ABSTRACT Within a wider research investigation of lateral stability of marine pipelines, a geotechnical investigation was carried out to examine lateral resistance to movement. It included extensive full-scale tests on a nominal 10-inch pipe on sand, under a range of loading conditions designed to simulate the loading history of a pipeline on the seabed. The results showed that the lateral resistance is actually higher than is generally assumed. The effect is particularly marked when the loading is cyclic with increasing amplitude, and when the pipe is partially embedded in the seabed. INTRODUCTION Lateral stability of a marine pipeline requires a balance between the hydrodynamic force induced by transverse velocities, the weight of the pipeline, and the resistance exerted by the seabed. If the seabed cannot exert enough sideways force to balance the horizontal component of the hydrodynamic force, the pipeline is swept sideways. The geotechnical question - how much resistance can the seabed supply? - and the hydrodynamic question - how large a force does the moving water create? - are therefore equally important. Remarkably, however, almost all research on stability has been devoted to hydrodynamics, though there are significant exceptions, notably the work of Lyons1, Karal2 and Lambrakos3 Pipeline designers almost invariably idealise the mechanical contact between the pipe and the bottom as one governed by friction, but both the idealisation and the customary choice of friction coefficients are open to criticism from the geotechnical point of view. Danish Hydraulic Institute (DHI) has recently completed a joint industry research program on pipeline stability. It was decided that the question of lateral resistance ought to form a major part of the program, and an extensive series of tests were carried out by Danish Geotechnical Institute (DGI), under subcontract to DGI and supervised jointly by DHI and by Andrew Palmer and Associates. The experimental results were used to construct a theoretical model. A wider study of the geotechnics of submarine pipelines was carried out in parallel with the experiments on lateral resistance. A distinct joint industry study on stability was in progress at the same time, at Norwegian Hydrodynamic Laboratories (NHL). That program also included geotechnical tests. The results were not available at the time the DHI program was planned and carried out, but some of them have since been published in two important papers4,5. A comparison with the DHI work is given later in this paper. CONCEPT Since the program had a relatively restricted budget, it was decided to concentrate the experimental work on sandy seabeds rather than clay. Sand covers much of the bottom of the North Sea, and was of primary concern to most of the participants. There is a parallel concern with soft silty clay, which occurs in wide areas in the deeper parts of the Norwegian sector, as well as in some areas in the UK sector, but was recognised from previous work on large deformations of soft clay6 that it would be difficult and expensive to secure uniformity and repeatability for a large number of large samples.