Abstract
A comprehensive set of guidelines for constructing linear models of single piles and pile groups for foundations of offshore structures is presented. These models are used as boundary conditions at the base of the superstructure, thus permitting independent analysis of the superstructure from its supporting foundation.This paper is a "how-to" text for piled foundation modeling. It is also of value to those in related disciplines, such as geotechnical specialists, who will gain insight into how their data is applied in analyzing structures supported by piled foundations. Discussions include the behavior and modeling of single piles and pile groups. Construction methods are presented for pile groups. Construction methods are presented for three types of pile models-matrix, springs, and equivalent pile. The advantages and disadvantages of each model type are described. Linear and nonlinear foundation behavior characteristics are treated in depth. Factors that influence the approach to a modeling problem are outlined. Emphasis is placed on providing the problem are outlined. Emphasis is placed on providing the reader with an understanding of the physical behavior of piled foundations and model construction. A step-by-step piled foundations and model construction. A step-by-step procedure for model synthesis is provided in an example. procedure for model synthesis is provided in an example.
Introduction
In a fixed offshore platform, the steel jacket superstructure and its supporting piled foundation are more conveniently analyzed if treated separately. There are major structural and behavior-al differences between the jacket and foundation, and the two do not lend themselves to similar analytical methods. This paper presents basic techniques for constructing linear models that simulate the foundation behavior at the superstructure/foundation boundary. Use of these models permits independent superstructure analyses. Selection of the model type and its degree of refinement are described from a global overview of the structure, available data, and ramification of analytical results. Construction of the foundation simulation model follows routine procedures using results of an independent foundation analysis.
Single Pile Behavior
The load-deflection behavior of a single pile crown is of key importance in model construction. Analysis of a pile embedded in soil is extremely difficult because of pile embedded in soil is extremely difficult because of the infinite dimensions, nonhomogeneity, and nonlinearity of the soil. As a practical necessity, the problem usually is simplified by treating the pile as a beam-column supported by nonlinear axial and lateral soil springs. Details of these analytical procedures are beyond the scope of this work, although application of the techniques presented here will require the availability of such an analytical tool. Although this material pertains to single piles, it forms the basis for understanding pertains to single piles, it forms the basis for understanding pile group behavior discussed later. pile group behavior discussed later. Fig. 1 depicts a right-hand orthogonal coordinate system, which is used throughout this paper. Displacements, s, and forces, F, are shown in each of the six degrees of freedom (DOF). The pile behavior is studied by observation of the force(s) required to produce displacement in each of the six DOF while all other displacements are held at zero. Of utmost importance is the effect of coupling-the interaction of forces (and displacements) in different DOF.First consider linear pile behavior, which is characteristic of small-magnitude loadings. Force and displacement are directly proportional; therefore, stiffness (force divided by displacement) remains constant for all values of displacement (Fig. 2).Fig. 3a shows that an axial displacement is produced by an axial force. This axial displacement requires no other forces in each of the remaining five DOF. Therefore, linear axial pile behavior is uncoupled. Similarly, a torsional displacement (Fig. 3b) requires only a torque along the same DOF and therefore is also lineally uncoupled.
SPEJ
p. 775
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