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DESIGN OF DEEP FOUNDATIONS

DESIGN OF DEEP FOUNDATIONS. George Goble Consulting Engineer. In this Lecture I Will Discuss the Deep Foundations Design Process Both Driven Piles and Cast-in-Place Systems Both Geotechnical and Some of the Structural Aspects.

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DESIGN OF DEEP FOUNDATIONS

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  1. DESIGN OF DEEP FOUNDATIONS George Goble Consulting Engineer

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. GENERAL STRUCTURAL DESIGNPROCESS

  9. ASD HISTORICAL BACKGROUND • Rational Analyses Appeared Early 1800’s • Analysis Linear Elastic Based - Steel • Well Developed by Late 1800 • Basic Concept – Do not Exceed Yield Stress • Produced an Orderly Basis for Design

  10. ASD BASIS STRESS y a STRAIN Define an ALLOWABLE STRESS a = Cy For Steel Beams C = 0.4 to 0.66

  11. 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

  12. STRENGTH DESIGN • Not All Structures Have Linear Load-Stress (or Load-Strength) Relationship • Example – Columns • Behavior Understood by Late 1800’s • Strength Non-Linear and Dependent on Slenderness Ratio and Can Be Calculated • Factor of Safety Introduced • Universally Used in Geotechnical Design • Still Called ASD

  13. 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

  14. ASD Qi = Rn/F.S. LRFD γij Qij = k Rnk Gravity Loads ASD - D + L LRFD - ACI: 1.2D + 1.6L LRFD - AASHTO: 1.25D + 1.75L

  15. 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-1980’s • Ontario Bridge Code Adopted 1977 • AASHTO Bridge Code Adopted LFD 1977 • AASHTO Bridge Code Adopted LRFD after Extensive Research Project, 1994

  16. STRENGTH AND LOAD DISTRIBUTION fR(R),fQ(Q) Load Effect (Q) Resistance (R) A a b R,Q Rn

  17. STRENGTH MINUS LOAD DISTRIBUTION

  18. 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

  19. FROM THE HANDLINGOF THE LOADS ALONE ITIS A BIG IMPROVEMENTOVER ASD

  20. LOAD FACTORS FOR SELECTED CODES

  21. ButThere Are Many LoadsAnd Load CombinationsFor Instance,Two Important OnesIn AASHTOStr I = 1.25D + 1.75 L + …Str IV = 1.50 D

  22. COMPARE F.S. WITH  FOR DIFFERENT L/D RATIOS γD QD + γL QL=  Rn (QD + QL)F.S.= Rn γD + γLQL/QD =  (1 + QL/QD)F.S. (γD + γLQL/QD)/ (1 + QL/QD) =  (F.S.)

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

  24. AASHTO Equivalent Resistance Factors for Given F.S., Function of L/D Dead L.F. = 1.25 Live L.F. = 1.75

  25. 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

  26. 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

  27. 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 Needn’t Know How to Obtain Resistance Factors from Probability

  28. 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

  29. 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

  30. 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

  31. FOUNDATION DESIGN PROCESS Plan and execute subsurface investigation Evaluate information and select foundation system Deep Foundation Shallow Foundation

  32. Foundation Design Process Deep Foundation Drilled Shaft Driven Pile Select Drilled Shaft

  33. 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?)

  34. Foundation Design Process Prepare Plans and Specifications Select Contractor Verify Shaft Constructability and Capacity Install and Inspect Production Shafts

  35. QUESTION Where does the Geotechnical Strength Variability come from?

  36. Foundation Design Process Deep Foundation Drilled Shaft Driven Pile Select Driven Pile

  37. 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

  38. DRIVEN PILE DESIGN PROCESS • Pile Depth is Defined by a Dense Layer or Rock • The Length is Easily Selected Based on the Depth to the Layer Penetration Well Defined

  39. 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

  40. 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

  41. 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

  42. 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

  43. 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

  44. 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

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

  46. 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

  47. DRIVEN PILE DESIGN PROCESS Change Driving System Select Contractor Contractor Advises Proposed Hammer and Driving System Perform Drivability Analysis Hammer Satisfactory? NO

  48. DRIVEN PILE DESIGN PROCESS • This is the same as above except the driving system is now known (given by Contractor)

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