Outline - National Chiao Tung...

Soil Compositions

ChihChih--Ping LinPing LinNational National ChiaoChiao TungTung [email protected]@mail.nctu.edu.tw

Soil Mechanics −

Outline

Soil : ThreeSoil : Three--phase Materialphase MaterialProportions of Each PhaseProportions of Each Phase-- Phase relationshipPhase relationship

Characteristics of Soil GrainsCharacteristics of Soil Grains-- Particle size, shape, and mineralParticle size, shape, and mineral

Interaction between Soil Particles and WaterInteraction between Soil Particles and Water-- AtterbergAtterberg limitslimits

Soil Structure and FabricSoil Structure and Fabric

Soils are different from other engineering material in that

Soils are Soils are natural materialsnatural materials--site characterization difficultsite characterization difficult

Soils are Soils are particulate materialparticulate material

Soils can contain all Soils can contain all three phasesthree phases of matterof matter--solid, liquid, and gassolid, liquid, and gas

Engineering properties depends largely on Engineering properties depends largely on the the interaction between particlesinteraction between particles,,for fine soils, for fine soils, interaction between particles and interaction between particles and the pore waterthe pore water,,and only secondly on their internal properties (i.e. and only secondly on their internal properties (i.e. molecular bonds).molecular bonds).

Soil : 3-phase Material

The physical properties of the 3-phase material includes:

Proportions of each phaseProportions of each phase-- Phase relationshipPhase relationship

Characteristics of each phaseCharacteristics of each phase-- Particle size, shape, and mineral Particle size, shape, and mineral

Interaction between phasesInteraction between phases-- AtterbergAtterberg limitslimits

Soil structure and fabricSoil structure and fabric

Soil : 3-phase Material

Solid

Air

Water

Mineral Skeleton

Idealization:Three Phase Diagram

Mineral Skeleton

Solid Particles

Voids

Phase Relationship

Three Phase Diagram

S : Solid Soil particle

W: Liquid Water (electrolytes) A: Air Air

Phase Relationship

Phase Parameters

In practice, we measure In practice, we measure VVtt, , MMtt, , MMww, , and and ρρss (or assumed), then we can (or assumed), then we can calculate the rest of variables in the phase diagramcalculate the rest of variables in the phase diagram..

So there are only So there are only 4 independent variables4 independent variables in the phase in the phase diagram. diagram. (3 independent variables in relative sense, (3 independent variables in relative sense, VVtt=1.0)=1.0)

However, we are more interested in However, we are more interested in the the ““relative proportionrelative proportion””, so several , so several phase parameters are defined for the phase parameters are defined for the sake of convenience in particular sake of convenience in particular problems. problems.

Phase Relationship

Volumetric Ratios (LHS)

((1) Void ratio 1) Void ratio e e (given in decimal, 0.65)(given in decimal, 0.65)

(2) Porosity (2) Porosity n n (given in percent 100%, 65%)(given in percent 100%, 65%)

(3) Degree of Saturation (3) Degree of Saturation S S (given in percent 100%, 65%)(given in percent 100%, 65%)

)V(solidsofVolume

)V(voidsofVolumee

s

v=

)V(samplesoilofvolumeTotal

)V(voidsofVolumen

t

v=

%100)V(voidsofvolumeTotal

)V(watercontainsvoidsofvolumeTotalS

v

w ×=

e1

e

)e1(V

eVn

s

s

+=

+=

Phase Relationship

Engineering Applications (e)Typical valuesTypical values Engineering applications:Engineering applications:

Volume change tendencyVolume change tendency

StrengthStrength

(Lambe and Whitman, 1979)

Simple cubic (SC), e = 0.91, Contract

Cubic-tetrahedral (CT), e = 0.65, Dilate

Link: the strength of rock joint

)itan(strengthShear n +φσ=

i

Phase Relationship

Engineering Implications (e)(Cont.)

Hydraulic conductivityHydraulic conductivity

Which packing (SC or Which packing (SC or CT) has higher hydraulic CT) has higher hydraulic conductivity?conductivity?

SC

e = 0.91

CT

e = 0.65

The fluid (water) can flow more easily through the soil with higher hydraulic conductivity

Phase Relationship

Engineering Applications (e)(Cont.)

