INTRODUCTION

Plate anchors are an economical alternative to gravity and other embedment anchors for resisting uplift forces in the marine environment Generally the plate is welded to a steel stem with an eye hook which accepts the external load. The ease with which plate anchors can be Installed (Mc Cormick, 1979), and the fact that grouting is not required make them. Ideal for use underwater, to secure pipelines, moorings, and cables to the seabed. Bobbit And Clemence (1987) reported the use of anchor sets having capacities of 45 KN to 150 KN to stabilize large diameter pipelines in Indonesia as illustrated in fig l a. Similarly on land, geogrids have been used to enhance the uplift capacity of burled pipelines (Selvadurai, 1989) To date no field cases have been reported from the U K sector Field experiments are expensive to set up, but ~t IS the authors' hope that when a reliable theory is produced on the basis of model tests, such those described in this paper some full-scale tests can be carried out to verify the theory.

Previous research into the uplift capacity of plate anchors embedded in cohesionless soil has shown that two distinct categories of anchor can be identified, shallow anchors and deep anchors. In the former, the anchor is installed close to the surface of the soil, and the failure surface in the soil extends from the tip of the anchor to the ground surface with significant surface movements. An increase in depth of embedment results in another type of failure in which the failure surface does not extend to the surface but instead forms locally around the anchor. This type of failure mechanism exemplifies the deep anchor mode

This paper presents the results of some model tests on shallow horizontal plate anchors and shows the effect on uplift resistance of varying the embedment depth and the relative density of the sand in which the anchor is embedded. The tests described are part of a wider study which has been in progress for some time at Glasgow University The complete study is aimed at assessing the behaviour of simple circular plate anchors embedded in sands and clays, and subjected to static and cyclic wave loading The following references describe some of this work Ponniah (1984) and Stewart (1988)

The aim of the wider study is to determine the parameters which affect pull-out resistance of a simple anchor, and eventually to produce a theory which will lead to more accurate predictions than are currently available. The comparisons of the results from the tests described in this paper with current theories ullustrate the variations In predicted uplift which can occur.

BACKGROUND

Most of the approaches used to predict anchor holding capacity are based upon the limit equilibrium concept The major difference amongst the various methods involves the assumed shape of the slip surface developed within the falling soil mass Limit equilibrium conditions.

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