teaching unsaturated soil mechanics as part of the ... · teaching unsaturated soil mechanics as...

Teaching Unsaturated Soil Teaching Unsaturated Soil Mechanics as Part of the Mechanics as Part of the

Undergraduate Civil Engineering Undergraduate Civil Engineering Curriculum Curriculum

Delwyn G. Fredlund, Visiting Professor

Kobe University, Kobe, Japan

Sapporo, Hokkaido, JapanFebruary 15, 2005

IntroductionIntroductionSoil mechanics textbooks do not address Soil mechanics textbooks do not address the full scope of problems encountered in the full scope of problems encountered in geotechnical engineeringgeotechnical engineeringTextbooks for undergraduate students Textbooks for undergraduate students focus on the behavior of focus on the behavior of saturated soilssaturated soilsUndergraduate students are generally Undergraduate students are generally taught very little about taught very little about unsaturated soilunsaturated soilbehaviorbehavior

Personal Observation Upon Personal Observation Upon Graduation From UniversityGraduation From University

•• After graduation I realized I had been taught soil After graduation I realized I had been taught soil mechanics courses related to saturated soil behavior mechanics courses related to saturated soil behavior and then found myself faced with attempting to solve and then found myself faced with attempting to solve many problems where the soil was unsaturated. After many problems where the soil was unsaturated. After 2 years I observed that about 90% of the problems I 2 years I observed that about 90% of the problems I had addressed involved soils with negative porehad addressed involved soils with negative pore--water water pressures. I began to feel that I had been taught soil pressures. I began to feel that I had been taught soil mechanics for saturated soils and then had to go out mechanics for saturated soils and then had to go out and practice soil mechanics for unsaturated soils. I and practice soil mechanics for unsaturated soils. I was illwas ill--equipped to face a world with unsaturated soil equipped to face a world with unsaturated soil mechanics problemsmechanics problems

Undergraduate Student NeedsUndergraduate Student NeedsUndergraduate engineering students need:Undergraduate engineering students need:

–– to be to be betterbetter--equippedequipped to face a geotechnical to face a geotechnical world with world with unsaturated soilsunsaturated soils problemsproblems

–– to better understand the to better understand the basic differencesbasic differencesbetween the behavior of between the behavior of saturatedsaturated and and unsaturated soilsunsaturated soils

–– to know the to know the concepts and fundamentals concepts and fundamentals behind behind unsaturated soilunsaturated soil behavior and learn to behavior and learn to ““think the way the unsaturated soil think the way the unsaturated soil behavesbehaves””

Is There a Need to Teach Is There a Need to Teach Unsaturated Soil Mechanics?Unsaturated Soil Mechanics?

Many Many Civil EngineeringCivil Engineering problems involve the problems involve the interaction between the interaction between the climate climate and the and the unsaturated soilunsaturated soil zone (i.e., flux boundary zone (i.e., flux boundary conditions)conditions)

–– FoundationsFoundations of many structures are near of many structures are near ground surfaceground surface

–– Expansive soilsExpansive soils problems impose a large problems impose a large financial burden on society in many countriesfinancial burden on society in many countries

–– Human BeingsHuman Beings usually contaminates the usually contaminates the environment starting at the ground surfaceenvironment starting at the ground surface

Teaching Unsaturated Soil Mechanics Teaching Unsaturated Soil Mechanics at the Undergraduate Levelat the Undergraduate Level

ToTo--date there has been little desire to teach date there has been little desire to teach unsaturated soilunsaturated soil mechanicsmechanics at the at the undergraduate levelundergraduate levelIt may be easier to introduce the It may be easier to introduce the basicbasicconcepts concepts of of unsaturated soilunsaturated soil mechanics at the mechanics at the undergraduate level than at the graduate level undergraduate level than at the graduate level It is suggested that the It is suggested that the basic conceptsbasic concepts of of saturatedsaturated--unsaturated soil mechanicsunsaturated soil mechanics be be taught using simple illustrative diagrams taught using simple illustrative diagrams

Saturated Soil Mechanics

Negative pore-waterpressures

Positive pore-waterpressures

Matric suction(ua - uw)