SC

e = 0.91

CT

e = 0.65

The finer particle cannot pass through the void

•Clogging

•Critical state soil mechanics

Filter

Phase Relationship

Engineering Applications (S)Completely dry soil S = 0 %Completely dry soil S = 0 %

Completely saturated soil S = 100%Completely saturated soil S = 100%

Unsaturated soil (partially saturated soil) 0% < S < 100%Unsaturated soil (partially saturated soil) 0% < S < 100%

Demonstration:Demonstration:

Effects of capillary forcesEffects of capillary forces

Engineering implications:Engineering implications:

Slope stabilitySlope stability

Underground excavationUnderground excavation

%100)V(voidsofvolumeTotal

)V(watercontainsvoidsofvolumeTotalS

v

w ×=

Phase Relationship

Engineering Applications (S) (Cont.)

• 80 % of landslides are due to erosion and “loss in suction” in Hong Kong.

• The slope stability is significantly affected by the surface water.

(Au, 2001)

Phase Relationship

Density and Unit WeightMass is a measure of a body's Mass is a measure of a body's inertia, or its "quantity of inertia, or its "quantity of matter". Mass is not changed at matter". Mass is not changed at different places. different places.

Weight is force, the force of Weight is force, the force of gravity acting on a body. The gravity acting on a body. The value is different at various value is different at various places (Newton's second law F places (Newton's second law F = ma)= ma)

The unit weight is frequently The unit weight is frequently used than the density is (e.g. in used than the density is (e.g. in calculating the overburden calculating the overburden pressure).pressure).

w

s

w

s

w

ss

3

2

g

gG

mkN8.9,Water

secm8.9g

gravitytodueonaccelerati:g

Volume

gMass

Volume

Weight,weightUnit

Volume

Mass,Density

γγ

=⋅ρ⋅ρ

=ρρ

=

=γ

⋅ρ=⋅ρ=γ

⋅==γ

=ρ

Gs ≈ 2.65 for most soils

Phase Relationship

Typical Values of Specific Gravity

(Lambe and Whitman, 1979)

(Goodman, 1989)

Phase Relationship

Density of Soil

a. Dry densitya. Dry density

b. Total, Wet, or Moist density b. Total, Wet, or Moist density (0%<S<100%, Unsaturated)(0%<S<100%, Unsaturated)

c. Saturated densityc. Saturated density(S=100%, (S=100%, VVaa =0)=0)

d. Submerged densityd. Submerged density(Buoyant density)(Buoyant density)


)M(solidssoilofMass

t

sd =ρ


)MM(samplesoilofMass

t

ws +=ρ


)MM(watersolidssoilofMass

t

wssat

++=ρ

wsat' ρ−ρ=ρ

Phase Relationship

Density of Soil (Cont.)

Submerged unit weight:Submerged unit weight:

Consider the buoyant Consider the buoyant force acting on the soil force acting on the soil solids:solids:

ArchimedeArchimede’’ss principleprinciple::The buoyant force on a body The buoyant force on a body

immersed in a fluid is equal to immersed in a fluid is equal to the weight of the fluid displaced the weight of the fluid displaced by that object. by that object.

wsat' γ−γ=γ

wsat

t

wtws

t

wwts

t

wwts

t

wss

V

VWW

V

WVW

%)100S(V

)VV(W

V

VW

γ−γ=

γ⋅−+=

+γ⋅−=

=γ⋅−−

=γ⋅−

Phase Relationship

Weight Ratios (RHS)

Water Content, Water Content, ww

For some organic soils For some organic soils ww>100%, up to 500 %>100%, up to 500 %

For quick clays, For quick clays, ww>100%>100%

%100)(

)(⋅=

s

w

MsolidssoilofMass

MwaterofMassw

%100)(

)(⋅=

s

w

WsolidssoilofWeight

WwaterofWeightw

or

Phase Relationship

Engineering Applications (w)For fineFor fine--grained soils, water plays a grained soils, water plays a

critical role to their engineering critical role to their engineering properties (discussed in the next properties (discussed in the next topic).topic).

For exampleFor example,,

The quick clay usually has a water The quick clay usually has a water content content w w greater thangreater than 100 % and a 100 % and a card house structure. It will behave card house structure. It will behave like a viscous fluid after it is fully like a viscous fluid after it is fully disturbed.disturbed.

