DESIGN OF DEEP FOUNDATIONS

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DESIGN OF DEEP FOUNDATIONS

• George Goble
• Consulting Engineer

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In this Lecture I Will Discuss the Deep
Foundations Design ProcessBoth Driven Piles and
Cast-in-Place Systems Both Geotechnical and Some
of the Structural Aspects
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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 Optimum
Structural Designand onthe Dynamics of Pile
DrivingManaged the Research that Developed
Dynamic Methods for Pile Capacity
PredictionFounded PDI and GRLNow Have a Bridge
Testing and Rating Business
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WHY DO THIS?

• 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
• Many job specs are poorly written

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FUNDAMENTAL 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
  depth required to get the capacity
• Capacity can be determined accurately by driving
  observations

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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 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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I Will Discuss the Basis for the Design.Since
early in the 19th Century a Design Approach
Called Allowable Stress Design (ASD) Has Been
Used. Will Discuss the Fundamental Basis for ASD
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GENERAL STRUCTURAL DESIGNPROCESS
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ASD HISTORICAL BACKGROUND

• Rational Analyses Appeared Early 1800s
• Analysis Linear Elastic Based - 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
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ALLOWABLE STRESS DESIGN

• Safe Stress or Load Permitted in Design
• Allowable Stress Determined by Dividing the Yield
  Strength of the Material by a Factor of Safety
  that is More than One
• The Factor Provides Safety Margin
• Factor Selected by Experience

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STRENGTH DESIGN

• Not All Structures Have Linear Load-Stress (or
  Load-Strength) Relationship
• Example Columns
• Behavior Understood by Late 1800s
• Strength 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
  Appendix
• 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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PROBABILITY RAISESITS UGLY HEAD

• Concept First Proposed in 1969 by Cornell in ACI
  Journal Article
• Extensive Research Developed Rational Load and
  Resistance Factors for Structural Elements
• AISC Code Adopted LRFD mid-1980s
• Ontario Bridge Code Adopted 1977
• AASHTO Bridge Code Adopted LFD 1977
• AASHTO Bridge Code Adopted LRFD after Extensive
  Research Project, 1994

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STRENGTH AND LOAD DISTRIBUTION
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STRENGTH MINUS LOAD DISTRIBUTION
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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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LOAD FACTORS FOR SELECTED CODES
Code Dead Load Live Load
AASHTO Bridge Code 1.25 1.75
ACI 318-02 1.20 1.60
AISC ANSI 577 1.20 1.60
Ontario Bridge Code 1.20 1.40
Canadian Code 1.20 1.60
Euro Code 1.35 1.50
Danish Code 1.00 1.30
Australian Code 1.25 1.50
API Code 1.30 1.50
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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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COMPARE F.S. WITH ? FOR DIFFERENT L/D RATIOS

• ?D QD ?L QL ? Rn ( QD QL)F.S. Rn
• ?D ?LQL/QD ? (1 QL/QD)F.S.
• (?D ?LQL/QD)/ (1 QL/QD) ? (F.S.)

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Resistance Factors as Function of L/D at
F.S.2.0 for Several Different Codes
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AASHTO Equivalent Resistance Factors for
Given F.S., Function of L/D Dead L.F. 1.25 Live
L.F. 1.75
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F.S.1.40
F.S.1.60
F.S.2.00
F.S.2.50
F.S.3.00
F.S.3.75
F.S.5.00
Str IV
Str I
Str I 1.25 D 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 Strength
• Load and Resistance Factors non-Unique
• Clearly Written, Unique Codes Necessary

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SUMMARY (Cont.)

• Probabilistic Load and Resistance Factor
  Determination Attractive
• Probabilistic Factors Must Be Based on a Clear
  Understanding of the Design Process
• Must Have Good Data!!!!!!
• Designer Neednt Know How to Obtain Resistance
  Factors from Probability

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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 structural engineer
• Loads will probably be reduced as design advances
• Improved (final) loads must be used in final
  design

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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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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 Geotechnical 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 - Divide 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

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FOUNDATION DESIGN PROCESS
Select Pile Type and Size Determine Unit Shaft
Friction and End Bearing With Depth Estimate
Required Pile Length Do a Preliminary Drivability
Check
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1DRIVEN 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

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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

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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

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

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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 Geotechnical Strength Variability
  come from?

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THE END