Net normal stress(σ - ua)

(σ - uw)Effective stress

Broad Categorization of Soil Mechanics Based on Broad Categorization of Soil Mechanics Based on Stress State VariablesStress State Variables

The “Real World” has a Moisture The “Real World” has a Moisture Flux Boundary ConditionFlux Boundary Condition

Saturated soil mechanicsSaturated soil mechanics has largely ignored has largely ignored ground surface moisture flux conditionsground surface moisture flux conditions

Changes in negative poreChanges in negative pore--water pressure water pressure (and consequently (and consequently matricmatric suction) in suction) in unsaturated soils can be caused by:unsaturated soils can be caused by:

–– precipitation and infiltrationprecipitation and infiltration–– evaporationevaporation–– transpirationtranspiration–– coverscovers

Saturated Soil

UnsaturatedSoil

Positivepore-waterpressures


Upwardflux

Downwardflux

Total stressPore-airpressures

Capillary fringe

Evaporation Evapotranspiration Precipitation

Hydrostatic

Visualization Visualization of the Role of the Role

of the Surface of the Surface Flux Flux

Boundary Boundary ConditionCondition

Long Term Response of the Water Long Term Response of the Water Table to Arid Climatic ConditionsTable to Arid Climatic Conditions




Arid regions

(Evaporation > Precipitation)

General ObjectivesGeneral Objectives of Teaching of Teaching Undergraduate Soil MechanicsUndergraduate Soil Mechanics

To present the To present the basic conceptsbasic concepts and and fundamental principlesfundamental principles of soil mechanics of soil mechanics based on the studentbased on the student’’s background in s background in mechanics, physics and mathematicsmechanics, physics and mathematicsTo To provide background knowledgeprovide background knowledge for a lifefor a life--time of learning geotechnical issuestime of learning geotechnical issuesTeaching should integrate modern learning Teaching should integrate modern learning principles, teaching techniques and use principles, teaching techniques and use learning aidslearning aids (From several recent Soil Mechanics (From several recent Soil Mechanics Textbooks)Textbooks)

More More Specific ObjectivesSpecific Objectives of Teaching of Teaching Undergraduate Soil MechanicsUndergraduate Soil Mechanics

Undergraduate engineering students should Undergraduate engineering students should learn:learn:

–– theoriestheories related to the physical and mechanical related to the physical and mechanical properties of soilsproperties of soils

–– means whereby relevant means whereby relevant measurementsmeasurements can be can be made in the laboratory or in the fieldmade in the laboratory or in the field

–– applicationapplication of the theories and measurements to of the theories and measurements to the analysis of geotechnical problems the analysis of geotechnical problems

–– procedures whereby the physical and mechanical procedures whereby the physical and mechanical properties can be properties can be estimated or approximatedestimated or approximated

Need for a New Paradigm for Need for a New Paradigm for Unsaturated Soil MechanicsUnsaturated Soil Mechanics

Implementation of Unsaturated Soil Implementation of Unsaturated Soil Mechanics Generally Requires:Mechanics Generally Requires:

–– the estimation of unsaturated soil property the estimation of unsaturated soil property functions, functions, USPFsUSPFs

–– USPFsUSPFs are estimated largely through use of are estimated largely through use of the soilthe soil--water characteristic curves, water characteristic curves, SWCCSWCC

–– a new paradigm or mindset is required that a new paradigm or mindset is required that respects estimation and approximation respects estimation and approximation procedures for procedures for USPFsUSPFs

Classification Tests(Grain Size Distribution)

Present theUnsaturated

Soil Property Functions

Estimation of Soil-WaterCharacteristic Curve

02040

60

80

100

0.001 0.01 0.1 1 10 100

Particle size (mm)

Perc

ent p

assi

ng (%

)

Fit CurveExperimental

1 10 100 1000 10000000

10

20

30

40

50

10000Soil suction (kPa)

Wat

er c

onte

nt, w

(%)

0

10

20

30

40

0.1 1 10 1000 100000 1000000

Wat

er c

onte

nt (%

)

Predicted from grain-sizeExperimental

100Soil suction (kPa)