Clay particle

Water

(Mitchell, 1993)

Phase Relationship

Summary of Phase Parameters

Mg/m3ρsat-ρwSubmerged density, ρ’

Mg/m3(Ms+Mw)/Vt ,S=100%

Saturated Density, ρsat

Mg/m3Mt /VtTotal (Wet) Density, ρt

Mg/m3Ms /VtDry Density, ρd

Mg/m3Ms /VsDensity of Solid, ρsDensity

%Mw /Ms Gravimetric Water Content, wMass Ratio

DimensionlessVw /VtVolumetric Water Content, θ%Vw /VvDegree of Saturation, S

DimensionlessVv /VtPorosity, n

DimensionlessVv /VsVoid Ratio, eVolumetric Ratio

UnitDefinitionTerminologyDescription of Phase

Phase Relationship

wρt/(1+w)ρt/(1+w)wρt/[(1+w)ρw]ρt/[(1+w)ρs]ρs, w, ρt

θρwρt-θρwθ(ρt-θρw)/ρsρs, θ, ρt

wρdρdwρd/ρwρd/ρsρs, w, ρd

θρwρdθρd/ρsρs, θ, ρd

MwMsVwVs

RHS of Phase DiagramLHS of Phase DiagramIndependent Parameters

1=

In a relative sense, let Vt=1.0, only three independent parameters areneeded to completely define the phase diagram. Mostly commonlyused are water content, soil density,and ρs.

Phase Relationship

Important Relationships

sGweS ⋅=⋅

e

en

+=

1

wets

d +=

+=

11

γγγ

ws

ws

sat

st

e

Ge

eGe

w

γγ

γγ

γγ

+−

=

++

=

++

=

1

1'

1

1

1

There are only three independent phase parameters, relationships among phase parameters can be derived.

Three most useful equations:

Phase Relationship

s

w

w

w

s

s

s

w

w

s

s

ws

s

w

s

v

v

w

s

V

V

VM

VM

M

M

M

MGw

V

V

V

V

V

VeS

GweS

=⋅=ρρ⋅=⋅

=⋅=⋅

⋅=⋅

e

e

VV

V

V

Vn

vs

vv

+=

+==

1

w

ww

V

W

V

WW

V

W

eVV

W

V

W

td

dtd

tw

tws

d

s

vs

ssd

+=∴

−=−=−=−

==

+=

+==

1

/1

1

γγ

γγγ

γγγ

γγ

Proofs:

Phase Relationship

Solving Phase-Relation Problems

Often encounter problems where some parameters are Often encounter problems where some parameters are known and others need to be determined. Two known and others need to be determined. Two approaches:approaches:

I. Use the 3 useful I. Use the 3 useful eqseqs and fundamental definitionsand fundamental definitions..

II. Go back to the fundamental and use the phase diagramII. Go back to the fundamental and use the phase diagram

1.1. Draw a phase diagram and annotate all of the Draw a phase diagram and annotate all of the dimensions presented in the problem.dimensions presented in the problem.

2.2. Assume either Assume either VVtt=1 or V=1 or Vss=1, =1, if not givenif not given..

3.3. Determine all the remaining dimensionsDetermine all the remaining dimensions

4.4. Calculate the required parametersCalculate the required parameters

Phase Relationship

Examples

Phase Relationship

Grain Size Distribution

Coarse-grained soils:

Gravel Sand

Fine-grained soils:

Silt Clay

0.075 mm(#200)

Sieve analysis Hydrometer analysis

2μm4.76 mm(# 4)

Particle-Size Distribution

Note:Clay-size particles

Clay minerals

For example:

Kaolinite, Illite, etc.

For example:

A small quartz particle may have the similar size of clay minerals.


Sieve Analysis

ASTM D422ASTM D422

This procedure is suitable for This procedure is suitable for coarse grained soils > 75 coarse grained soils > 75 μμmm

E.g. No.10 sieve E.g. No.10 sieve ……. has 10 . has 10 apertures per linear inchapertures per linear inch


Hydrometer Analysis

ASTM D422, Also called Sedimentation Analysis

Used for smaller particlesUsed for smaller particles (75μm – 1 μm)

Analysis based on Analysis based on StokeStoke’’ss Law, velocity proportional Law, velocity proportional to diameter.to diameter.


( )

)/(

182

scm

DGG

v wws

ηγ−

= ( ))(

18

cm

GG

vD

wws γη

−=

Stoke’s Law

v = L/t P (%)=R/Ms

L = 16.3-0.1641Rgrams of soil still in Suspension for Gs=2.65 and Gw=1.0, may also require zero and temperature correction

Hydrometer Analysis (Cont.)


