DESIGN OF DEEP FOUNDATIONS George Goble Goble PileTest, Inc.

DESIGN OF DEEP DESIGN OF DEEP FOUNDATIONSFOUNDATIONS

George GobleGeorge Goble

Goble PileTest, Inc.Goble PileTest, Inc.

In this lecture I will discuss the deep In this lecture I will discuss the deep foundations design process forfoundations design process for

driven piles and to a lesser degree driven piles and to a lesser degree cast-in-place systems, cast-in-place systems,

both geotechnical, structural aspects both geotechnical, structural aspects and some other topics within time and some other topics within time

limitationslimitations

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

Experience in Construction and Several Years as a Experience in Construction and Several Years as a Structural DesignerStructural 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 Minimum Cost Structural DesignResearch on Minimum Cost Structural Design

and on the Dynamics of Pile Drivingand on the Dynamics of Pile DrivingManaged the Research that Developed Dynamic Managed the Research that Developed Dynamic

Methods for Pile Capacity PredictionMethods for Pile Capacity PredictionFounded PDI and GRLFounded PDI and GRL

Now Have a Bridge Testing and Rating BusinessNow Have a Bridge Testing and Rating BusinessIn Goble Pile Test, I’m Developing an Easy to Use In Goble Pile Test, I’m Developing an Easy to Use

Dynamic Pile Testing SystemDynamic Pile Testing System

WHY MAKE THIS PRESENTATION?WHY MAKE THIS PRESENTATION?

• 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– Drivability analyses not competently done Drivability analyses not competently done – Many job specs are poorly writtenMany job specs are poorly written

THE ADVANTAGES OF THE THE ADVANTAGES OF THE DRIVEN PILEDRIVEN 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 blow count necessary penetrates to the blow count necessary to get the required capacityto get the required capacity

• Capacity can be determined quite Capacity can be determined quite accurately by driving observations, accurately by driving observations, usually conservative (setup)usually conservative (setup)

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

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

BASIS FOR DESIGNBASIS FOR 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 and Stress Design (ASD) Has Been and

Is Still Used in Some Codes. Is Still Used in Some Codes. The Fundamental Basis?The Fundamental Basis?

ASD HISTORICAL BACKGROUNDASD HISTORICAL BACKGROUND

• Rational Linear Elastic Analyses Rational Linear Elastic Analyses Appeared Early 1800’sAppeared Early 1800’s

• Linear Elastic Analysis Based on SteelLinear Elastic Analysis Based on 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

Factor of Safety? How is Stress Measured?

ALLOWABLE STRESS DESIGNALLOWABLE STRESS DESIGN

• ““Safe” Stress or Load Permitted in DesignSafe” Stress or Load Permitted in Design– Allowable Stress Determined by Allowable Stress Determined by

Multiplying the Yield Stress of the Multiplying the Yield Stress of the Material by a Safety Margin that is Less Material by a Safety Margin that is Less than Onethan One

– The Factor Provides Safety MarginThe Factor Provides Safety Margin– Factor Selected by Experience of about Factor Selected by Experience of about

150 Years150 Years

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 – Columns, or ConcreteExample – Columns, or Concrete• Behavior Understood by Late 1800’sBehavior Understood by Late 1800’s• But for Columns, But for Columns, StrengthStrength is Non-Linear and is Non-Linear and

Dependent on Slenderness Ratio and Can Be Dependent on Slenderness Ratio and Can Be CalculatedCalculated

• 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 Alternate Appendix (Strength Design)an Alternate Appendix (Strength Design)• 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 Loads

ASD - D + LASD - D + L

LRFD - ACI: 1.2D + 1.6LLRFD - ACI: 1.2D + 1.6L

LRFD - AASHTO: 1.25D + 1.75LLRFD - AASHTO: 1.25D + 1.75L

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

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

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

between strength types Strengthbetween strength types Strength

• Load and Resistance Factors non-Load and Resistance Factors non-UniqueUnique

• Clearly Written, Unique Codes Clearly Written, Unique Codes NecessaryNecessary

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 the Preliminary loads defined by the structural engineerstructural engineer

• 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

• (Anaheim Example) (Anaheim Example)

Plan and execute subsurfacePlan and execute subsurfaceinvestigationinvestigation


Evaluate information andEvaluate information andselect foundation systemselect foundation system

Deep FoundationDeep Foundation Shallow FoundationShallow Foundation

COST EVALUATIONCOST EVALUATION

A Cost Evaluation Should Always Be Made If More Than One Deep Foundation System Is Possible. It

Is Not Difficult For Deep Foundations And Cost Savings Can

Be Very Large.

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 Strength Where does the Strength Variability come from?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 - Multiply by Get Factored Load - Multiply 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 LayerLayer

• Select pile type oirjmoi;ernj;somSelect pile type oirjmoi;ernj;som

Penetration Well DefinedPenetration Well Defined

FOUNDATION DESIGN PROCESSFOUNDATION DESIGN PROCESSPenetration Not Well DefinedPenetration Not Well Defined

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

End Bearing With Depth By Static AnalysisEnd Bearing With Depth By Static AnalysisEstimate Required Pile LengthEstimate Required Pile Length

Do a Preliminary Drivability CheckDo a Preliminary Drivability CheckCan The Pile Be Driven To The Required Can The Pile Be Driven To The Required

Depth And CapacityDepth And CapacityIs The Pile Satisfactory StructurallyIs The Pile Satisfactory Structurally

DRIVEN PILE DESIGN PROCESSDRIVEN PILE DESIGN PROCESSGENERALGENERAL

• 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


• 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

Loads (Costs??)Loads (Costs??)

• 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 Rock or Dense Length Will Be Obvious if Piles to Rock or Dense

Layer (Howard Franklin)Layer (Howard Franklin)


• 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

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

THE ENDTHE END

DESIGN OF DEEP FOUNDATIONS George Goble Goble PileTest, Inc.

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