DESIGN OF DEEP FOUNDATIONS

DESIGN OF DEEP DESIGN OF DEEP FOUNDATIONSFOUNDATIONS

George GobleGeorge GobleConsulting EngineerConsulting Engineer

In this Lecture I Will Discuss the Deep In this Lecture I Will Discuss the Deep Foundations Design ProcessFoundations Design Process

Both Driven Piles and Cast-in-Place Both Driven Piles and Cast-in-Place Systems Systems

Both Geotechnical and Some of the Both Geotechnical and Some of the Structural AspectsStructural Aspects

MY BACKGROUNDMY BACKGROUNDStructural Engineer – Minor in Soil MechanicsStructural Engineer – Minor in Soil Mechanics

Experience in Construction and Several Years as Experience in Construction and Several Years as a Structural Designera Structural Designer

Designed Several Large Pile FoundationsDesigned Several Large Pile FoundationsThirty Years as a College Professor Teaching Thirty Years as a College Professor Teaching

Structures and Mechanics, Emphasizing DesignStructures and Mechanics, Emphasizing DesignResearch on Optimum Structural DesignResearch on Optimum Structural Design

and onand onthe Dynamics of Pile Drivingthe Dynamics of Pile Driving

Managed the Research that Developed Dynamic Managed the Research that Developed Dynamic Methods for Pile Capacity PredictionMethods for Pile Capacity Prediction

Founded PDI and GRLFounded PDI and GRLNow Have a Bridge Testing and Rating BusinessNow Have a Bridge Testing and Rating Business

WHY DO THIS?WHY DO THIS?

• Driven Pile Design is Often Not Well DoneDriven Pile Design is Often Not Well Done– Not dangerous but excessively conservativeNot dangerous but excessively conservative– Design process not clearly understoodDesign process not clearly understood– Large cost savings possibleLarge cost savings possible– Capabilities of modern hammers not Capabilities of modern hammers not

recognizedrecognized– Many job specs are poorly writtenMany job specs are poorly written

FUNDAMENTAL ADVANTAGES FUNDAMENTAL ADVANTAGES OF THE DRIVEN PILEOF THE DRIVEN PILE

• We know the material that we put in the We know the material that we put in the ground before we driveground before we drive

• Because it is driven each pile Because it is driven each pile penetrates to the depth required to get penetrates to the depth required to get the capacitythe capacity

• Capacity can be determined accurately Capacity can be determined accurately by driving observationsby driving observations

FOUNDATION DESIGN PROCESSFOUNDATION DESIGN PROCESS

• Process is Quite Complex (Unique)Process is Quite Complex (Unique)• Not Complete Until the Driving Criterion is Not Complete Until the Driving Criterion is

Established in the FieldEstablished in the Field• Structural Considerations can be CriticalStructural Considerations can be Critical

– But Structural Properties Known in Advance of But Structural Properties Known in Advance of Pile InstallationPile Installation

• Factor of Safety (Resistance Factor) Factor of Safety (Resistance Factor) Dependent on Methods of Capacity Dependent on Methods of Capacity Determination and Installation Quality ControlDetermination and Installation Quality Control

I Will Discuss the Basis for the I Will Discuss the Basis for the Design.Design.

Since early in the 19Since early in the 19thth Century a Century a Design Approach Called Allowable Design Approach Called Allowable

Stress Design (ASD) Has Been Stress Design (ASD) Has Been Used. Used.

Will Discuss the Fundamental Basis Will Discuss the Fundamental Basis for ASDfor ASD

GENERAL STRUCTURAL DESIGNGENERAL STRUCTURAL DESIGN

PROCESSPROCESS

ASD HISTORICAL BACKGROUNDASD HISTORICAL BACKGROUND

• Rational Analyses Appeared Early Rational Analyses Appeared Early 1800’s1800’s

• Analysis Linear Elastic Based - SteelAnalysis Linear Elastic Based - Steel• Well Developed by Late 1800Well Developed by Late 1800• Basic Concept – Do not Exceed Yield Basic Concept – Do not Exceed Yield

StressStress• Produced an Orderly Basis for DesignProduced an Orderly Basis for Design