0 .0 0 0 1 0 .0 0 1 0 .0 1 0 .1 1 1 0 1 0 00

2 0

4 0

6 0

8 0

1 0 0

P a r tic le s ize (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

Grain Size Distribution Curves (Grading Curves)


Log scale

Fin

er

Effective size D10: 0.02 mmD30 D60

Characteristics of Grading Curve

45002.0

9

10

60 ===D

DC

uniformityoftCoefficien

u

(Holtz and Kovacs, 1981)

2)9)(02.0(

)6.0(

))((

)( 2

6010

230 ===

DD

DC

curvatureoftCoefficien

c


Characteristics of Grading Curve(1) Central tendency: D10 (2) Dispersion: Cu

(2) Curvature: Cc

)sandsfor(

6Cand3C1

)gravelsfor(

4Cand3C1

soilgradedWell

uc

uc

≥<<

≥<<−

CriteriaCriteria

Cc ↓

Cc ↑

Cu=1 for one grain sizeThe smaller 10% dominate


Grain Size Distribution (Cont.)Engineering applicationsEngineering applications−− It will help us It will help us ““feelfeel”” the soil texture (what the soil is) the soil texture (what the soil is)

and it will also be used for the soil classification (next and it will also be used for the soil classification (next topic).topic).

−− It can be used to define the grading specification of a It can be used to define the grading specification of a drainage filter (clogging).drainage filter (clogging).

−− It can be a criterion for selecting fill materials of It can be a criterion for selecting fill materials of embankments and earth dams, road subembankments and earth dams, road sub--base materials, base materials, and concrete aggregates.and concrete aggregates.

−− Effective Size, DEffective Size, D1010, can be correlated with the hydraulic , can be correlated with the hydraulic conductivity (describing the permeability of soils). conductivity (describing the permeability of soils). (Hazen(Hazen’’s Equation).(s Equation).(Note: controlled by small particlesNote: controlled by small particles))

The grain size distribution is more important to coarse-grained soils.


Particle Shape

−− Important for granular soilsImportant for granular soils

•• Angular soil particle Angular soil particle →→ higher frictionhigher friction

•• Round soil particle Round soil particle →→ lower frictionlower friction

•• Note that clay particles are sheetNote that clay particles are sheet--like.like.

Rounded Subrounded

Subangular Angular(Holtz and Kovacs, 1981)

Coarse-grained soils

Particle-Shape

Granular Soils

i.e. silt, sand, graveli.e. silt, sand, gravel

Effect of gravity >> effect of electrochemical Effect of gravity >> effect of electrochemical forces between particlesforces between particles

WaterWater--particle interaction negligibleparticle interaction negligible

Engineering properties not affected by waterEngineering properties not affected by water(Except for influence due to change of pore (Except for influence due to change of pore pressure)pressure)

Granular Soil vs. Clay

Granular Soils (Cont.)

Engineering properties of granular soil Engineering properties of granular soil primarily depends on how dense the soil isprimarily depends on how dense the soil is

Relative DensityRelative Density

%100

%100

dmindmax

dminddmax

minmax

max

×−−

×=

×−−

=

γγγγ

γγ

d

r ee

eeD

Dr Classification0-15 Very loose15-35 Loose35-65 Medium dense65-85 Dense85-100 Very dense


ClayBecause of small particle size and plateBecause of small particle size and plate--like shape of like shape of clays, the surface area to mass ratio (specific surface) is clays, the surface area to mass ratio (specific surface) is much greater than other soils. much greater than other soils. e.g. As of e.g. As of MontmonrilloniteMontmonrillonite≈≈ 800 m800 m22/g. /g. Large specific surface Large specific surface more contact area more contact area between particles and greater affinity for absorbing between particles and greater affinity for absorbing water.water.Effect of gravity < effect of electrochemical forces Effect of gravity < effect of electrochemical forces between particles.between particles.Engineering properties vary with water content.Engineering properties vary with water content.Most clays are mixed with silt and/or sand, small Most clays are mixed with silt and/or sand, small percentage of clay significantly affects the behavior of percentage of clay significantly affects the behavior of soils.soils.


Granular Soils vs. Clay(Holtz and Kovacs, 1981)


Polar Water MoleculesStructure Polar molecule

H(+) H(+)

O(-)

Hydrogen bond Salts in aqueous solution

hydration

Soil-Water Interaction

Adsorbed layers

3 monolayers

Clay-Water Interaction1. Hydrogen bond

Kaolinite

Oxygen Hydroxyl Cla

y S

urfa

ces

Free water

Bulk water

The water molecule “locked” in the adsorbed layers has different properties compared to that of the bulk water due to the strong attraction from the surface.

O OH

HO

HO

H

OH

H


Clay-Water Interaction (Cont.)

The water molecules wedge into the interlayer after adding water

2. Ion hydration

Dry condition

(Interlayer)

Clay layers

cation

The cations are fully hydrated, which results in repulsive forces and expanding clay layers (hydration energy).

