Design Analysis for Pile Foundation in Liquefied Ground DFI Seminar, April 3, 2009

1. Design Analysis forPile Foundation in Liquefied GroundDFI Seminar, April 3, 2009 Project Funded by PEER Po-Lam - EarthMechanics Pedro Arduino UW Peter Mackenzie-Helnwein UW Ahmed Elgamaal UCSD Greg Fenves UT/UCB

2. Presentation Outline Overview Kinematic load case from ground deformation/displacement. Focus on the three layer liquefaction lateral spread problem and its impact on pile performance

3. Design Issues Related to Kinematic Pile Loading due to Ground Deformation or Displacement Two Generic Soil Conditions Encountered for Liquefaction Design Problems

4. Elements for Designing the Pile Foundation Against the Kinematic Ground Displacement Loading Problem • Define the amplitude of ground displacement, including accounting for pile pinning effects. • Define the ground displacement profile • Conduct soil-pile interaction analysis • Assess the pile performance

5. Analysis Approaches For the Kinematic Pile Loading Problem Beam on Winkler Spring Models Fixed-Fixed Beam Models Finite Element Models

6. Comments Regarding the Finite Element Approaches • Finite element model potentially be most powerful and be more rigorous, but they are impractical as a design tool. Furthermore, from experience they tend to be misused, leading to mistakes. • Best to be used as research tool for developing guidances to designers.

7. Comments Regarding the Fixed-Fixed Beam Model • The fixed-fixed beam equation model is the simplest, but requires gross assumptions and tends to be overly conservative. • Best to be used as preliminary screening tool, recognizing that the approach is conservative. • Further discussions to be provided.

8. Comments Regarding the Beam On Winkler Spring Model • Beam on Winkler spring model, probably is the most widely used approach favored by designers. Some softwares are available and designers are familiar with them. • We found significant problems with the approach as there are shortcomings in conventional way to define the ground displacement profile and also difficulty in defining p-y curves. • Further discussions to be provided.

9. Presentation of a PEER OPENSEES Research Project • Description of the research concept and results to-date. • Development of steps for simplified step-by-step procedure that can easily be implemented to-date for designing a wide range of kinematic pile loading problems. • Calibration of prior listed widely used analysis solutions from beam on Winkler spring and fixed-fixed beam model. • Outline future research program for further improvements.

10. New approach: Beam-Solid Contact Element Pile: beam elements Soil: solid elements Pile-Soil Interface: Beam-Solid Contact Element

11. Laterally Loaded Piles(comparison with LPILE) 3x Magnification

12. 3-D Finite Element Pushover Solutions of a 3-Layer Liquefied Lateral Spreading Ground Soil-Pile Interaction Problem Single 3-D linear elastic pile 3 layer soil with liquefied middle layer Linear elastic soil model, Es at non-liq. Free-field shear beam displacement profile on side boundaries (common assumptions by Geotechs) Near field soil displacement is automatically reinforced by the pile (pile curvature is continuous to the 4th order). An important issue!! Amplitude of displacement increased monotonically in pushover solution Es Es The FE Mesh was designed for the basic Single Solitary Pile Loading Problem where The Liquefied Layer is Sufficiently Deep And away from the Basic Inertia Load Case.

13. Paramatric Finite Element Solutions 24-In Prestressed Concrete Piles 54-Inch Prestressed Concrete Piles (Coronado Bay Bridge Dumbarton Bridge Antioch Bridge) 2.5-m Diameter Steel Piles (SFOBB East Span, New Carquinez, and New Benicia Bridge)

14. Parametric Study Scope and Schedule Constraint Project to Basic Problem Understanding • Pile diameters • D1=2.50 m, D2=54in., and D3=24in. • Soft Layer Thicknesses • T1=1D, T2 = 2D, and T4=4D. • Piles stiffness, EI • (scale factors for base EI values • “E-3”=0.125, “E-2”=0.25, “E-1”=0.50, “E0”=1.0, “E1”=2.0, “E2”=4.0, and “E3”=8.0. • Total cases = 84 cases

15. Typical Solutions for a 24-Inch 0.61-m Pile, in 0.61-m Thick Liquefied Soil for Reference EI and Es

16. Characteristic Results of Parametric studymaxM and location

17. Development of Simplified Solution Approach Design Needs: Location of max. moment Equation to solve for max. moment, or curvature Equation to solve for max. shear Definition of Embedment Depth, Lem

18. Es modulus of elasticity of stiff soil layer EI stiffness of pile T thickness of liquefiable layer D outer diameter of pile Identification of the Liq. Soil-Pile Interaction Parameter

19. Embedment Depth, Lem versus Characteristic Parameter 

20. Dimensionless Moment as Function of  Curvature = Moment/EI

21. Dimensionless Shear as Function 

22. Comparison of Proposed Solutions to Fixed-Fixed Beam Equation

23. Comparison of Proposed Solution to Beam on P-Y Curve Solutions 24-Inch Concrete Pile Solution in blue from conventional API (Reese’s P-y curves) yielding 38,570 kN-m/ m disp. vs. 7,180 kN-m from study, or 5.4 times correct sol. Solution in red (softened p-y from Vesic’s) yielded 11,000 kN-m as compared to 7,180 kN-m, or 1.5 Times correct sol. Due to conservatism in infinite curvature implicit in shear beam displacement function (implicit in free-field) which ignores pile EI reinforcing effects

24. Comparison of Proposed Solution to Beam on P-Y Curve Solutions 2.5-m (10-ft) pile Solution in red (softened p-y from Vesic’s yielded 208,000 kN-m as compared to 138,000 kN-m, or 1.5 times correct sol. Due to conservatism in infinite curvature implicit in shear beam displacement function (implicit in free-field) which ignores pile EI reinforcing effects. Such curvature reinforcing effect has never been addressed by past Researches!

25. Aspects of Reinforcing Effects of Piles on Ground Curvature

26. Aspects of Reinforcing Effects of Piles on Ground Curvature

27. Conclusions • Complex 3-D finite element analyses were conducted by expert analysts with careful checking. • Resultant solutions were used for development of simplified solution charts to support practical design needs. • Comparisons have been made to conventional widely used analysis approaches (fixed-fixed beam and beam on Winkler spring models). • Comparison showed some problems with conventional approaches, that they tend to be overly conservative. • Reasons on potential errors have been identified. • Use of proposed method leads to more rational design decisions (e.g. big diameter piles be beneficial for soft or liquefied soil sites).

28. THANK YOU!