vs

number of blows in log scale)

91

Second Method

Fall Cone Method BS1377


92


Liquid Limit (LL) at d = 20 mm

Fall Cone Method BS1377

93

94

Plastic Limit

The minimum water content at which a soil will just beginto crumble when it is rolled into a thread of approximately3 mm in diameter.

95

Plastic Limit w% procedure

Take 15~20g of soil used for LL test

Prepare several ellipsoidal-shaped soil masses by quizzing

the soil with your hand.

Put the soil in rolling device, and roll the soil until the thread

reaches 3.0~3.2mm.

Continue rolling until the thread crumbles into several pieces.

Determine the moisture content of about 6g of the crumbled

soil (obtain 3 data and take the average).

The plasticity index (PI) is the difference between the liquid

limit and the plastic limit of a soil: PI = LL-PL

Plasticity index indicates the degree of plasticity of a soil

The greater the difference between liquid and plastic limits,

the greater is the plasticity of the soil

A cohesion-less soil has zero plasticity index. Such soils are

termed non-plastic.

Fat clays are highly plastic and possess a high plasticity

index.

The plasticity index is important in classifying fine-grained

soils. It is fundamental to the Casagrande plasticity chart,

which is currently the basis for the Unified Soil Classification

System

Plasticity index (PI)

97

Plasticity index (PI)

98

Prof. Donald M.Burmister (1949) classified the

plasticity index in a qualitative manner as follows

99

1) Plasticity Index (PI), =Ip = LL - PL

2) Liquidity Index (LI)PL

PL

LL

LI

Important consistency relationships :

The relative consistency of a cohesive soil in the natural state

can be defined by liquidity index (LI). w in situ moisture

content of soil. The in situ moisture content for a sensitive clay

may be greater than the liquid limit. In this case: LI < 1

Soil deposits that are heavily over consolidated may have a

natural moisture content less than the plastic limit.

In this case: LI > 1

100

3) Consistency Index (CI)PL

LL

LLCI

If w = LL, the CI= 0 and if w = PI, then CI = 1

4) Flow Index = If = Slope of the flow curve(flow curve in the plot of water content vs number of blows in

log scale)

101

Plasticity Chart (USCS)Casagrande (1932) studied the relationship of the plasticity

index to the liquid limit of a wide variety of natural soils and

proposed a plasticity chart

20 100500

20

0

40

60

Liquid Limit

Liq

uid

Lim

it

SiltsClays

High

plasticity

Low

plasticity

35

Intermediate plasticity

102

103

LL Values < 16 % not realistic

16 Liquid Limit, %

PI, %

104

LL Values > 50 - HIGH

Liquid Limit, %

PI, %

50

H

105

LL Values < 50 - LOW

Liquid Limit, %

PI, %

50

L

106

Plasticity Chart

(Holtz and Kovacs, 1981)

LL

PI

HL

The A-line generally

separates the more

claylike materials

from silty materials,

and the organics

from the inorganics.

The U-line indicates

the upper bound for

general soils.

Note:

If the measured limits of soils are on the left of U-line, they should be rechecked.

107

Unified Soil Classification System (USCS)

Origin of USCS:

This system was first developed by Professor A. Casagrande (1948) for the

purpose of airfield construction during World War II. Afterwards, it was

modified by Professor Casagrande, the U.S. Bureau of Reclamation, and

the U.S. Army Corps of Engineers to enable the system to be applicable to

dams, foundations, and other construction (Holtz and Kovacs, 1981).

Soil classification determined base on the soil parameter;

Diameter of soil particle

Gravel : pass sieve no.3 but retained at sieve no. 4

Sand : pass sieve no. 4 but retained at sieve no. 200

Silt and Clay : pass sieve no. 200

Coefficient of soil uniform (Cu, Cc)

Atterberg Limits

Unified Soil Classification System (USCS

109

Symbols:

Soil symbols:

G: Gravel

S: Sand

M: Silt

C: Clay

O: Organic

Pt: Peat

Liquid limit symbols:

H: High LL (LL>50)

L: Low LL (LL

110

USCS-Summary

(Holtz and Kovacs, 1981)

THE FLOW CHART OF USCS METHOD

Make visual examination of soil to determine

whether it is HIGHLY ORGANIC, COARSE

GRAINED, or FINE GRAINED, ini borderline

cases determine amount passing No. 200 sieve

HIGHLY ORGANIC SOIL (Pt)

Fibrous texture, color, odor, very high

moisture content, particle of vegetable

matter (sticks, leaves, etc.)

COARSED GRAINED

50% or less pass No.200 sieve

FINE GRAINED

More than 50% pass No.200 sieve

111

FLOWCHART OF USCS METHOD (CONTINUED)

COARSED GRAINED

50% or less pass No.200 sieve

Run sieve analysis

GRAVEL (G)

Greater percentage of coarse

fraction retained on No. 4 sieve

SAND (S)

Greater percentage of coarse

fraction pass on No. 4 sieve

Less than 5%

pass No. 200

sieve *

Between 5% and 12%

pass No. 200 sieve

more than 12%

pass No. 200

sieve

Examine grain size

curve

Borderline. to have double

symbol appropriate to grading

and plasticity characteristic,

e.g. GW-GM

Run LL and PL on

minus No. 40

sieve fraction

Well

Graded

Poorly

Graded

GW GP

Below A line and

hatched zone on

plasticity chart

Limits plot in

hatched zone on

plasticity chart

Above A line and

hatched zone on

plasticity chart

GM GM-GC GC

Less than 5%

pass No. 200

sieve *

Between 5% and 12%

pass No. 200 sieve

more than 12%

pass No. 200

sieve

Examine grain size

curve

Borderline. to have double

symbol appropriate to grading

and plasticity characteristic,

e.g. GW-GM

Run LL and PL on

minus No. 40

sieve fraction

Well

Graded

Poorly

GradedBelow A line and

hatched zone on

plasticity chart

Limits plot in

hatched zone on

plasticity chart

Above A line and

hatched zone on

plasticity chart

SW SP SM SM-SC SC

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113

Between 5% and 12%?

(passing sieve No:200)

FINE GRAINED

More than 50% pass

No.200 sieve

Run LL and PL on minus No.40

sieve material

L

Liquid Limit

less than 50

H

Liquid Limit

more than 50

Below A line and hatched

zone on plasticity chart

Limits plot in hatched

zone on plasticity

chart

Above A line and hatched

zone on plasticity chart

Color, odor, possibly LL

and PL on oven dry soil

Organic Inorganic

Below A line on

plasticity chart

Above A line on

plasticity chart

Color, odor, possibly LL

and PL on oven dry soil

Inorganic Organic

OL ML ML-CL CL MH OH CH

FLOWCHART USCS METHOD (CONTINUED)

114

Soil group Symbol Recommended name

Coarse soils 35% fines Liquid limit%

SILT M

MG Gravelly SILT

MS Sandy SILT

ML, MI... [Plasticity subdivisions as for CLAY]

CLAY C

CG Gravelly CLAY

CS Sandy CLAY

CL 90 CLAY of extremely high plasticity

Organic soils O [Add letter 'O' to group symbol]

Peat Pt [Soil predominantly fibrous and organic]

British Soil Classification System

115

Example :

A sample of soil was tested in the laboratory with the following results:

Liquid limit = 30%

Plastic limit = 12%

Sieve analysis data:

Classify the soil by the Unified Soil Classification System

U.S. Sieve Size Percentage Passing

3/8 in. 100.0

No. 4 76.5

No. 10 60.0

No. 40 39.7

No. 200 15.2

Example Soil A

Soil Mechanics

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