Na+ crystal radius: 0.095 nm

radius of hydrated ion: 0.358 nm


Clay-Water Interaction (Cont.)

The concentration of cations is higher in the interlayers (A) compared with that in the solution (B) due to negatively charged surfaces. Because of this concentration difference, water molecules tend to diffuse toward the interlayer in an attempt to equalize concentration.

3. Osmotic pressure

From Oxtoby et al., 1994

A B


Diffuse Double Layer

Clay particle with negatively charged surface

x Distance xCon

cent

rati

on

Exponential decay

Cations

Anions

-

-

++-

-

-

--


O

A1000

Clay WaterSoil-Water Interaction

Thickness of Double Layer

Thickness of double layer K

Valence:

eargchElectron:e

ionconcentratCation:n

eTemperatur:T

ttanconsBoltzman:k

typermittivilativeRe:

vacuumintyPermittivi:

en2

kTK

0

0

2/1

220

0

ν

κε

⎟⎟⎠

⎞⎜⎜⎝

⎛ν

⋅κε=

K ↑ repulsion force ↑

n0↑ K ↓ repulsion force ↓

v ↑ K ↓ repulsion force ↓

T ↑ K ↑ repulsion force ↑(?) κ decreases with increasing temperature


Atterberg LimitsThe presence of The presence of waterwater in in finefine--grained soilsgrained soils can significantly affect can significantly affect associated engineering behavior, so we need a associated engineering behavior, so we need a reference indexreference index to clarify to clarify the effects.the effects.


In percentage


Liquid Limit-LL

Cone Penetrometer Cone Penetrometer MethodMethod

••(BS 1377: Part 2: (BS 1377: Part 2: 1990:4.3)1990:4.3)

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

••Multipoint testMultipoint test

••OneOne--point testpoint test

CasagrandeCasagrande MethodMethod

••(ASTM D4318(ASTM D4318--95a)95a)

••Professor Professor CasagrandeCasagrandestandardized the test and standardized the test and developed the liquid developed the liquid limit device.limit device.

••Multipoint testMultipoint test

••OneOne--point testpoint test


Liquid Limit-LL (Cont.)

Dynamic shear testDynamic shear test

•• Shear strength is about 1.7 Shear strength is about 1.7 ~2.0 ~2.0 kPakPa..

•• Pore water suction is about Pore water suction is about 6.0 6.0 kPakPa. .

•• (review by Head, 1992; Mitchell, 1993).(review by Head, 1992; Mitchell, 1993).

Particle sizes and waterParticle sizes and water

•• Passing No.40 Sieve (0.425 Passing No.40 Sieve (0.425 mm).mm).

•• Using Using deionizeddeionized water.water.

The type and amount of The type and amount of cationscations can significantly can significantly affect the measured results.affect the measured results.


Casagrande Method

N=25 blows

Closing distance = 12.7mm (0.5 in)

•Device

The water content, in percentage, required to close a distance of 0.5 in (12.7mm) along the bottom of the groove after 25 blows is defined as the liquid limit


Casagrande Method (Cont.)

( ).log

)(/log

,12

21

contNIw

valuepositiveachooseNN

wwIindexFlow

F

F

+−=

−=

Multipoint Method

(c)2

001

Bro

oks/

Col

e, a

div

isio

n of

Tho

mso

n L

earn

ing,

Inc

. T

hom

son

Lea

rnin

g ™is

a tr

adem

ark

used

he

rein

und

er li

cens

e.


Casagrande Method (Cont.)One-point Method

• Assume a constant slope of the flow curve.

• The slope is a statistical result of 767 LL tests.

Limitations:• β is an empirical coefficient,

so it is not always 0.121.• Good results can be obtained

only for blow number around 20 to 30.

121.0tan

25

tan

=β==

⎟⎠⎞

⎜⎝⎛=

β

contentmoistureingcorrespondw

blowsofnumberN

NwLL

n

n


Cone Penetrometer Method•Device

(Head, 1992)

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


Cone Penetrometer Method (Cont.)

Multipoint Method


Cone Penetrometer Method (Cont.)

44094.140LL,094.1Factor

%,40w,mm15depthnPenetratio

≈⋅====

(Review by Head, 1992)

One-point Method (an empirical relation)

Example:


Casagrande vs. Cone Penetrometer

Littleton and Farmilo, 1977 (from Head, 1992)

A good correlation between the two methods can be observed as the LL is less than 100.


Plastic Limit-PL

The plastic limit PL is defined as the water content at which a soil thread with 3.2 mm diameter just crumbles.

