Mos Lecture1

7/30/2019 Mos Lecture1

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Assoc. Prof. Derin N. URAL

2/16/2009 Mechanics of Soils 1

Lecture 1

SECTION 1

z Soil Formation

z Particle Size Distribution

z Soil Classification

SECTION 2

z


z 3-phase material

z Soil Characterization (particle size, soilplasticity)


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Soil Mechanics

z Soil mechanics is the branch of science that deals with

of soil masses subjected to various types of forces.

z Classify soils and rocks

z Establish engineering properties


z Ascertain the shear strength

According to Terzaghi (1948):

Soil Mechanics is the application of laws of

mechanics and hydraulics to engineering problems

dealing wi th sediments and other unconsolidatedaccumulations of solid particles produced by the

mechanical and chemical disintegration of rocks


admixture of organic constituent.


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Soil Formation

Parent Rock

Residual soil Transported soil


physical & chemical

agents) of parent rock

~ wea ere antransportedfar away

by wind, water and ice.

Soil Formation

~ formed by one of these three different processes

igneous sedimentary metamorphic

formed b coolin of formed by gradual

molten magma (lava) deposition, and in layers

of igneous &

sedimentary rocks by

pressure/temperaturee.g., limestone, shale

e.g., marble

e.g., granite


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Determination of Particle Size

Distribution

z Mechanical analysis is used in the determination of the

,

percentage of the total dry weight.

z There are two methods that generally utilized to

determine the particle size distribution of soil:


z Sieve Analysis (for particle sizes > 0.075mm in diameter)z Hydrometer Analysis ( < 0.075mm )

Particle Size Distributions and Soil

Particle Characteristics

z Particle size distribution curve is a representation in graphical or

tabular form of the various (diameter) grain sizes in a soil,

determined through sieving and sedimentation.


z The particle diameters are plotted in log scale, and the

corresponding percent finer in arithmetic scale.


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Particle Size Distribution Curve


SILT & CLAY SAND GRAVEL

Sieve Analysis

z It is performed by shaking

set of sieves having

progressively smaller

openings.



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Hydrometer Analysis

z It is based on the principle of sedimentation of soil grains

n water.


By David Airey, The University of Sydney

Hydrometer Analysis

Also called SedimentationAnalysis

Stokes Law

)(2 Lsw GGDv

=



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Some commonly used measures are:

a) Effective size : )( 10D

corresponding to 10% finer. (maximum size of the smallest 10%

of the soil)

b) Uniformity Coefficient:

It is the ratio of the maximum diameter of the smallest 60% to

1060/DDCu =


e e ec ve s ze.

A well graded soil will have

sandsfor6C

gravelsfor4

u

u

>

>C

Some commonly used measures are:

c) Coefficient of Curvature:2

: Diameter corresponding the 30% finer

d) Clay Fraction: (CF)

It is the percentage by dry mass of particles smaller than

0.002mm (2m), and is an index property frequently quoted

c =

30D


63m). It has a strong influence on the engineering properties

of fine grained soils.


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Definitions

e) Well-Graded Material Contains particles of a wide range of

sizes The smaller articles fill the s aces left between the lar er.

particles; therefore the soil has greater strength than a poorly

graded soil, and lower permeability.

f) Poorly Graded Material Contains a large portion of uniformly

sized particles. This particular soil has larger voids in its structure


.


Soil A: Well Graded

Soil B: Poorly Graded

Soil C: Uniform

By David Airey, The University of Sydney


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Soil Plasticity & Consistency Limitsz In the early 1900s a Swedish scientist Atterberg developed a

method to describe the consistency offine grained soils with

var in de ree of moisture content .

z If a soil is gradually dried from a slurry, it passes from state of

viscous liquid to a plastic state; then to a semi-solid, and finally into

a solid state. The moisture contents at which the soil passes from

one state to the next are known as consistency limits (also called

Atterberg Limits)


z Consistency limits are utilized to compare soils from different

locations and different depths.

