DESIGN OF DEEP FOUNDATIONS

1. DESIGN OF DEEP FOUNDATIONS George Goble Goble PileTest, Inc.

2. 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

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 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, I’m Developing an Easy to Use Dynamic Pile Testing System

4. 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

5. 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)

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 Are Known in Advance of Pile Installation • Factor of Safety (Resistance Factor) Dependent on Methods of Capacity Determination and Installation Quality Control

7. 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?

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

9. 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?

10. 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

11. STRENGTH DESIGN • Not All Structures Have Linear Load-Stress (or Load-Strength) Relationship • Example – Columns, or Concrete • Behavior Understood by Late 1800’s • 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

12. 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

13. 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

14. 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

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

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

19. 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

20. 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

21. 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

22. 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)

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

24. 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.

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

26. 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?)

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

28. QUESTION Where does the Strength Variability come from?

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

30. 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

31. 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 • Select pile type oirjmoi;ernj;som Penetration Well Defined

32. 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

33. 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

34. 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

35. 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??)

36. 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)

37. 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

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

39. 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

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

41. DRIVEN PILE DESIGN PROCESS Hammer Satisfactory? YES Set driving criteria Drive test pile to criteria Verify test pile capacity Capacity/stress satisfactory? NO

42. 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

43. QUESTION Where does the Strength Variability come from?

45. THE END