ABSTRACT

A physical model study was undertaken to develop a better understanding of pile performance under axial cyclic loading in clay soils. Centrifuge model test results are presented for load controlled pile tests and an interaction diagram for combining static and cyclic loads is proposed. The pile exhibits very stiff cyclic load/ displacement behaviour and since excess pore pressures do not develop until the pile/soil bond is broken there is a critical level of cyclic loading below which the pile is stable. The results are compared to field scale model pile tests performed using cyclic loads which gave a constant rate of extraction and always displayed degradation since the movement always generated pore pressures.

INTRODUCTION

Most platforms installed in the UK sector of the North Sea are steel template structures (jackets) founded on groups of driven open ended piles. Piled foundations have been used as a means of supporting 'permanent' production and drilling platforms in the North Sea since 1968, drawing on experience gained in the Gulf of Mexico. It is likely that steel jackets will still dominate in ice-free waters at depths between 150 and 250m since there is no alternative design which can complete with the piled jacket for flexibility in design and construction and, above all, economics (Van Leeuwen (1983)). With the discovery of condensate reserves beneath greater water depths the use of floating systems has appeared more attractive than fixed production systems. These comprise structures which are mainly of the type that rely on buoyancy for vertical support whilst movement is restricted by an extensible link to the seabed of low flexural stiffness. The tension-leg platform (TLP), tethered buoyant platform (TBP), and guyed column type structures are typical of these. To date only one TLP has been installed in the North Sea (Conoco's North West Hutton Unit) although Saga Petroleum has chosen a TLP production facility for the southern part of Snorre Field (Offshore Engineer (1987)). A guyed column type of structure has been installed in the Gulf of Mexico at Lena Field (Boening and Howell (1984». Both these types of structure use piles to provide anchorage of the relatively large mooring forces at the seabed. These forces comprise a sustained vertical component with superimposed fluctuating vertical and horizontal components.

There is relatively little experience of the use of piles on land to resist sustained and fluctuating tensile loading. Experimental data on the behaviour of piles subjected to tensile loading are limited. In order to increase the data base a research program was carried out by the UK Building Research Establishment, (BRE) to examine some of the factors affecting the performance of piles subjected to tensile loads at approximately one tenth scale on land. At the same time centrifuge model tests were carried out to examine pile behaviour under both compressive and tensile axial cyclic loads and horizontal loading (University of Manchester (1982), Lambson (1987)). The BRE work is summarised by Department of Energy (1986).

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