Abstract

A program of research on laterally loaded piles for offshore structure has included field test with an instrument pile, laboratory model testing, and development of correlation for design. The work has been sponsored by a group of five oil companies.

Three loading condition are considered to be particularly pertinent to the design of laterally loaded pile in soft normally consolidated marine clay. These are (1) short time static loading, (2) cyclic loading such as would occur during the progressive build-up of the storm, and (3) subsequent reloading with forces less than previously maximums.

Good general agreement exist between conventional static-loading ultimate-resistance concept and experimental results, provide due allowances are made for the reduced vertical restraint at shallow depth, where it is insufficient to cofine plastic flow to horizontal planes. Force-deformation characteristic based on approximate theory produce satisfactory agreement between computed and experimental behavior of the pile-soil system.

The mechanism of cyclic loading characteristic are qualitatively illustrated by typical results from laboratory model studies. Deterioration in resistance because of cyclic loading is most severe at shallow depth and with large lateral deflection of the pile. A correlation based primarily on result with the instrumented pile tested at Sabine, Texas, gives satisfactory prediction of pile deflection and moment over a wide range of loading condition.

Estimates of response for reloading after cycling at a higher load made by considering that most of the lateral soil resistance is estimated for deflection smaller than those previous attained.

The correlation are summarized and recommendation given for their use in design.

Introduction

The ability to make reasonable estimates of the behavior of laterally loaded piles is an important consideration in the design and construction of many offshore installations. This is particularly true in the Gulf of Mexico where large lateral forces are produced by winds and waves associated with hurricane and where the foundation materials in the critical zone near the mudline are often found to be very weak clays.

A program of research on laterally loaded piles sponsored by five oil companies is the primary basis for the correlation in this paper. At this time it is intend only that the results be summarized in a form directly in design, but publication of the background research is planned for the near future.

Requirement for Analysis and Design

There are many different ways in which piles or caissons may be subjected to effect of lateral forces. One such case is show in Fig 1a represent a pile and a leg of a jacket-type structure.

The structural analysis problem amounts to that of a complex beam-column on an inelastic foundation. For pile separated by spacing of several diameter or more, the Winkler assumption is useful to facilitate the analysis. This means that the soil is considered as a series of independent layers in providing resistance p to pile deflection y (Fig 1b)

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