Analysis Of Three-Dimensional Pile Groups With Nonlinear Soil Response And Pile-Soil-Pile Interaction
Authors: O&apos;Neill Michael W. , Ghazzaly Osman I. , Ha Ho  Boo 
Published: October 2008

Present methods of pile group analysis are either limited to special geometric cases or do not consider pile-soil-pile interaction. An analytical procedure is described in this paper wherein arbitrary piling geometry can be assigned, nonlinear soil response in-axial, lateral, and torsional modes can be considered, and pile-soil-pile interaction can be introduced.

The application of this procedure is illustrated in three example analyses, and design implications of the results are outlined.

INTRODUCTION

The requirement for sitting fixed offshore platforms in progressively deeper water and in locations characterized by severe environmental loadings has resulted in the consideration of very deep and often complex pile foundation systems. Concurrently, the need has evolved for establishing procedures that will permit analysis of these systems in a rational, general manner. The primary objective of any such analytical procedure relative to the analysis of the foundation is to determine the loads and deformations along each pile in the system through specific consideration of piles with variable sections; nonlinear reactions of soil against piles; the pile-soil-pile interaction, or "group effect", that occurs as a result of the influence of soil reactions against one pile on the soil deformation in the vicinity of the others in the group; and coupling with the superstructure. Furthermore, such an analytical scheme should offer a method for obtaining the pile-head response relationships for use in superstructure analysis. In order to be general enough to be used by design professionals, the effects described above should be modeled in three dimensions, and modeling of the true three dimensional conditions of loading and pile geometry should be permitted.

Existing procedures, while quite useful in special cases, are incomplete relative to the requirements outlined above. Present typical analytical techniques include (1) two-dimensional approximation of the pile-soil system and calculation of loads and deformations in the piles by a modified Hrennik off model that considers nonlinear pile response to axial and lateral loads, decoupled relationships between axial and lateral modes of loading in an individual pile, and neglects pile-soil-pile interaction,18 (2) three-dimensional models that consider linear, uncoupled behavior in the axial, lateral, and torsional modes of loading in which pile-soil-pile interaction is neglected,19,20 (3) three-dimensional solutions that model actual pile geometry and loadings, consider uncoupled, nonlinear behavior in an individual pile, consider deformable foundation-structure coupling, but neglect pile-soil-pile interaction,1 (4) procedures that include nonlinear soil response and pile-soil-pile interaction but that are limited to- simple conditions of pile geometry and loading.6,14,22

In the past, analysis of foundations of structures situated in relatively shallow water has proceeded on the basis that main piles are spaced sufficiently far apart to preclude the necessity for considering group effects. However, as pile penetrations increase such that the ratio of penetration to spacing increases, and as piles are used more frequently in clusters, it becomes necessary to include group effects in the analysis.