Lateral Group Behavior of Piles in Offshore Soil Conditions
Authors: Horsnell M.R. , Aldridge T.R. , Erbrich C. 
Published: October 2008

As North Sea development moved into deeper water during the nineteen seventies, foundation design became more dependent upon the behavior characteristics of piles clustered around the legs of the jacket. Structural designers were faced with the problem of adequately representing the behavior of these pile groups within their overall structural model. In many cases the problem was compounded by the non-uniformity of the foundation soils and their associated non-linear behavior over the load range under consideration for design. Geotechnical consultants, familiar with assessing the behavioral characteristics of single isolated piles, now had to address problems associated with pile groups. During this period and subsequently many pile group computer programs have been developed. The ability of these programs to adequately model pile group behavior is a function of the simplifying assumptions which many of them incorporate. The influence of these assumptions may be transparent to the user and little or no guidance is given on the selection or effect of governing soil parameters. This paper reviews the development of pile group analysis methodology concentrating on problems associated with lateral group behavior in North Sea soil conditions. Limitations of current design methods and computer programs are discussed and details of a more rigorous method of analysis developed by the Authors is presented.

Introduction

The aim of the pile group response analysis is to obtain relationships between pile group loads and deflections such that for a given deflected position (?) it is possible to define the pile group loads (p). The required relationship therefore takes the form:-

(MATHEMATICAL EQUATION AVAILABLE IN FULL PAPER)

For a given deflection and pile cap rotation, the position and rotation of the head of each pile within the group can be obtained from geometrical considerations. The response of the piles within the group to axial, lateral and rotational displacement of the pile head will consist of:-single pile axial loads, Vp'single pile lateral loads, hp' andsingle pile head bending moments, mp

At a given deflected position of the pile cap, it is possible to sum the response of the individual piles to obtain the overall pile group resistance, ie.

(Mathematical equations) (Available in full paper)

where li = lever arm from pile i to the axis of rotation. (This is shown diagrammatically in Fig. 1).

The analysis of the loads and moments generated at the head of each pile are conventionally carried out on the assumption that the lateral and axial load deflection behavior are independent of each other, ie. that changes in lateral load applied to the pile do not cause any axial deflection and vice versa. The Vp terms in equation (2) and (4) are thus derived from axial group analysis and the hp and mp terms in equations (3) and (4) are deriver from an independent lateral group analysis. The predominant stresses on an individual pile arise from the bending moments induced under lateral loading. This paper, therefore, concentrates on the methodology associated with lateral group analysis.