ASD BASISASD BASIS

STRAIN

STRESS

a

y

Define an ALLOWABLE STRESS

a = C y

For Steel Beams C = 0.4 to 0.66

ALLOWABLE STRESS DESIGNALLOWABLE STRESS DESIGN

• ““Safe” Stress or Load Permitted in Safe” Stress or Load Permitted in DesignDesign– Allowable Stress Allowable Stress DeterminedDetermined by by

Dividing the Yield Strength of the Dividing the Yield Strength of the Material by a Factor of Safety that is Material by a Factor of Safety that is More than OneMore than One

– The Factor Provides Safety MarginThe Factor Provides Safety Margin– Factor Selected by ExperienceFactor Selected by Experience

STRENGTH DESIGNSTRENGTH DESIGN

• Not All Structures Have Linear Load-Stress Not All Structures Have Linear Load-Stress (or Load-Strength) Relationship(or Load-Strength) Relationship

• Example – ColumnsExample – Columns• Behavior Understood by Late 1800’sBehavior Understood by Late 1800’s• StrengthStrength Non-Linear and Dependent on Non-Linear and Dependent on

Slenderness Ratio and Can Be CalculatedSlenderness Ratio and Can Be Calculated• Factor of Safety IntroducedFactor of Safety Introduced• Universally Used in Geotechnical DesignUniversally Used in Geotechnical Design• Still Called ASDStill Called ASD

WHY LRFD?WHY LRFD?• First Adopted by ACI Building Code – 1956 in First Adopted by ACI Building Code – 1956 in

an Appendixan Appendix• Adopted 1963 as Equal to ASDAdopted 1963 as Equal to ASD• Strength Design Necessary for Particularly for Strength Design Necessary for Particularly for

Concrete ColumnsConcrete Columns• Desirable to Split Safety Margin on Both Loads Desirable to Split Safety Margin on Both Loads

and Strengthand Strength• Adopted Different Factors on Different Load Adopted Different Factors on Different Load

TypesTypes• Adopted in Practice in about Two YearsAdopted in Practice in about Two Years• All Factors Determined HeuristicallyAll Factors Determined Heuristically

ASDASD

QQi i = R= Rnn/F.S./F.S.

LRFDLRFD

γγijij Q Qijij = = k k RRnknk

Gravity LoadsGravity LoadsASD - D + LASD - D + L

LRFD - ACI: 1.2D + 1.6LLRFD - ACI: 1.2D + 1.6LLRFD - AASHTO: 1.25D + 1.75LLRFD - AASHTO: 1.25D + 1.75L

PROBABILITY RAISESPROBABILITY RAISESITS UGLY HEADITS UGLY HEAD

• Concept First Proposed in 1969 by Cornell in Concept First Proposed in 1969 by Cornell in ACI Journal ArticleACI Journal Article

• Extensive Research Developed Rational Extensive Research Developed Rational Load and Resistance Factors for Structural Load and Resistance Factors for Structural ElementsElements

• AISC Code Adopted LRFD mid-1980’sAISC Code Adopted LRFD mid-1980’s• Ontario Bridge Code Adopted 1977Ontario Bridge Code Adopted 1977• AASHTO Bridge Code Adopted LFD 1977AASHTO Bridge Code Adopted LFD 1977• AASHTO Bridge Code Adopted LRFD after AASHTO Bridge Code Adopted LRFD after

Extensive Research Project, 1994Extensive Research Project, 1994

STRENGTH STRENGTH ANDAND LOAD DISTRIBUTION LOAD DISTRIBUTION

fR(R),fQ(Q)

Load Effect (Q)

Q

Resistance (R)A

R,Qa bRn R

STRENGTH MINUS LOAD DISTRIBUTIONSTRENGTH MINUS LOAD DISTRIBUTION

R-Q

f R-Q

0

R-Q

R-Q

UNDERSTAND THE UNDERSTAND THE LIMITATIONSLIMITATIONS

• Load and Resistance Factors not UniqueLoad and Resistance Factors not Unique– Several Factors Selected Based on One ConditionSeveral Factors Selected Based on One Condition

• Design Process Must Be Well-Understood by Design Process Must Be Well-Understood by Code DevelopersCode Developers

