Teaching Unsaturated Soil Mechanics as Part of the ... · Teaching Unsaturated Soil Mechanics as...
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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
Negative pore-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
Saturated Soil Mechanics
Negative pore-waterpressures
Positive pore-waterpressures
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
Soil Property Functions
Indirectly Compute theUnsaturated
Soil Property Functions
Indirectly Compute theUnsaturated
Soil Property Functions
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)
Matric suction, (ua -uw) (kPa)
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
Matric suction, (ua -uw) (kPa)
Coe
ffici
ent o
f per
mea
bilit
y,k w
(m/s
)W
ater
con
tent
, w (%
) Start of desaturationfor a clayey silt
Matric suction, (ua -uw) (kPa)
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’
Net normal stress, (σ - ua)
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
Net normal stress, (σ - ua)
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
Matric suction, (ua -uw) (kPa)
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
Matric suction, (ua -uw) (kPa)
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
Matric suction(ua - uw)
Net normal
stress
(σ - u a)
am
Net normal
stress
(σ - u a)
SWCC
Matric suction(ua - uw)
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
Saturated Soil Mechanics
Negative pore-waterpressures
Positive pore-waterpressures
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)