ASTM D4318-95a, BS1377: Part 2:1990:5.3



Shrinkage Limit-SL

Definition of shrinkage limit:

The water content at which the soil volume ceases to change is defined as the shrinkage limit.


Shrinkage Limit-SL (Cont.)

)100)((M

VV)100(

M

MM

(%)w(%)wSL

w2

fi

2

21

i

ρ⎟⎟⎠

⎞⎜⎜⎝

⎛ −−⎟⎟

⎠

⎞⎜⎜⎝

⎛ −=

Δ−=


Shrinkage Limit-SL (Cont.)

““Although the shrinkage limit was a popular classification test Although the shrinkage limit was a popular classification test during the 1920s, it is subject to considerable uncertainty and during the 1920s, it is subject to considerable uncertainty and thus is no longer commonly conducted.thus is no longer commonly conducted.””

““One of the biggest problems with the shrinkage limit test is One of the biggest problems with the shrinkage limit test is that the amount of shrinkage depends not only on the grain that the amount of shrinkage depends not only on the grain size but also on the initial fabric of the soil. The standard size but also on the initial fabric of the soil. The standard procedure is to start with the water content near the liquid limprocedure is to start with the water content near the liquid limit. it. However, especially with sandy and silty clays, this often However, especially with sandy and silty clays, this often results in a shrinkage limit greater than the plastic limit, whiresults in a shrinkage limit greater than the plastic limit, which ch is meaningless. is meaningless. CasagrandeCasagrande suggests that the initial water suggests that the initial water content be slightly greater than the PL, if possible, but content be slightly greater than the PL, if possible, but admittedly it is difficult to avoid entrapping air bubbles.admittedly it is difficult to avoid entrapping air bubbles.””(from Holtz and Kovacs, 1981) (from Holtz and Kovacs, 1981)


Typical Values of Atterberg Limits

(Mitchell, 1993)


Atterberg Limit of Clay Minerals

Lambe and Whitman, 1979

Na-montmorillonite

•Thicker double layer

•LL=710

Ca-montmorillonite

•Thinner double layer

•LL=510

The thickness of double layer increases with decreasing cation valence.


IndicesPlasticity index PIPlasticity index PI

For describing the range of For describing the range of wwover which a soil was plasticover which a soil was plastic

PI = LL PI = LL –– PLPL

Liquidity index LILiquidity index LI

For scaling the natural For scaling the natural ww of a of a soil sample to the Limits.soil sample to the Limits.

contentwatertheiswPLLL

PLw

PI

PLwLI

−−

=−

=

LI <0 (A), brittle fracture if sheared

0<LI<1 (B), plastic solid if sheared

LI >1 (C), viscous liquid if sheared


Indices (Cont.)

Sensitivity SSensitivity Stt (for clays)(for clays)

strengthshearUnconfined

)disturbed(Strength

)dundisturbe(StrengthSt =

(Holtz and Kavocs, 1981)

Clay particle

Water

w > LL


Indices (Cont.)Activity AActivity A ((SkemptonSkempton, 1953), 1953)

mm002.0:fractionclay

)weight(fractionclay%

PIA

<

=

Normal clays: 0.75<A<1.25Normal clays: 0.75<A<1.25

Inactive clays: A<0.75Inactive clays: A<0.75

Active clays: A> 1.25Active clays: A> 1.25

High activity:High activity:

large volume change when wettedlarge volume change when wetted

Large shrinkage when driedLarge shrinkage when dried

Very reactive (chemically)Very reactive (chemically)

Purpose

Both the type and amount of clay in soils will affect the Atterberglimits. This index is aimed to separate them.

Mitchell, 1993


Soil classificationSoil classification

The The AtterbergAtterberg limits are usually correlated with some engineering limits are usually correlated with some engineering properties such as the permeability, compressibility, shear streproperties such as the permeability, compressibility, shear strength, and ngth, and others.others.−− In general, clays with high plasticity have lower permeability, In general, clays with high plasticity have lower permeability, and they are and they are

difficult to be compacted.difficult to be compacted.−− The values of SL can be used as a criterion to assess and prevenThe values of SL can be used as a criterion to assess and prevent the t the

excessive cracking of clay liners in the reservoir embankment orexcessive cracking of clay liners in the reservoir embankment or canal.canal.

Engineering Applications

−The Atterberg limit enable clay soils to be classified.


Soil Structure and FabricThe The structurestructure of a soil is taken to mean both the of a soil is taken to mean both the geometric geometric arrangementarrangement of the particles or mineral grains as well as the of the particles or mineral grains as well as the interparticleinterparticle forcesforces which may act between them. which may act between them.