• Strength Data May Be Dependent on Strength Data May Be Dependent on Undefined VariablesUndefined Variables

FROM THE HANDLINGFROM THE HANDLINGOF THE LOADS ALONE OF THE LOADS ALONE

ITITIS A BIG IMPROVEMENTIS A BIG IMPROVEMENT

OVER ASDOVER ASD

LOAD FACTORS FOR SELECTED CODESLOAD FACTORS FOR SELECTED CODES

CodeCode Dead LoadDead Load Live LoadLive LoadAASHTO Bridge CodeAASHTO Bridge Code 1.251.25 1.751.75

ACI 318-02ACI 318-02 1.201.20 1.601.60

AISC & ANSI 577AISC & ANSI 577 1.201.20 1.601.60

Ontario Bridge CodeOntario Bridge Code 1.201.20 1.401.40

Canadian CodeCanadian Code 1.20 1.20 1.601.60

Euro CodeEuro Code 1.351.35 1.501.50

Danish CodeDanish Code 1.001.00 1.301.30

Australian CodeAustralian Code 1.251.25 1.501.50

API CodeAPI Code 1.301.30 1.501.50

ButButThere Are Many LoadsThere Are Many LoadsAnd Load CombinationsAnd Load Combinations

For Instance,Two Important OnesFor Instance,Two Important OnesIn AASHTOIn AASHTO

Str I = 1.25D + 1.75 L + …Str I = 1.25D + 1.75 L + …Str IV = 1.50 DStr IV = 1.50 D

COMPARE F.S. WITH COMPARE F.S. WITH FOR FOR DIFFERENT L/D RATIOSDIFFERENT L/D RATIOS

γγDD Q QDD + + γγL L QQLL= = R Rn n (( QQDD + + QQLL)F.S.)F.S. = R= Rn n

γγDD + + γγLLQQLL/Q/QDD = = (1 + Q (1 + QLL/Q/QDD)F.S.)F.S.

((γγDD + + γγLLQQLL/Q/QDD)/ (1 + Q)/ (1 + QLL/Q/QDD) = ) = (F.S.) (F.S.)

Resistance Factors as Function of L/D at F.S.=2.0 for Several Different Codes

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1(L/D)

ACI 318-95

AASHTO

Australian Code

Eurocode

API Code

AISC, ANSI 577, andCanadian BridgeCodeOntario BridgeCode

Danish FoundationCode

AASHTOAASHTOEquivalentEquivalentResistanceResistance

Factors for GivenFactors for GivenF.S., Function ofF.S., Function of

L/DL/DDead L.F. = 1.25Dead L.F. = 1.25Live L.F. = 1.75 Live L.F. = 1.75

Phi As A Function of L/D for Various F.S.Load Cases Str I and IV

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

L/D

Phi

F.S.=1.40F.S.=1.40

F.S.=1.60F.S.=1.60

F.S.=2.00F.S.=2.00

F.S.=2.50F.S.=2.50

F.S.=3.00F.S.=3.00F.S.=3.75F.S.=3.75

F.S.=5.00F.S.=5.00

Str I = 1.25 D + 1.75 L Str IV = 1.50 DStr I = 1.25 D + 1.75 L Str IV = 1.50 D

Str IVStr IV StrStr I I

SUMMARYSUMMARY

• LRFD Is an Improvement Based on LRFD Is an Improvement Based on the Split Safety Margins Alonethe Split Safety Margins Alone– Both between Load Types and Both between Load Types and

StrengthStrength• Load and Resistance Factors non-Load and Resistance Factors non-

UniqueUnique• Clearly Written, Unique Codes Clearly Written, Unique Codes

NecessaryNecessary

SUMMARY (Cont.)SUMMARY (Cont.)

