DESIGN OF DEEP FOUNDATIONS

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DESIGN OF DEEP FOUNDATIONS

• George Goble
• Goble PileTest, Inc.

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In this lecture I will discuss the deep
foundations design process fordriven piles and
to a lesser degree cast-in-place systems, both
geotechnical, structural aspects and some other
topics within time limitations
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MY BACKGROUNDStructural Engineer Minor in Soil
MechanicsExperience in Construction and Several
Years as a Structural DesignerDesigned Several
Large Pile FoundationsThirty Years as a College
Professor Teaching Structures and Mechanics,
Emphasizing DesignResearch on Minimum Cost
Structural Designand on the Dynamics of Pile
DrivingManaged the Research that Developed
Dynamic Methods for Pile Capacity
PredictionFounded PDI and GRLNow Have a Bridge
Testing and Rating BusinessIn Goble Pile Test,
Im Developing an Easy to Use Dynamic Pile
Testing System
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WHY MAKE THIS PRESENTATION?

• Driven Pile Design is Often Not Well Done
• Not dangerous but excessively conservative
• Design process not clearly understood
• Large cost savings possible
• Capabilities of modern hammers not recognized
• Drivability analyses not competently done
• Many job specs are poorly written

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THE ADVANTAGES OF THE DRIVEN PILE

• We know the material that we put in the ground
  before we drive
• Because it is driven each pile penetrates to the
  blow count necessary to get the required capacity
• Capacity can be determined quite accurately by
  driving observations, usually conservative (setup)

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FOUNDATION DESIGN PROCESS

• Process is Quite Complex (Unique)
• Not Complete Until the Driving Criterion is
  Established in the Field
• Structural Considerations can be Critical
• But Structural Properties Are Known in Advance of
  Pile Installation
• Factor of Safety (Resistance Factor) Dependent on
  Methods of Capacity Determination and
  Installation Quality Control

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BASIS FOR DESIGNSince Early in the 19th
Century a Design Approach Called Allowable Stress
Design (ASD) Has Been and Is Still Used in Some
Codes. The Fundamental Basis?
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ASD HISTORICAL BACKGROUND

• Rational Linear Elastic Analyses Appeared Early
  1800s
• Linear Elastic Analysis Based on Steel
• Well Developed by Late 1800
• Basic Concept Do not Exceed Yield Stress
• Produced an Orderly Basis for Design

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ASD BASIS
STRESS
?y
?a
STRAIN
Define an ALLOWABLE STRESS ?a C ?y For Steel
Beams C 0.4 to 0.66 Factor of Safety? How is
Stress Measured?
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ALLOWABLE STRESS DESIGN

• Safe Stress or Load Permitted in Design
• Allowable Stress Determined by Multiplying the
  Yield Stress of the Material by a Safety Margin
  that is Less than One
• The Factor Provides Safety Margin
• Factor Selected by Experience of about 150 Years

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STRENGTH DESIGN

• Not All Structures Have Linear Load-Stress (or
  Load-Strength) Relationship
• Example Columns, or Concrete
• Behavior Understood by Late 1800s
• But for Columns, Strength is Non-Linear and
  Dependent on Slenderness Ratio and Can Be
  Calculated
• Factor of Safety Introduced
• Universally Used in Geotechnical Design
• Still Called ASD

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WHY LRFD?

• First Adopted by ACI Building Code 1956 in an
  Alternate Appendix (Strength Design)
• Adopted 1963 as Equal to ASD
• Strength Design Necessary for Particularly for
  Concrete Columns
• Desirable to Split Safety Margin on Both Loads
  and Strength
• Adopted Different Factors on Different Load Types
• Adopted in Practice in about Two Years
• All Factors Determined Heuristically

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• ASD
• ?Qi Rn/F.S.
• LRFD
• ??ij Qij ?k Rnk
• Gravity Loads
• ASD - D L
• LRFD - ACI 1.2D 1.6L
• LRFD - AASHTO 1.25D 1.75L

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UNDERSTAND THE LIMITATIONS

• Load and Resistance Factors not Unique
• Several Factors Selected Based on One Condition
• Design Process Must Be Well-Understood by Code
  Developers
• Strength Data May Be Dependent on Undefined
  Variables

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FROM THE HANDLINGOF THE LOADS ALONE ITIS A BIG
IMPROVEMENTOVER ASD
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ButThere Are Many LoadsAnd Load
CombinationsFor Instance,Two Important OnesIn
AASHTOStr I 1.25D 1.75 L Str IV 1.50 D
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SUMMARY

• LRFD Is an Improvement Based on the Split Safety
  Margins Alone
• Both between Load Types and between strength
  types Strength
• Load and Resistance Factors non-Unique
• Clearly Written, Unique Codes Necessary

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FOUNDATION DESIGN PROCESS

• Combined effort of geotechnical, structural and
  construction engineer
• Local contractor may provide input
• Large design capacity increases are often
  possible for driven piles
• Both design and construction practice need
  improvement

