Monotonic and cyclic pushover analyses of pile foundations in laterally spreading ground.

MONOTONIC AND CYCLIC BEAM ON NONLINEAR WINKLER FOUNDATION ANALYSES OF PILE FOUNDATIONS IN LATERALLY SPREADING GROUND S. J. Brandenberg 1 , R. W. Boulanger 2 , B. L. Kutter 2 , and D. Chang 3 ABSTRACT Static beam on nonlinear Winkler foundation (BNWF) analyses are compared with data from a suite of centrifuge tests of pile groups in liquefied and laterally spreading ground. Analyses that utilize monotonic load paths are shown to reasonably predict bending moments, and pile cap displacements were reasonably predicted when limited to less than about 0.4 meters. However, the analyses under-predicted larger pile cap displacements, with the error being attributed primarily to the accumulation of displacement during repeated loading cycles due to cyclic ratcheting. The influence of cyclic ratcheting is explored using cyclic BNWF analyses, and predicted cap displacements are larger for cyclic load paths compared with linear load paths in cases where the pile tips fail in the axial mode (i.e. plunging or pullout). Introduction Extensive damage to pile foundations has been caused by liquefaction and lateral spreading of the surrounding soils [e.g., Japanese Geotechnical Society (JGS) 1996, 1998]. Lessons learned from case histories have provided important guidance for selecting input parameters and loading conditions to use in design computations, but the number of case histories is not sufficient to fully validate analytical methods. Furthermore, case histories typically do not provide the detailed data required to measure fundamental loading mechanisms, and it is therefore difficult to ascertain just how general observations from case histories are, and whether they would hold true for different conditions. Model studies can provide sufficient detailed data to identify fundamental loading mechanisms that occur during lateral spreading, thereby supplementing case histories to provide a rational basis for deriving robust analytical approaches. Pile groups in lateral spreading soils are often analyzed using a static-seismic beam on nonlinear Winkler foundation (BNWF) analytical approach, in which the foundation components are modeled as beam-column elements, and soil-structure interaction is modeled using p-y materials for lateral subgrade reaction, t-z materials for pile shaft friction, and q-z materials for pile tip end bearing. As shown in Fig. 1, demands from laterally spreading layers can be represented by imposing free-field soil displacements Post-Doctoral Scholar, Dept. of Civil and Environ. Eng., University of California, Davis , CA 95691 Professor, Dept. of Civil and Environ. Eng., University of California, Davis, CA 95691 Graduate Researcher, Dept. of Civil and Environ. Eng., University of California, Davis, CA 95691