• Probabilistic Load and Resistance Probabilistic Load and Resistance Factor Determination Attractive Factor Determination Attractive – Probabilistic Factors Must Be Based on a Probabilistic Factors Must Be Based on a

Clear Understanding of the Design Clear Understanding of the Design ProcessProcess

– Must Have Good DataMust Have Good Data!!!!!!!!!!!!• Designer Needn’t Know How to Obtain Designer Needn’t Know How to Obtain

Resistance Factors from Probability Resistance Factors from Probability

FOUNDATION DESIGN PROCESS FOUNDATION DESIGN PROCESS

• Combined effort of geotechnical, Combined effort of geotechnical, structural and construction engineerstructural and construction engineer

• Local contractor may provide inputLocal contractor may provide input• Large design capacity increases are Large design capacity increases are

often possible for driven pilesoften possible for driven piles• Both design and construction Both design and construction

practice need improvementpractice need improvement

FOUNDATION DESIGN PROCESS FOUNDATION DESIGN PROCESS

Establish requirements for structuralEstablish requirements for structuralconditions and site characterizationconditions and site characterization

Obtain general site geologyObtain general site geology

Collect foundationCollect foundationexperience from the areaexperience from the area

Plan and execute subsurfacePlan and execute subsurfaceinvestigationinvestigation


• Preliminary loads defined by structural Preliminary loads defined by structural engineerengineer

• Loads will probably be reduced as Loads will probably be reduced as design advancesdesign advances

• Improved (final) loads must be used in Improved (final) loads must be used in final designfinal design

Plan and execute subsurfacePlan and execute subsurfaceinvestigationinvestigation


Evaluate information andEvaluate information andselect foundation systemselect foundation system

Deep FoundationDeep Foundation Shallow FoundationShallow Foundation

Foundation Design ProcessFoundation Design Process

Deep FoundationDeep Foundation

Driven PileDriven Pile Drilled ShaftDrilled Shaft

Select Drilled ShaftSelect Drilled Shaft


Drilled ShaftDrilled Shaft

Select Shaft Type andSelect Shaft Type andFactor of Safety or Resistance FactorFactor of Safety or Resistance Factor

By Static Analysis, Estimate UnitBy Static Analysis, Estimate Unit Shaft Friction and End BearingShaft Friction and End Bearing

Select Cross Section andSelect Cross Section and Length for Required CapacityLength for Required Capacity

(Structural Engineer?)(Structural Engineer?)


Prepare Plans and SpecificationsPrepare Plans and Specifications

Select ContractorSelect Contractor

Verify Shaft ConstructabilityVerify Shaft Constructabilityand Capacityand Capacity

Install and Inspect Production Install and Inspect Production ShaftsShafts

QUESTIONQUESTION

Where does the Geotechnical Where does the Geotechnical Strength Variability come from?Strength Variability come from?


Deep FoundationDeep Foundation

Driven PileDriven Pile Drilled ShaftDrilled Shaft

Select Driven PileSelect Driven Pile


Determine Working Loads and Loads Times Factor of SafetyDetermine Working Loads and Loads Times Factor of SafetyGives Required Ultimate or Nominal Resistance for ASDGives Required Ultimate or Nominal Resistance for ASD

For LRFD Determine Loads Times Load FactorsFor LRFD Determine Loads Times Load FactorsGet Factored Load - Divide by Get Factored Load - Divide by Factor to Factor to

Get Required Nominal ResistanceGet Required Nominal Resistance

Define Subsurface ConditionsDefine Subsurface ConditionsSelect Capacity Determination MethodSelect Capacity Determination Method

Select Quality Control ProceduresSelect Quality Control Procedures Determine Safety Factor or Resistance FactorDetermine Safety Factor or Resistance Factor

Penetration Well DefinedPenetration Well Defined Penetration Not Well DefinedPenetration Not Well Defined

DRIVEN PILE DESIGN DRIVEN PILE DESIGN PROCESSPROCESS

• Pile Depth is Defined by a Pile Depth is Defined by a Dense Layer or RockDense Layer or Rock

• The Length is Easily Selected The Length is Easily Selected Based on the Depth to the Based on the Depth to the Layer Layer

Penetration Well DefinedPenetration Well Defined


Select Pile Type and SizeSelect Pile Type and SizeDetermine Unit Shaft Friction andDetermine Unit Shaft Friction and

End Bearing With DepthEnd Bearing With DepthEstimate Required Pile LengthEstimate Required Pile Length

Do a Preliminary Drivability Check Do a Preliminary Drivability Check

11DRIVEN PILE DESIGN PROCESSDRIVEN PILE DESIGN PROCESS

GENERALGENERAL

• Capacity Verification MethodCapacity Verification Method– More Accurate Methods Justify a Smaller More Accurate Methods Justify a Smaller