Soil Soil fabricfabric refers only to the refers only to the geometric arrangementgeometric arrangement of particles of particles (from (from Holtz and Kovacs, 1981).Holtz and Kovacs, 1981).

The The interparticleinterparticle forces (or surface forces) are relatively important forces (or surface forces) are relatively important for finefor fine--grained soils at low confinement (low state of stress).grained soils at low confinement (low state of stress).““Although the behavior of a coarseAlthough the behavior of a coarse--grained soil can often be related grained soil can often be related to particle size distribution, the behavior of a finedto particle size distribution, the behavior of a fined--grained soil grained soil usually depends much more on geological history and structure thusually depends much more on geological history and structure than an on particle sizeon particle size”” (from (from LambeLambe and Whitman, 1979)and Whitman, 1979)..

*Fabric and structure are used interchangeably sometimes.

Soil Fabric

Microfabric Features in Natural Soils

1.1. Elementary particle arrangementsElementary particle arrangements, which consist of single , which consist of single forms of particle interaction at the level of individual clay, forms of particle interaction at the level of individual clay, silt, or sand particles or interaction between small groups of silt, or sand particles or interaction between small groups of clay platelets or clothed silt and sand particles.clay platelets or clothed silt and sand particles.

2.2. Particle assemblagesParticle assemblages, which are units of particle , which are units of particle organization having definable physical boundaries and a organization having definable physical boundaries and a specific mechanical function. Particle assemblages consist of specific mechanical function. Particle assemblages consist of one or more forms of elementary particle arrangements or one or more forms of elementary particle arrangements or smaller particle assemblages.smaller particle assemblages.

3.3. Pore spacesPore spaces within and between elementary particles within and between elementary particles arrangements and particle assemblages.arrangements and particle assemblages.

Collins and McGown, 1974 (from Holtz and Kovacs, 1981)

Soil Fabric

Elementary ParticlesIndividual clay platelet interaction

Individual silt or sand particle interaction

Clay platelet group interaction

Clothed silt or sand particle interaction Particle discernible


Soil Fabric

Particle Assemblages


Soil Fabric

Pore Space Types

Collins and McGown, 1974 (from Mitchell, 1993)

Soil Fabric

Terminology

Dispersed: No faceDispersed: No face--toto--face association of clay particlesface association of clay particles

Aggregated: FaceAggregated: Face--toto--face association (FF) of several clay particles.face association (FF) of several clay particles.

Flocculated: EdgeFlocculated: Edge--toto--Edge (EE) or edgeEdge (EE) or edge--toto--face (EF) associationface (EF) association

Deflocculated: No association between aggregatesDeflocculated: No association between aggregates

Face (F)

Edge (E)

Clay Particle

van Olphen, 1991 (from Mitchell, 1993)

Soil Fabric-Clay

Particle Associations

Dispersed and deflocculated Aggregated but deflocculated

Edge-to-face flocculated and aggregated

Edge-to-edge flocculated and aggregated

Edge-to-face and edge to edge flocculated and aggregated

Edge-to-edge flocculated but dispersed

Edge-to-face flocculated but dispersed

van Olphen, 1991

Soil Fabric-Clay

Interaction of Clay Particles (or Layers)

Layer

InterlayerInterparticle

Particle

Net force between clay particles (or interlayers)

= van der Waals attraction +

Double layer repulsion (overlapping of the double layer)+

Coulombian attraction (between the positive edge and negative face)

DLVO forces

Soil Fabric-Clay

Thickness of Double Layer

Thickness of double layer K

Valence:

eargchElectron:e

ionconcentratCation:n

eTemperatur:T

ttanconsBoltzman:k

typermittivilativeRe:

vacuumintyPermittivi:

en2

kTK

0

0

2/1

220

0

ν

κε

⎟⎟⎠

⎞⎜⎜⎝

⎛ν

⋅κε=

K ↑ repulsion force ↑

n0↑ K ↓ repulsion force ↓

v ↑ K ↓ repulsion force ↓

T ↑ K ↑ repulsion force ↑(?) κ decreases with increasing temperature

Soil Fabric-Clay

Interaction of Clay ParticlesDispersed fabric

The net interparticle force between surfaces is repulsive

IncreasingElectrolyte concentration n0

Ion valence vTemperature T (?)

