Simplified Method For Evaluating Soil-Pile-Structure Interaction Effects
Authors: Berger Ernest , Mahi Stephen A. , Pyke Robert 
Published: October 2008

A simplified analytical procedure for evaluating soi1-pi1e-structure interaction effects for pi1esupported offshore structures is presented. This procedure has been developed principally for seismic analyses but may also find use in analyses of soil pile-structure interaction under wave loadings. The foundation is modeled in the analysis by a full structural representation of the piles to which a series of springs and dashpots are attached. These springs and dashpots represent the stiffness and wave propagation effects associated with relative movement of the piles and the soil. In the case of seismic analyses, the free-field motions are directly applied to the free ends of the springs and dash pots, eliminating the need for explicit representation of the free-field in the analytical model. The foundation model used is two-dimensional but it is shown that three-dimensional effects can be satisfactorily represented. In particular, the three-dimensional propagation of energy in the system, commonly referred to as radiation damping, is adequately represented. This is demonstrated by the results of analyses of a one pile model subjected to earthquake loading. In the examples given, equivalent linear properties are used; however, the foundation model can also be used with nonlinear time-dependent soil properties as presently installed in the nonlinear structural analysis program DYNAS which is capable of analyzing multiple pile single or stacked frames.

INTRODUCTION

Foundation compliance has been taken into account in analyses of pile-supported platforms subjected to wave loadings for some years, usually by means of sets of linear elastic springs placed at the base of each leg. Because the behavior of the foundation is actually nonlinear, this requires an iterative procedure to determine compatible loadings and spring properties. This kind of approach is also possible when considering earthquake loadings, but in addition to the fact that the iterative approach can become time consuming, there are several aspects of behavior under earthquake loading which are difficult to represent by simple sets of springs. This is particularly true if nonlinear analyses of the structure are to be conducted in order to investigate the reserve capacity or collapse mechanism under extreme loadings. Hence, a number of more sophisticated foundation models have been proposed which make it possible to conduct an integrated analysis of the structure and the foundation.

In such analyses it is desirable that the actual pile section and length be used and that the following aspects of the foundation behavior be taken into account:The change in soil properties with depth;The nonlinear stress-strain behavior of soils which will tend to be modified as a result of earthquake shaking;The nonlinear behavior of any gaps or yielding that develop at the soil-pile interface;The variation of motion with depth in the soil profile which will tend to control the deformation pattern of the piles;The three-dimensional displacement pattern of the soil as the piles tend to push into the soil;The three-dimensional propagation of energy in the soil due to soil-pile interaction effects.