Mechanisms of load transfer between pile groups and laterally spreading nonliquefied crust layers.
International Symposium on Earthquake Engineering Commemorating Tenth Anniversary of the 1995 Kobe Earthquake (ISEE Kobe 2005) Kobe and Awaji, January 13 16, 2005 MECHANISMS OF LOAD TRANSFER BETWEEN PILE GROUPS AND LATERALLY SPREADING NONLIQUEFIED CRUST LAYERS S. J. Brandenberg 1 R. W. Boulanger 2 D. Chang 3 and B. L. Kutter 4 Department of Civil Environmental Engineering, University of California at Davis, USA Email: sjbrandenberg@ucdavis.edu Department of Civil Environmental Engineering, University of California at Davis, USA Email: rwboulanger@ucdavis.edu Department of Civil Environmental Engineering, University of California at Davis, USA Email: ddchang@ucdavis.edu Department of Civil Environmental Engineering, University of California at Davis, USA Email: blkutter@ucdavis.edu SUMMARY Observations of pile damage from the 1995 Kobe earthquake provided clear illustrations of the importance of loads imposed by laterally spreading ground. Analyses of these and other case histories have shown that the loads of greatest concern are often those imposed by nonliquefied surface (crust) layers that spread laterally over a liquefied layer. To study the mechanisms of load transfer between pile groups and laterally spreading nonliquefied crust layers, a series of dynamic centrifuge model tests were performed on a 9 m radius centrifuge. Pile groups consisting of 6 piles, with prototype diameters of either 0.73 or 1.17 m, connected together by a pile cap were embedded in soil profiles that consisted of gently sloping nonliquefied crusts over liquefiable loose sand over dense sand. Models were shaken with earthquake motions having peak base accelerations from 0.13 g to 1.00 g. Time series for the soil pile and soil pile cap loads were either measured directly or determined by back calculation from the dense instrumentation arrays. The relative displacements between the free field soil and pile cap that were required to mobilize the peak horizontal loads from the crust were much larger than expected based on analogies to static loading conditions. The mechanisms responsible for this softer than expected lateral load transfer behavior are explained and a simple model for describing the observed lateral load transfer behavior between a pile group and nonliquefied crust is developed. 1. INTRODUCTION Loads from laterally spreading ground have been a major cause of damages to pile foundations in past earthquakes, including the 1995 Kobe earthquake (e.g. Tokimatsu et al. 1996, Karube and Kimura 1996, Hamada and Wakamatsu 1996, Matsui and Oda 1996) Analyses of these case histories have shown that the loads of greatest concern are often those imposed by nonliquefied surface (crust) layers that are spreading laterally over an underlying liquefied layer. This paper describes the lateral load transfer behavior between pile groups and laterally spreading soil during earthquake shaking based on a series of dynamic centrifuge model tests. The centrifuge models involved nonliquefied surface layers of clay spreading laterally against pile groups. The observed lateral load transfer behavior is shown to be much softer than expected based on current design practice. A simple model is developed that describes the mechanisms by which liquefaction causes the lateral load transfer behavior from the clay crust to be softened relative to the static load transfer behavior in non liquefied ground. This simple model is used to illustrate how other factors, beyond those covered in the centrifuge tests, are expected to affect the lateral load transfer behavior. Implications of these findings for pile foundation design are discussed. ISEE Kobe 2005 B 153 Brandenberg et al.