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FOUNDATION DESIGN PROCESS
Establish requirements for structuralconditions
and site characterization
Obtain general site geology
Collect foundation experience from the area
Plan and execute subsurface investigation
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FOUNDATION DESIGN PROCESS

• Preliminary loads defined by the structural
  engineer
• Loads will probably be reduced as design advances
• Improved (final) loads must be used in final
  design
• (Anaheim Example)

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FOUNDATION DESIGN PROCESS
Plan and execute subsurface investigation
Evaluate information and select foundation system
Deep Foundation
Shallow Foundation
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COST 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.
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Foundation Design Process
Deep Foundation
Drilled Shaft
Driven Pile
Select Drilled Shaft
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Foundation Design Process
Drilled Shaft
Select Shaft Type and Factor of Safety or
Resistance Factor
By Static Analysis, Estimate Unit Shaft Friction
and End Bearing
Select Cross Section and Length for Required
Capacity (Structural Engineer?)
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Foundation Design Process
Prepare Plans and Specifications
Select Contractor
Verify Shaft Constructability and Capacity
Install and Inspect Production Shafts
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QUESTION

• Where does the Strength Variability come from?

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Foundation Design Process
Deep Foundation
Drilled Shaft
Driven Pile
Select Driven Pile
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FOUNDATION DESIGN PROCESS
Define Subsurface Conditions Select Capacity
Determination Method Select Quality Control
Procedures Determine Safety Factor or Resistance
Factor
Determine Working Loads and Loads Times Factor of
Safety Gives Required Ultimate or Nominal
Resistance for ASD For LRFD Determine Loads Times
Load Factors Get Factored Load - Multiply by ?
Factor to Get Required Nominal Resistance
Penetration Not Well Defined
Penetration Well Defined
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DRIVEN PILE DESIGN PROCESS
Penetration Well Defined

• Pile Depth is Defined by a Dense Layer or Rock
• The Length is Easily Selected Based on the Depth
  to the Layer
• Select pile type oirjmoiernjsom

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FOUNDATION DESIGN PROCESSPenetration Not Well
Defined
Select Pile Type and Size Determine Unit Shaft
Friction and End Bearing With Depth By Static
Analysis Estimate Required Pile Length Do a
Preliminary Drivability Check Can The Pile Be
Driven To The Required Depth And Capacity Is The
Pile Satisfactory Structurally
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DRIVEN PILE DESIGN PROCESSGENERAL

• Capacity Verification Method
• More Accurate Methods Justify a Smaller Safety
  Factor (Larger Resistance Factor)
• Choices
• Static load test
• Dynamic test
• Wave equation
• Dynamic formula

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DRIVEN PILE DESIGN PROCESSGENERAL

• Q. C. Method
• As Q.C. is Improved, Factor of Safety can
  decrease (Resistance Factor can Increase)
• e.g., Better Capacity Determination Method
• Increased Percentage of Piles Statically or
  Dynamically Tested
• Critical Piles Tested

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DRIVEN PILE DESIGN PROCESSGENERAL

• Make Pile Static Capacity Prediction
• Predict Unit Shaft Friction and End Bearing with
  Depth
• Prediction Should Be Best Possible
• Do Not Adjust with Resistance Factor
• Note Any Minimum Depth Requirements
• Pile Size Determined With Knowledge of Loads
  (Costs??)

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DRIVEN PILE DESIGN PROCESSGENERAL

• Pile Size Selection Should Consider Loads
• Structural Limit State Must Also Be Considered
  Lateral Loads
• Close Structural and Geotechnical Coordination
  Necessary
• Maybe Pile Size Selection by Structural Engineer
  Foundation Engineer
• Length Will Be Obvious if Piles to Rock or Dense
  Layer (Howard Franklin)

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DRIVEN PILE DESIGN PROCESS

• At this stage a proposed foundation design is
  complete
• All other strength limit states must be checked
• Drivability must be checked
• All serviceability limit states also checked

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DRIVEN PILE DESIGN PROCESS
Evaluate Drivability
Design Satisfactory?
NO
YES
Prepare plans and specifications
Select Contractor
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DRIVEN PILE DESIGN PROCESS

• Drivability usually evaluated by wave equation
• Must satisfy driving stress requirement
• Blow count must be reasonable
• Hammer and driving system assumed
• If dynamic formula used it will determine
  required blow count
• Dynamic formula will not detect excessive driving
  stresses

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DRIVEN PILE DESIGN PROCESS
Change Driving System
Select Contractor
Contractor Advises Proposed Hammer and Driving
System
Perform Drivability Analysis
Hammer Satisfactory?
NO
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DRIVEN PILE DESIGN PROCESS
Hammer Satisfactory?
YES
Set driving criteria
Drive test pile to criteria
Verify test pile capacity
NO
Capacity/stress satisfactory?
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DRIVEN PILE DESIGN PROCESS
Capacity/stress satisfactory?
NO
YES
Drive production piles
Undertake construction control and monitor
installation
Resolve pile installation problems and
construction procedures
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QUESTION

• Where does the Strength Variability come from?

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THE END