z There are 4 basic states

Consistency of fine-grained soil varies in proportion to the water content

Atterberg Limits

Plastic limit

Liquid limit

plastic

liquid

PlasticityIndex

(pea soup)

(peanut butter)


Shrinkage limitsolid

semi-solid (cheese)

(hard candy)

By P. Jayawickrama, Texas Tech University


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Consistency Limits

Volume

Viscous

Liquid


Moisture Content (%)

LLSL PL

Solid-

Solid

Definitions

a) Liquid Limit (LL) : is the minimum moisture content at which the

soil will flow under its own weight. The moisture content (in %)

.

groove after 25 blows is thell.

b) Plastic Limit (PL): is the moisture content (in %) at which the soilwhen rolled into threads of 3.2mm in diameter, crumbles. PL is

the lower limit of the plastic stage of the soil. The test is simple

and performed by repeated rollings of ellipsoidal size soil mass by


.

c) Shrinkage Limit (SL): is the moisture content (in %) at which the

volume change of the soil mass ceases.


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Definitions

d) Plasticity Index (PI): is a measure of the range of the moisture

contents over which a soil is plastic.

e) Liquidity Index (LI): The relative consistency of a cohesive soil

in a natural state can be defined by the ratio called LI.

PL-LLPI=

PL)-(LL/PL)-(wLI=


f) Activity : is the ratio of PI to the clay fraction (% by dry weight ofparticles < 2m)

fraction%)(Clay/PIA =

CLASSIFICATION OF SOILS

z The sizes of particles that make up soil may vary widely

.

classified as :

1) Gravel

2) Sand

3) Silt

4) Clay


z The most comprehensive is the Unified Soil Classification

System (USCS).


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USCS

z This system classifies soils under two broad

categories:z Coarse Grained Soils -are gravelly and sandy in

nature with


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USCS Table

Give typical names: indicate ap-proximate percentages of sandand gravel: maximum size:

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

GWWide range of grain size and substantialamounts of all intermediate particlesizes

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

estimated weights)

Groupsymbols

1Typical names

Information required fordescribing soils

Laboratory classificationcriteria

C = Greater than 4D

D----60

10U

(D )302

ecurve

.075mm

ymbols

se gravels

orno

es)

Unified soil classification (including identification and description)

angularity, surface condition,and hardness of the coarsegrains: local or geological nameand other pertinent descriptiveinformation and symbol in

parentheses.

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

Example:

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

GP

GM

GC

SW

SP

SM

SC

Predominantly 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 intermediate

particle 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)

Dry strengthcrushing

character-istics

Dilatency(reaction

to shaking)

Toughness(consistencynear plastic

limit)

Inor anic silts and ver fine sands

C = B et we en 1 a nd 3D x D----------------------

10c

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 6D

D----60

10U

C = B et we en 1 an d 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

eterminepercentagesofgravelandsandfromgrainsiz

ecurveinidentifyingthefractionsasgivenunderfieldidentification

Dependingonpercentagesoffines(fractionsmallerthan

sievesize)coarsegrainedsoilsareclassifiedasfollows

Lessthan5%

Morethan12%

5%to12%

GW,

GP,SW,S

P

GM,

GC,

SM,

SC

Bordelinecaserequiringuseofduals

sievesizeisaboutthesmallestparticlevisibletothenakedeye

allerthan

Coarsegrainedsoils

Morethanhalfofmaterialislargerthan

.075mmsievesize

andclays

uidlimit

than50

Sands

Morethanhalfofcoarse

fractionissmallerthan

2.36mm

Gravels

Morethanhalfofcoars

fractionislargerthan

2.36mm

Sandswith

fines

(appreciable

amountoffines)

Cleansands

(littleorno

fines)

Gravelswith

fines

(apreciable

amountoffines)

Clean

(littlefi

n

Identification procedure on fraction smaller than .425mmsieve size

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)


