Geotechnical Modeling and Capacity Assessment

Transcription

1 Geotechnical Modeling and Capacity Assessment by Geoffrey R. Martin Chapter 6 : Geotechnical Modeling and Capacity Assessment Foundation Modeling (Evaluation Methods D and E) Equivalent linear stiffness models Capacity models Shallow footing, piles, shafts, abutments Ground Displacement Demands Settlement (Appendix B) Liquefaction Induced Lateral Spreads 1

2 Geotechnical Modeling and Capacity Assessment Seismic Bridge Response May Be Significantly Influenced by the Stiffness and Strength of the Foundation System (abutments, footings, pile foundations) Stiffness and Strength of Foundations can Influence Both the Force and Displacement on the Bridge and the Distribution of Loads to the Structure and Foundation Components Because of the Cost of Retrofit Construction, More Detailed Foundation Analysis May be Warranted for Retrofit Projects than for New Construction Foundation Modeling 2

3 Foundation Modeling Stiffness and Capacity: Shallow Footing Foundations Assume idealized elastoplastic behavior Use uncoupled spring model for rigid footings or Winkler spring model Winkler spring model can capture progressive mobilization of plastic capacity during rocking behavior Upper and lower bound approaches capture uncertainties in soil properties 3

4 Shallow Footing Foundations : Stiffness Parameters Uncoupled stiffness parameters obtained from theoretical solutions for a rigid plate on a semiinfinite elastic halfspace Shallow Footing Foundations : Stiffness Parameters (cont.) 4

5 Shallow Footing Foundations : Stiffness Parameters (cont.) Shallow Footing Foundations : Capacity Parameters Winkler springs assumed for capacity evaluation Soil yield, rocking and uplift can reduce ductility demands on bridge structure Settlement < few inches for F v >2.5 5

6 Shallow Footing Foundations : Capacity Parameters (cont.) Non-linear moment rotation behavior is generated by yield and uplift Lengthens fundamental period dissipates energy by soil yielding Shallow Footing Foundations : Capacity Parameters (cont.) Column shear forces resisted by friction at base and passive resistance at footing face 6

7 Pile Footing Foundations : Stiffness and Capacity Pile Group footing or pile cap normally uncoupled from the piles contributions from two components evaluated separately Pile Cap treated as a footing but base friction neglected Primary source of lateral resistance stiffness and capacity is passive pressure on vertical face Pile Lateral Load nonlinear load-deflection characteristics determined by pushover analysis assuming nonlinear p-y Winkler springs Axial nonlinear load-deflection characteristics uncoupled from lateral load behavior, and govern moment-rotation behavior Pile-Pile Cap connection details influence lateral loaddeflection characteristics Pile Footing Foundations : Stiffness and Capacity (cont.) 7

8 Pile Footing Foundations : Stiffness and Capacity (cont.) Pile Footing Foundations : Stiffness and Capacity (cont.) 8

9 Pile Head Stiffness : Lateral Loading Non-linear p-y curves may be linearized to determine coupled 6x6 stiffness matrix for a pile group Stiffness coefficients obtained from beam-column solutions for single piles assuming elastic subgrade reaction theory Pile Head Stiffness : Lateral Loading (cont.) 9

10 Pile Head Stiffness : Lateral Loading (cont.) Pile Head Stiffness : Lateral Loading (cont.) 10

11 Pile Head Stiffness : Lateral Loading (cont.) Pile Head Stiffness : Lateral Loading (cont.) - Example 11

12 Pile Head Stiffness : Axial Loading A graphical simplified solution may be used for initial estimates Alternatively, a simple estimate is given by αae/l, where: α=1 for end bearing piles on rock α=2 for friction piles Pile Group Stiffness : Example Calculations Example: 3x3 pile group 1-ft. diameter pipe piles (0.25in wall thickness filled with concrete) driven into sand (φ=30 o ) 70 ft. 1. Obtain single pile axial and lateral stiffness Superimpose single pile stiffness Superimpose pile cap stiffness 12

13 Pile Group Stiffness : Example Calculations (cont.) Lateral Stiffness of Pile: Comments 13

14 Lateral Stiffness of Pile: Comments (cont.) Lateral Stiffness of Pile: Issues 14

15 Pile Group: Moment-Rotation Capacity Moment Capacity of a pile group depends on Number of piles and spatial dimensions Axial capacity of piles in compression and uplift The study on the right illustrates a modeling procedure Pile Group: Moment-Rotation Capacity (cont.) Analytical studies have shown that for pile footings where the static factor of safety for dead load is >3, the settlement generated from mobilizing moment capacity during earthquake is unlikely to be significant. 15

16 Pile Group: Moment-Rotation Capacity (cont.) Traditional Moment Retrofit Example Abutments: Stiffness and Capacity The stiffness and capacity of abutments under longitudinal inertial loading depends on the structural design of the abutment walls and the resistance of mobilized fill soils. 16

17 Abutments: Stiffness and Capacity (cont.) Abutment Stiffness Longitudinal Direction: K K effl effl Pp = 0.02H (integral) Pp = (Seat) 0.02H + D ( g ) H Approach Slab Granular Drainage Material Passive Pressure Zone o o Footing Active Pressure Zone Tie Default Passive Pressures: Non-plastic backfill P p = 2H/3 ksf Cohesive backfill P p = 5 ksf (UCS > 4 ksf) Force PP Keff1 Ki Keff2 Dg Deff Seat Abutments H Actual Behavior K i = K eff1 Force PP Keff2 Deflection Deff Integral or Diaphragm Abutments Abutments: Stiffness and Capacity (cont.) Knock-off backwalls are commonly used to avoid damage to piled abutment foundations or to avoid backward tilt of walls 17

18 Free Standing Full Height Abutment Retaining Walls 6.3 Ground Displacement Demands on Foundations Earthquake Induced Settlement (Appendix B) Liquefaction Induced Lateral Spreads Large rigid body movements of abutments and piers Can cause catastrophe damage to piers and foundations and/or unseat the superstructure 1990 Philippines Earthquake 2m lateral spread 18

19 Ground Displacement Demands on Foundations 1987 Edgecombe N.Z. Earthquake 2m lateral spread Piles resisted passive pressure Ground Displacement Demands on Foundations (cont.) 1995 Kobe Earthquake Plastic hinge development at interfaces between liquefied and nonliquefied layers 19