Similar Grain-size and Corresponding Soil-water

Characteristic Curves

Database Mining

1001010.1Particle size (mm)

0.0010.0001

20

0

40

60

80

100

0.01

Perc

ent

pass

ing

(%)

Series 1Series 2Series 3Series 4Series 5Series 6

Soil

1001010.1Soil suction (kPa)

100001000

10

0

40

20

30

Wat

er c

onte

nt, (

%)

Series 1Series 2Series 3Series 4Series 5Series 6

1000000

50

Soil-Water Characteristic Curve Measurement

Direct MeasurementThrough Experiments

Soil

Matric suction, (kPa) (ua - uw)0 100 200 300 400 500

100

200

300

Shea

r str

engt

h, t

(kPa

)

Shear strengthenvelope

c’ Indirectly Compute theUnsaturated


Indirectly Compute theUnsaturated


Indirectly Compute theUnsaturated


Determination of Unsaturated Soil Property Functions

Wide range of applied Wide range of applied suctionssuctionsApplies total stressesApplies total stressesMeasures Measures water and total water and total volume changevolume changeMeasure Measure diffused airdiffused airTest Test individual specimensindividual specimensNullNull--type type initial suctioninitial suctionDryingDrying and and wettingwetting modesmodes

Fifteen bar Pressure Fifteen bar Pressure Plate equipment Plate equipment manufactured by manufactured by GCTS, U.S.AGCTS, U.S.A..

What is an Unsaturated Soil?What is an Unsaturated Soil?Definition:Definition:

–– a soil that has water and air in the voids a soil that has water and air in the voids separated by a contractile skin (airseparated by a contractile skin (air--water water interinter--phase)phase)

–– a soil where the porea soil where the pore--water pressures are water pressures are negative relative to the porenegative relative to the pore--air pressuresair pressures

Note:Note: the smallest amount of air renders a soil the smallest amount of air renders a soil unsaturated but it is the relative pressures unsaturated but it is the relative pressures between air and water that is most importantbetween air and water that is most important

Partly Saturated & Partially Saturated terms seldom used

Legend

Dry soil

Saturated soil

Unsaturated soil

- Solids- Water- Air

Two fluid phases

Capillary fringe

SubdivisionSubdivisionof Soil of Soil above above

the Waterthe WaterTable basedTable based

on on Variation Variation

in Degree of in Degree of SaturationSaturation

-Discontinuous water phase

-Water filling the voids

Dry soil

Two fluid phases-Continuous water phase

Capillary fringe

Saturated soil

Unsaturated soil

-Air in a dissolved state

-Air filling mostvoids

-Continuous air phase

-Discontinuous air phase-Water filling most voids

Categorization of Soil Categorization of Soil Mechanics Based on the Mechanics Based on the

Nature of the Fluid PhaseNature of the Fluid Phase

Aeolian

Lacustrine

Alluvial

Others(Fluvial, Glacial,

etc.)Residual

Bedrock

Natural or remolded

(compacted states)

Is of SecondaryImportance!

Categorization of Soil Mechanics Categorization of Soil Mechanics Based on Geological OriginsBased on Geological Origins

Example Where ‘Soil Classification’ Example Where ‘Soil Classification’ Pertains to Unsaturated Soil MechanicsPertains to Unsaturated Soil Mechanics

Shrinkage curve:Shrinkage curve:–– subdivides soils into states:subdivides soils into states:

»» the liquid statethe liquid state»» the plastic statethe plastic state»» the solid statethe solid state»» the semithe semi--solid statesolid state

–– is the response of an initially is the response of an initially slurriedslurried soil to soil to an increase in soil suction an increase in soil suction

–– relates water contents to the SWCC relates water contents to the SWCC

Subdivisions between“states” relate to

soil suction levels

Relationship between Relationship between AtterbergAtterberg Limits and Limits and the Shrinkage Curve for a Highly Plastic the Shrinkage Curve for a Highly Plastic

ClayClay

2.5

1.5

1.0

0.5

Void

ratio

, e

0 20 40 60 80 100

3.0

Saturation line

Plastic limit

Shrinkage limit

Residual water content

Liquid limit

Air entry value

2.0

0

Water content (%)

Example of the Relevance of the Example of the Relevance of the ‘Grain‘Grain--Size Distribution’ to Size Distribution’ to

Unsaturated Soil Mechanics?Unsaturated Soil Mechanics?