ML

CL,CI

OL

MH

CH

OH

Pt

None toslight

Medium tohigh

Slight tomedium

Slight tomedium

High to veryhigh

Medium tohigh

Readily identified by colour, odourspongy feel and frequently by fibroustexture

Quick toslow

None to veryslow

Slow

Slow tonone

None

None to veryhigh

None

Medium

Slight

Slight tomedium

High

Slight tomedium

,rock flour, silty or clayeyfine sands with slight plasticityInorganic clays of low to medium

plasticity, gravelly clays, sandyclays, silty clays, lean clays

Organic silts and organic silt-clays of low plasticity

inorganic silt s, 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)

Usegrainsiz

The.07

5mm

Finegrainedso

ils

Morethanhalfofmaterialissm

.075mmsievesize

Siltsandclays

liquidlimit

greaterthan

50

Silts

liq

less

Highly organic soils

0 10 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 00Liquid limit

0

10

20

30

40

50

60

Plasticityindex

CH

OH

or

MHOL

MLor

CL

"A" l

ine

Comparing soils at equal liquid limit

Toughness and dry strength increase

with increasing plasticity index

Plasticity chartfor laboratory classification of fine grained soils

CI

CL-MLCL-ML

Classification Procedure

z Coarse Grained Materialsz

sieve, the soil is classified as coarse. The following stepsare then followed to determine the appropriate 2 lettersymbol

z Determine the1st letter of the symbol If more than half of the coarse fraction is sand then use prefix

S

If more than half of the coarse fraction is gravel then use prefix


G

z Determine the 2nd letter of symbol

This depends on the uniformity coefficient Cu and thecoefficient of curvature Cc obtained from the grading curve, onthe percentage of fines, and the type of fines.


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z First determine the percentage of fines, that is the % of materialpassing the 75 m sieve.

z Then if % fines is

z < 5% use W or P as suffix

z > 12% use M or C as suffix

z between 5% and 12% use dual symbols. Use the prefix from abovewith first one of W or P and then with one of M or C.

z If W or P are required for the suffix then Cu and Cc must beevaluated


z If prefix is G then suffix is W if Cu > 4 and Cc is between 1 & 3otherwise use P

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

otherwise use P


z If M or C are required they have to be determined fromthe rocedure used for fine rained materials discussedbelow. Note that M stands for Silt and C for Clay. This isdetermined from whether the soil lies above or below theA-line in the plasticity chart.

z For a coarse grained soil which is predominantly sandthe following symbols are possible


z SW, SP, SM, SC

z SW-SM, SW-SC, SP-SM, SP-SC


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z These are classified solely according to the results from.

and Liquid Limit are used to determine a point in theplasticity chart. The classification symbol is determinedfrom the region of the chart in which the point lies.

Examples

z CH High plasticity clay

z CL Low plasticity clay


z

g p as c y sz ML Low plasticity silt

z OH High plasticity organic soil (Rare)

z Pt Peat

Casagrande Plasticity Chart

60

Comparing soils at equal liquidlimit

20

30

40

Plasticity

index

CH

OH

orCLOLCL

"A"

lineoug nessan rystrengt ncrease

withincreasingplasticity index


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

0

MHML

or

ML


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3-Phase Material

Aira er


SolidBy P. Jayawickrama, Texas Tech University

The Mineral Skeleton

Volume

Solid Particles

Voids (air or water)



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Three Phase Diagram

Water


o

Mineral Skeleton Idealization:

Three Phase Diagram

Fully Saturated Soils

Water

Solid


Fully SaturatedMineral Skeleton


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Dry Soils

Air

Solid


Mineral Skeleton Dry Soil

Partly Saturated Soils

r

Water


o

Mineral Skeleton Partly Saturated Soils


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Phase Diagram

Solid

r

WaterWT

Ws

Ww

a~

Vs

a

Vw

Vv

VT


Volume Weight

Objectives of a Phase Diagram

To compute the weights (or masses) and volumes of

the three different phases.

NotationM = mass or weight

V = volumes = soil grains

Va Wa=0

WwWt

VwVv

Vt

Air

Water

38

w = water

a = air

v = voids

t = total

Vs Ws

Phase Diagram

Solid


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Volume Relationships

Void ratio (e): is a measure of the void volume.