Safety Factor (Larger Resistance Factor)Safety Factor (Larger Resistance Factor)• ChoicesChoices

– Static load testStatic load test– Dynamic testDynamic test– Wave equationWave equation– Dynamic formulaDynamic formula

DRIVEN PILE DESIGN PROCESSDRIVEN PILE DESIGN PROCESSGENERALGENERAL

• Q. C. MethodQ. C. Method– As Q.C. is Improved, Factor of Safety As Q.C. is Improved, Factor of Safety

can decrease (Resistance Factor can can decrease (Resistance Factor can Increase)Increase)• e.g., Better Capacity Determination Methode.g., Better Capacity Determination Method• Increased Percentage of Piles Statically or Increased Percentage of Piles Statically or

Dynamically TestedDynamically Tested• Critical piles testedCritical piles tested


• Make Pile Static Capacity PredictionMake Pile Static Capacity Prediction– Predict Unit Shaft Friction and End Bearing Predict Unit Shaft Friction and End Bearing

with Depthwith Depth– Prediction Should Be Best PossiblePrediction Should Be Best Possible

• Do Not Adjust with Resistance FactorDo Not Adjust with Resistance Factor– Note Any Minimum Depth RequirementsNote Any Minimum Depth Requirements– Pile Size Determined With Knowledge of Pile Size Determined With Knowledge of

LoadsLoads

• Pile Size Selection Should Consider LoadsPile Size Selection Should Consider Loads• Structural Limit State Must Also Be Considered – Structural Limit State Must Also Be Considered –

Lateral LoadsLateral Loads• Close Structural and Geotechnical Coordination Close Structural and Geotechnical Coordination

NecessaryNecessary• Maybe Pile Size Selection by Structural Engineer Maybe Pile Size Selection by Structural Engineer

– – Foundation EngineerFoundation Engineer• Length Will Be Obvious if Piles to RockLength Will Be Obvious if Piles to Rock


• At this stage a proposed foundation At this stage a proposed foundation design is completedesign is complete

• All other strength limit states must be All other strength limit states must be checkedchecked

• Drivability must be checkedDrivability must be checked• All serviceability limit states also All serviceability limit states also

checkedchecked

DRIVEN PILE DESIGN PROCESSDRIVEN PILE DESIGN PROCESS

NO NO

Evaluate DrivabilityEvaluate Drivability

DesignDesignSatisfactory?Satisfactory?

YES YES

Prepare plans and specificationsPrepare plans and specifications



• Drivability usually evaluated by wave Drivability usually evaluated by wave equationequation– Must satisfy driving stress requirementMust satisfy driving stress requirement– Blow count must be reasonableBlow count must be reasonable– Hammer and driving system assumedHammer and driving system assumed

• If dynamic formula used it will determine If dynamic formula used it will determine required blow countrequired blow count– Dynamic formula will not detect excessive Dynamic formula will not detect excessive

driving stressesdriving stresses



NO NO

Contractor Advises ProposedContractor Advises ProposedHammer and Driving SystemHammer and Driving System

Perform Drivability AnalysisPerform Drivability Analysis

Hammer Hammer SatisfactorySatisfactory??


ChangeDrivingSystem

• This is the same as above except the This is the same as above except the driving system is now known (given by driving system is now known (given by Contractor)Contractor)


NO NO

Set driving criteriaSet driving criteria

Drive test pile to criteriaDrive test pile to criteria

Capacity/stressCapacity/stresssatisfactory?satisfactory?

Verify test pile capacityVerify test pile capacity

Hammer Hammer Satisfactory?Satisfactory?

YES YES


Capacity/stressCapacity/stresssatisfactory?satisfactory?

NO NO

YES YES

Drive production pilesDrive production piles

Undertake construction controlUndertake construction controland monitor installationand monitor installation

Resolve pile installation problemsResolve pile installation problemsand construction proceduresand construction procedures


QUESTIONQUESTION

Where does the Geotechnical Where does the Geotechnical Strength Variability come from?Strength Variability come from?

THE ENDTHE END

Documents

DESIGN OF DEEP FOUNDATIONS