DecreasingPermittivity κSize of hydration ionpHAnion adsorption

•Reduce the double layer repulsion (only applicable to some cases)

•Flocculated or aggregated fabric

Flocculated fabric

Edge-to-face (EF): positively charged edges and negatively charged surfaces (more common)

Edge-to-edge (EE)

Aggregated fabric

Face-to-Face (FF) Shifted FF

Soil Fabric-Clay

Fabric of Natural Clay Soils

““The individual clay particles seem to always be aggregated or The individual clay particles seem to always be aggregated or flocculated together in submicroscopic fabric units called flocculated together in submicroscopic fabric units called domainsdomains. Domains then in turn group together to form . Domains then in turn group together to form clustersclusters, , which are large enough to be seen with a visible light microscopwhich are large enough to be seen with a visible light microscope. e. Clusters group together to form Clusters group together to form pedspeds and even groups of and even groups of pedspeds. . PedsPeds can be seen without a microscope, and they and other can be seen without a microscope, and they and other macrostructuralmacrostructural features such as joints and fissures constitute the features such as joints and fissures constitute the macrofabricmacrofabric systemsystem”” (from Holtz and Kovacs, 1981).(from Holtz and Kovacs, 1981).

Domain Domain →→ Cluster Cluster →→PedPed

Soil Fabric-Clay

Fabric of Natural Clay Soils (Cont.)

Enlargement

Domains and clusters with micropores

1.Domain

2.Cluster

3.Ped

4.Silt grain

5.Micropore

6.Macropore

Yong and Sheeran (1973) (from Holtz and Kovacs, 1981)

Soil Fabric-Clay

Fabric of Natural Clay Soils (cont.)

MacrostructureMacrostructure, including the , including the stratigraphystratigraphy of fineof fine--grained soil deposits, has grained soil deposits, has an important influence on soil behavior in engineering practice.an important influence on soil behavior in engineering practice. Joints, Joints, fissures, silt and sand seams, root holes, fissures, silt and sand seams, root holes, varvesvarves, and other defects often , and other defects often control the engineering behavior of the entire soils mass.control the engineering behavior of the entire soils mass.

The The microstructuremicrostructure reflects the depositional history and environment of the reflects the depositional history and environment of the deposit, its weathering history (both chemical and physical), andeposit, its weathering history (both chemical and physical), and stress d stress history.history.

Clay particle

Water

(From Holtz and Kovacs, 1981)

Soil Fabric-Clay

PackingDense packingLoose packing

Honeycombed fabric•Meta-stable structure

•Loose fabric

•Liquefaction

•Sand boil

Holtz and Kovacs, 1981

Soil Fabric-Granular Soil

Packing (Cont.)-Sand Boil

Loose sand

Kramer, 1996


Packing (Cont.)

““Contrary to popular belief, it is not possible to drown Contrary to popular belief, it is not possible to drown in quicksand, unless you really work at it, because the in quicksand, unless you really work at it, because the density of quicksand is much greater than that of water. density of quicksand is much greater than that of water. Since you can almost float in water, you should easily Since you can almost float in water, you should easily be able to float in quicksand be able to float in quicksand ““((from Holtz and Kovacs, 1981from Holtz and Kovacs, 1981).).

Help!


Load TransferLoading

The black particles carry most of load. The remaining particles prevent the buckling of the load-carrying chains (From Santamarina et al., 2001).


The Relative Density (Dr)

The relative density DThe relative density Drr is used to characterize the is used to characterize the density of natural granular soil.density of natural granular soil.

%100

%100ee

eeD

mindmaxd

mindd

d

maxd

minmax

maxr

×γ−γγ−γ

×γγ

=

×−−

=

(Lambe and Whitman, 1979)The relative density of a natural soil deposit very strongly affects its engineering behavior. Consequently, it is important to conduct laboratory tests on samples of the sand at the same relative density as in the field ( from Holtz and Kovacs, 1981). (compaction)


The Relative Density (Dr) (Cont.)“The relative density (or void ratio) alone is not sufficient to characterize the engineering properties of granular soils” (Holtz and Kovacs, 1981). Two soils with the same relative density (or void ratio) may contain very different pore sizes. That is, the pore size distribution probably is a better parameter to correlate with the engineering properties (Santamarina et al., 2001).

2 1:Holtz and Kovacs, 1981


Loess

Capillary force

• Loess is a type of aeolian soils, and the particles are predominantly silt-size. The soil structure is mainly stabilized by (1) the capillary force and (2) light cementation arising from the salt and fines (e.g. clay) precipitation around the contacts (Holtz and Kovacs, 1981; Santamarina, 2001).

Cementation

• After loess is submerged, collapse of the soil structure occurs due to loss of suction and cementation

Capillary force cementation

Why?

The interaction between water and salts and clay


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