Load transfer between pile groups and laterally spreading ground during earthquakes.

13 th World Conference on Earthquake Engineering Vancouver, B.C., Canada August 1-6, 2004 Paper No. 1516 LOAD TRANSFER BETWEEN PILE GROUPS AND LATERALLY SPREADING GROUND DURING EARTHQUAKES Scott J. BRANDENBERG 1 , Ross W. BOULANGER 2 , Bruce L. KUTTER 2 , Dan W. WILSON 3 , Dongdong CHANG 4 SUMMARY The lateral load transfer between pile groups and laterally spreading ground during earthquakes is evaluated based on results of dynamic model tests on a 9-m-radius centrifuge. The pile groups consisted of six piles, with prototype diameters of either 0.73 m or 1.17 m, connected together by a pile cap embedded in a soil profile consisting of a gently sloping nonliquefied crust over liquefiable loose sand over dense sand. Realistic earthquake motions with peak base accelerations ranging from 0.13 g to 1.00 g were applied to each of the models. The observed patterns of deformation throughout the centrifuge models are described in detail to illustrate several important features of behavior. A typical dynamic response is presented, showing components of the soil-pile and soil-pile cap interaction forces that were measured directly or obtained by data processing. Procedures for estimating the total horizontal loads on an embedded pile cap (i.e., passive loads plus friction along the base and sides) are discussed. Relative displacements between the free-field soil and pile cap required to mobilize the peak horizontal loads from the crust are shown to be much larger than expected for static loading conditions. The reasons for this softer-than-expected lateral load transfer behavior are discussed, and a simple idealization is used to illustrate the mechanism by which liquefaction of the underlying sand affected the load transfer behavior of the overlying crust. A simple relation for describing the observed lateral load transfer behavior between the pile caps and nonliquefied crusts is subsequently presented. INTRODUCTION Loads imposed on pile foundations by laterally spreading ground during earthquakes have been a major cause of past damages and are consequently a major concern in design practice. The cost can be very large to construct a new pile foundation, or retrofit an existing pile foundation, to resist the expected loads from laterally spreading ground, especially when a relatively strong surface layer is spreading over an underlying liquefied layer. However, the actual loading mechanisms between laterally spreading ground and pile foundations are only approximately understood. Graduate student, University of California, Davis. Email: sjbrandenberg@ucdavis.edu Professor, University of California, Davis Facility Manager, Center for Geotechnical Modeling, University of California, Davis Graduate student, University of California, Davis.