The grainThe grain--size distribution curve: size distribution curve: –– provides a provides a measure of the soil solids measure of the soil solids

distributiondistribution–– The inverse of the soil solids distribution The inverse of the soil solids distribution

(i.e.,(i.e., distribution of the voids)distribution of the voids) forms the forms the basis for the estimation of the SWCCbasis for the estimation of the SWCC

–– Require the use of the Capillary Theory Require the use of the Capillary Theory to calculate the to calculate the SWCCSWCC

Comparison between the GrainComparison between the Grain--Size Size Distribution Curve and the SoilDistribution Curve and the Soil--Water Water

Characteristic Curves for SandCharacteristic Curves for Sand

0

20

40

60

80

100

0.0001 0.001 0.01 0.1 1 10 100

Particle size (mm)

Perc

ent p

assi

ng, (

%) Fit Curve

Experimental

0

10

20

30

40

0.1 1 10 100 1000 10,000 100,000 1,000,000Soil suction (kPa)

Volu

met

ric w

ater

con

tent

Predicted from grain-sizeExperimental

The soil-water characteristic curve for sand

(after Fredlund, 1987)

Grain-size distribution for sand

Topics of Unsaturated Soil Mechanics Topics of Unsaturated Soil Mechanics Covered in Classical Soils Mechanics Covered in Classical Soils Mechanics

Soil compaction and volumeSoil compaction and volume--mass mass relationships relationships Reveals that Reveals that two volumetwo volume--mass mass constitutive relationships are necessaryconstitutive relationships are necessaryto compute changes in soil properties to compute changes in soil properties during any processduring any process

S e = w Gs

2500

2700

2300

2100

1900

1700

1500

1300

1100

900

7000 10 20 30 40 50Water content, w (%)

0.2

0

0.4

0.1

1.4

0

0.2

0.6

0.3

0.5

0.8

1.01.2

1.6

2.3

1.82.0

2.860.7

0.74

0.6

Void

ratio

, e

Poro

sity

, n

S= 50%

S= 70%

0.4

S= 90%S= 80%S=

10%

S= 20%

S= 100%

Den

sity

, kg/

m3

S= 60%

Specific gravity, G = 2.70Dry density, Constant total density lines, Density of water, = 1000 (kg/m )

s

d

w

ρρw

ρ 3

S= 40%

S=0%

S= 30%

Saturated Soil

VolumeVolume--Mass Mass

Relations Relations for a Soil for a Soil

Specific Specific Gravity = Gravity =

2.702.70

The Centrality of Stress State The Centrality of Stress State VariablesVariables

Undergraduate students are generally taught Undergraduate students are generally taught about stress tensors in classes such as about stress tensors in classes such as strength of materialsstrength of materialsStress state variablesStress state variables combine to form of two combine to form of two independent independent stress tensorsstress tensors for unsaturated for unsaturated soilssoilsA soil always behaves in response to the A soil always behaves in response to the stress state variablesstress state variables and changes in the and changes in the stress state variables stress state variables

(σz - ua)

(σy - ua)(ua - uw)

(ua - uw)

τzx

τzy

τxz

τxy

τyzτyx

Unsated So e

nics

S at atc s

Soil ha nic

hru aat c

edru

(σz - ua)

(σy - ua)

τzx

τzy

τxz

τxy

τyzτyx

Stress State at a Point Stress State at a Point above and below the above and below the

Water TableWater Table(ua - uw)

il M

Me

(σx - ua)

(σx - ua)

Saturated Soil Mechanics as a Special Saturated Soil Mechanics as a Special Case of Unsaturated Soil MechanicsCase of Unsaturated Soil Mechanics

There is a smooth change from the There is a smooth change from the stress state for an stress state for an unsaturated soilunsaturated soil to to that of that of saturated soilsaturated soil::

–– water pressure approaches to air pressurewater pressure approaches to air pressure–– matricmatric suction stress tensor drops outsuction stress tensor drops out–– stress state reverts to the single effective stress state reverts to the single effective

stress tensorstress tensorSmooth transitions should also exist for Smooth transitions should also exist for all constitutive relationshipsall constitutive relationships

What is the Primary Need and What is the Primary Need and Responsibility of the University Responsibility of the University Professor of Geotechnical Professor of Geotechnical Engineering?Engineering?