S

V

V

Ve =

a a

WwWt

Vw

Vv

Vt

Water


s Ws

Phase Diagram

o


Porosity (n): is also a measure of the void volume,

expressed as a percentage.

T

V

V

V

n = X 100%

Va Wa=0

WwWt

Vw

Vv

Vt

Air

Water


Theoretical range: 0 100%Vs Ws

Phase Diagram

Solid


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Degree of saturation (S): is the percentage of the void

volume filled by water.

V

W

V

VS= X 100%

Va Wa=0

WwWt

Vw

Vv

Vt

Air

Water


ange:

DrySaturated

Vs Ws

Phase Diagram

Solid

Weight Relationships

Water content (w): is a measure of the water

present in the soil.

S

W

W

W

w = X 100%

Va Wa=0

WwWt

Vw

Vv

Vt

Air

Water


.

Range = 0 100%.

Vs Ws

Phase Diagram

Solid


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Unit Weight Relationships

Natural Unit Weight (): is the

density of the soil in the

current state.

Dry Unit Weight (d): is the unit

weight of the soil in dry state.

T

T

V

W=

Va Wa=0

WwWt

Vw

Vv

Vt

Air

Water


T

Sd

V

W=

ss

Phase Diagram

Unit Weight Relationships

Saturated Unit Weight (sat): is theunit weight of the soil when the

.

Submerged Unit Weight (sub): isthe effective unit weight of the soil

T

vs

V

VWsat

w*

+=

Vs

a a=

Ws

WwWt

Vw

Vv

Vt

Solid

Water


.

Phase Diagramwsatsub =


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Phase Relations

Consider a fraction of the soil where Vs = 1.

The other volumes canbe obtained from theprevious definitions.

The weights can beobtained from:

SewSe

e

Air

Water

Weights = Unit Weights x Volume 1 Gsw

Phase Diagramvolumes weights

Solid

Phase RelationsFrom the previous definitions,

W SeW==

Air

SS GW

ee

VVn

T

V

+==

11 G

SewSe

e

Solid

Water




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Phase Relations

ST SeGW +== Air

T eV +1

WS

T

Tsat

e

eG

V

W

+

+==

11

SewSe

e

Solid

Water


WS

T

Sd

e

G

V

W

+==

1

s w


Definitions

Bulk (natural), saturated, dry and submerged

densities () are defined in a similar manner.

Here, you can also use mass (kg) instead of weight (kN).

/ g = = M/V


kg/m3N/m3 m/s

2


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Specific Gravity

WaterofVolumeEqualanofWeight

ceSubsaofWeightGS

tan=

WaterofWeightUnit

ceSubsaofWeightUnitGS

tan=


, wz w = 1.0 g/cm

3 (strictly accurate at 4 C)

z w = 9.81 kN/m3

In Terms of Density

i. Density of water : w = 1000kg/m3

ii. Dry density of soil :

iii. Bulk density of unsaturated or saturated soil:

WS

T

Sd

e

G

V

M

+==

1

WS

T

T

e

SeG

V

M

+

+==

1


iv. Air content (A) :

e

wGe

V

VA S

T

a

+

==

1


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Relationship between parameters

z These definitions can be used to determine any desired

,

determine void ratio, degree of saturation, etc. That can

not be measured directly by laboratory tests. Some

relationships are as follows:


Relationship between parameters

z For unsaturated soils:

W SeWw == [1]wG

e S=

z For saturated soils: S = 1 then;

z Bulk density;

SS

wGe S=

WS

T

T

e

SeG

V

M

+

+==

1 e

wG

e

GwG wSw

SS

+

+=

+

+=

1

)1(

1

)(

GM


z Degree of Saturation;

W

T

deV

+==

1 =+ wd

+=

wS

S

Gw

wGS

)1(


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z Try not to memorize the equations. Understand

the definitions, and develop the relations from the

phase diagram with VS = 1;

z Assume G (2.6-2.8) when not given;

By N. Sivakugan


z Do not mix densities and unit weights;

z Soil grains are incompressible. Their mass and

volume remain the same at any void ratio.



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