To teach all students the fundamental To teach all students the fundamental concepts of concepts of saturatedsaturated--unsaturated soilunsaturated soilbehavior and thereby teach the behavior and thereby teach the students to students to ““think the way saturatedthink the way saturated--unsaturated soil systems behaveunsaturated soil systems behave””!!

Differences between Unsaturated and Differences between Unsaturated and Saturated Soil MechanicsSaturated Soil Mechanics

Unsaturated soil propertiesUnsaturated soil properties are highly are highly nonlinear nonlinear Constitutive relationsConstitutive relations for the classic for the classic areas of soil mechanics need to be areas of soil mechanics need to be extended to embrace unsaturated soilsextended to embrace unsaturated soilsFormulationsFormulations need to be extendedneed to be extendedSolutionsSolutions need to be obtained through need to be obtained through numerical modeling using a computernumerical modeling using a computer

UnsaturatedSoil

Seepage

Volume change

Shearstrength

v = kw (-uw) ∂h/ ∂y

SaturatedSoil

v = kw ∂h/ ∂y

τ = c’ + (σn - ua) tan φ’ + (ua - uw) tan φ b

τ = c’ + (σn - uw) tan φ’

de = a1 d (σ - ua) + a2 d (ua - uw)

de = av d (σ - uw)

Constitutive Equations for the Classic Constitutive Equations for the Classic Areas of Soil MechanicsAreas of Soil Mechanics

Coefficient of permeability(m/s)

kS

10-9

Water content(%)

10-810-6 10 10 -10 0 10 20 30 40 50

Soil-water characteristic

curve

contentTwofluid phases

Air entry valuePermeability

function

Dry soil

-7

1,000,000

100,000

10,000

1000

100

10

1

Residual water

Visualization Visualization of the of the

Coefficient of Coefficient of Permeability Permeability Function in Function in

the the Unsaturated Unsaturated

Soil Zone

Unsaturated SoilSoil suction (kPa)

Capillary fringe

Soil ZoneSaturated Soil

0 10 20 30 40Water content (%)

NegativePore-waterpressures

Wetting (Capillary rise)

Drying (Capillary draining)

Soil

0

10

20

30

40

50

1 10 100 1000 10000 100000 1000000Soil suction (kPa)

Wat

er c

onte

nt (%

)

Wetting curve (Adsorption)

Residualwater content

Air- entry value

Drying curve (Desorption)

Illustration of Water Illustration of Water Holding Capacity of a Soil

SWCC Laboratory test

Holding Capacity of a Soil

Pressure Plate Apparatus

mw2

1

Matric suction, (ua -uw) (kPa)

Coe

ffici

ent o

f per

mea

bilit

y,k w

(m/s

)

Soil-water characteristic curve

Wat

er c

onte

nt, w

(%)

Start of

Start of desaturation for

a clayey silt

100 200 300 400 500

10-5

10-7

10-8

10-9

10-10

100 200 300 400 500

0

00

10

20

30

40

50

Saturation coefficient ofpermeability (fine sand)


desaturationfor a fine sand

Relationship Relationship between between

SoilSoil--Water Water Characteristic Characteristic Curve and the Curve and the PermeabilityPermeability

for a for a Fine Sand Fine Sand

and a and a Clayey Silt

10-6

Saturation coefficient ofpermeability (clayey silt)

Clayey Silt

Clayey silt

Fine sand

1 10 100 1000

Clayey silt

kw=ks

1+a(ua- uw)n

10

10-10

Fine sand

Start of desaturation for a

fine sand

Permeability function

203040

50

1 10 100 1000

10-9

10-8

10-7

10-6

10-5

Gardner’s equation n clayey silt

nsand

Typical Typical Gardner Gardner

Empirical Empirical Permeability Permeability Functions Functions

for a for a Fine Sand Fine Sand

and a and a Clayey SiltClayey Silt


Coe

ffici

ent o

f per

mea

bilit

y,k w

(m/s

)W

ater

con

tent

, w (%

) Start of desaturationfor a clayey silt


Steps Involved Implementing an Steps Involved Implementing an Engineered SolutionEngineered Solution

Seepage flow laws

Cover

Waste

Liner

Numerical model “what if...” analyses

( ) )dydh( ) )

dyg

uYd w

+ ρw

( ukuk wwww −=− (uv w −−=

kk ww =v= dydh

dyg

uYd w

+ ρw

+++ 0=

+kw∂

2∂ xhw

2 ∂2∂ y

hw2 ∂

∂ xkw ∂

∂ xhw ∂

∂ yk w ∂

∂ yhw

+ 0=

+kw∂

2∂ xhw

2 ∂2∂ y

hw2

Formulation

Engineered solutionto ‘real’ problem

Constitutive

Solution

Boundary conditions and formulation

Modified MohrModified Mohr--Coulomb Shear Strength Coulomb Shear Strength EnvelopesEnvelopes

Net normal stress, (σ - ua)

φ’

Shea

r str

engt

h, τ

c’


Shea

r str

engt

h, τ

c’

φ’

φb

(u a- u w

)

Planar envelope

Shea

r str

engt

h, τ

c’

φ’

Unsated Soil e nics

Sat atcs

M

Soil haniecM

hru aat c

edru


Soil water characteristic curves

Wat

er c

onte

nt, w

(%)

10

20

30

40

50

200 300 400 50000 100

Start of

a fine sand

Start of desaturation for

a clayey silt

desaturation for

Fine sand


Relationship Relationship between between

SWCC and SWCC and Shear StrengthShear Strength

for a Fine for a Fine Sand and a Sand and a Clayey Silt

Clayey silt

0c’Sh

ear s

tren

gth,

τ (k

Pa)

100 200 300 400 500

100

200

300

Shear strength envelope

φ’ Fine sand

Clayey Siltφ’ Claey silt


VolumeVolume--Mass Changes for an Mass Changes for an Unsaturated SoilUnsaturated Soil

Two threeTwo three--dimensionaldimensional plots are plots are required to visualize volumerequired to visualize volume--mass behaviormass behavior

VolumeVolume--mass change problems mass change problems involving unsaturated soils are involving unsaturated soils are difficultdifficult to solveto solve

a) at

Loading with geotechnical quipment


Net normal

stress

(σ - u a)

am

Net normal

stress

(σ - u a)

SWCC


bt bm

Void

ratio

, e

av

Effective

stress

(σ - u w)

Unsated Soil ec nics

Saturatecs

M

Soil hanid ecM

ha rut a

Water content, w Void ratio, e

Void Ratio Void Ratio and Water and Water

Content Content Constitutive Constitutive

SurfacesSurfaces

Examples of Engineered Structures Examples of Engineered Structures Commonly Placed above the Water TableCommonly Placed above the Water Table

Spread footingfoundation

Unsaturated Soil Retainingwall

Roadway

Light structure

Flux boundaryconditions

Saturated Soil

Examples of Seepage Problems Examples of Seepage Problems Involving Unsaturated SoilsInvolving Unsaturated Soils

Structures can suffer distressed from infiltration Structures can suffer distressed from infiltration of water into an of water into an expansive or collapsibleexpansive or collapsible soilsoilMoisture flux at ground surface influences the Moisture flux at ground surface influences the movement of contaminantsmovement of contaminantsCovers designsCovers designs involve an analysis of the involve an analysis of the transmission and storage of water transmission and storage of water Extended infiltrationExtended infiltration on the surface of an earth on the surface of an earth dam may cause the instability dam may cause the instability Predictions related to Predictions related to ““ClosureClosure”” of mining of mining operation are controlled by the surface fluxoperation are controlled by the surface flux

TwoTwo--dimensional seepage analysis through an dimensional seepage analysis through an earthfillearthfill dam with a clay core.dam with a clay core.

Equipotential lines

Optimized mesh for saturated-unsaturated seepage analysis

Examples of the Movement of Water Examples of the Movement of Water through a Cover and Flow in the through a Cover and Flow in the Unsaturated Zone below a LinerUnsaturated Zone below a Liner

Groundwater table

Compacted liner Unsaturated Soil

Waste

Compactedcover

Water movement

Flux boundary conditionsEvaporation Precipitation

Bedrock

Groundwater table

Completely weathered

Residual soil

Surface drain Anchor for membrance

Rainfall

Plastic membranceRunoff

Collection system for runoff footings

Unsaturated SoilHighlyweathered

Examples of the Control of Infiltration Examples of the Control of Infiltration through the use of through the use of GeomembrancesGeomembrances

Examples of Shear Strength ProblemsExamples of Shear Strength ProblemsNatural slopesNatural slopes often fail following extended often fail following extended levels of precipitation. levels of precipitation. Loosely compacted fillsLoosely compacted fills can collapse and can collapse and result in high velocity mass movement upon result in high velocity mass movement upon wettingwettingCuts or trenchesCuts or trenches for laying pipelines can for laying pipelines can collapse collapse Some Some backfill materialsbackfill materials used behind earth used behind earth retaining structures can change volume and retaining structures can change volume and shear strength due to the intake of watershear strength due to the intake of waterBearing capacityBearing capacity of shallow footings may of shallow footings may change significantly due to infiltrationchange significantly due to infiltration

Examples of Volume Change ProblemsExamples of Volume Change Problems

Footings and slabsFootings and slabs--onon--groundground should be should be simulated using realistic moisture flux simulated using realistic moisture flux conditionsconditionsShrinkage problemsShrinkage problems can occur due to drying can occur due to drying or vegetationor vegetationCollapse Collapse of the soil structure can occur as a of the soil structure can occur as a result of a decrease in suctionresult of a decrease in suctionPredictions of the Predictions of the depth of crackingdepth of crackingVolume change predictionsVolume change predictions of compacted of compacted fills and covers needs to be analyzedfills and covers needs to be analyzed

Concluding RemarksConcluding RemarksUndergraduate engineering students Undergraduate engineering students can be taught the concepts of can be taught the concepts of unsaturated soil mechanicsunsaturated soil mechanics at the at the undergraduate levelundergraduate levelOnce the concepts are taught, and Once the concepts are taught, and saturated soil systemssaturated soil systems can be shown can be shown to be a special case, much of the to be a special case, much of the remaining time can be spent on remaining time can be spent on solving saturated soil mechanics solving saturated soil mechanics problemsproblems

Delwyn G. Fredlund

UnsaturatedSoils

Group

Observations from the Elliptical Observations from the Elliptical Geotechnical WorldGeotechnical World

Unsaturated Soils Mechanics:Unsaturated Soils Mechanics:–– applies above the applies above the phreaticphreatic lineline–– soil has negative poresoil has negative pore--water pressureswater pressures–– porepore--air pressure may or may not be atmosphericair pressure may or may not be atmospheric–– stress state variables are:stress state variables are:

»» net normal stress, (s net normal stress, (s -- uuaa))»» matricmatric suction (suction (uuaa -- uuww))

Saturated Soils Mechanics:Saturated Soils Mechanics:–– applies below the applies below the phreaticphreatic lineline–– soil has positive poresoil has positive pore--water pressureswater pressures–– stress state variable is effective stress, (s stress state variable is effective stress, (s -- uuww))

The Need to Quantify “RealThe Need to Quantify “Real--World” World” Moisture Flux Boundary ConditionMoisture Flux Boundary Condition

Changes in Changes in matricmatric suction causes serious suction causes serious distress to the light engineered structuresdistress to the light engineered structuresClimate influences the location of the Climate influences the location of the groundwater table and poregroundwater table and pore--water water pressurespressuresClimate Climate ““DrivesDrives”” many geotechnical many geotechnical engineering problemsengineering problems

Long Term Response of the Water Long Term Response of the Water Table to a Temperate, Humid ClimateTable to a Temperate, Humid Climate




Temperate, Humid regions (Evaporation ~ Precipitation)

Teaching within the New ParadigmTeaching within the New ParadigmA new attitude and mindset must be adopted by the A new attitude and mindset must be adopted by the geotechnical engineer when considering unsaturated soil geotechnical engineer when considering unsaturated soil behavior. The estimation of unsaturated soil property behavior. The estimation of unsaturated soil property functions is done largely with the assistance of functions is done largely with the assistance of SWCCsSWCCs. .

It can be said that geotechnical engineering for It can be said that geotechnical engineering for unsaturated soils must operate in a new paradigm of unsaturated soils must operate in a new paradigm of estimations and approximationsestimations and approximations

The new protocols do not nullify the importance of The new protocols do not nullify the importance of modeling unsaturated soils; rather, these estimations add modeling unsaturated soils; rather, these estimations add a new and improved understanding of the behavior of a new and improved understanding of the behavior of saturatedsaturated--unsaturated soil systems.unsaturated soil systems.

Zones of Zones of UnsaturationUnsaturationThere are three zones of There are three zones of unsaturationunsaturation::

–– occluded air bubbles in the capillary zoneoccluded air bubbles in the capillary zone–– the zone where both air and water phases are the zone where both air and water phases are

continuouscontinuous–– the zone where the water phase become the zone where the water phase become

discontinuousdiscontinuousThere can be There can be continuity or discontinuitycontinuity or discontinuity of of the air and water phases under various the air and water phases under various levels of soil suction and this makes levels of soil suction and this makes unsaturated soil mechanicsunsaturated soil mechanics complex complex

Relationship of Seepage to the SWCCRelationship of Seepage to the SWCC

Coefficient of permeability (or hydraulic Coefficient of permeability (or hydraulic conductivity) of the soil is:conductivity) of the soil is:

–– constant throughout the capillary zoneconstant throughout the capillary zone–– decreases with increase in soil suctiondecreases with increase in soil suction–– decreases logarithmically between the air decreases logarithmically between the air

entry value and the residual conditionentry value and the residual condition–– various over several orders of magnitudevarious over several orders of magnitude

Hydraulic flow continues until the Hydraulic flow continues until the residual suction is reached residual suction is reached Water flows only where there is water in Water flows only where there is water in the soil!the soil!

Relationship of Coefficient of Relationship of Coefficient of Permeability and the SWCCPermeability and the SWCC

Coefficient of permeability responds Coefficient of permeability responds logarithmically to an arithmetic change in logarithmically to an arithmetic change in water contentwater contentAir Entry ValueAir Entry Value and and Residual ConditionsResidual Conditionsbecome the most important parts of a become the most important parts of a SWCCSWCCIt is possible for a sand to have a lower It is possible for a sand to have a lower coefficient of permeability than a claycoefficient of permeability than a clay

Shear Strength of an Unsaturated SoilShear Strength of an Unsaturated SoilShear strength envelope is Shear strength envelope is threethree--dimensionaldimensionalShear strength is Shear strength is linearlinear over a limited soil over a limited soil suction rangesuction rangeShear strength becomes Shear strength becomes nonlinear nonlinear over a over a wide range of soil suctions wide range of soil suctions Shear strength increases in Shear strength increases in accordance with accordance with the effective angle of internal frictionthe effective angle of internal friction below below the air entry value of the soilthe air entry value of the soilShear strength remains Shear strength remains constantconstant after after residual conditionresidual condition

Illustration Illustration of the of the

Potential Potential for Swelling for Swelling

Versus Versus Depth and Depth and

Soil SuctionSoil Suction

Unsaturated Soil

Saturated Soil

Two fluid phase

Dry soil

Dep

th

106 0 5 10

Capillary fringe

Potential for swelling, %Soil suction, kPa10

510110

2103

104

100 15 20 25

Swelling = f (suction change, overburden)

teaching unsaturated soil mechanics as part of the ... · teaching unsaturated soil mechanics as...

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