ABSTRACT

This paper concerns the lateral stability of submerged pipelines in soft clay. Although considerable work has been published with respect to model testing of pipes in sands and theoretical treatment of the lateral stability problem, to the best of the authors knowledge little experimental data have been published previously with respect to soft clays.

The material presented in this paper is derived from a laboratory and field investigation using model pipelines in cohesive soils. The field investigation, sponsored by Shell Development Company, was carried out in Harris County, Texas, near Galveston Bay. The study was accomplished using 2.875 and 4.50 inch diameter pipes which were weighted with steel rods to obtain different specific gravities of pipe. A follow up investigation was conducted in the laboratory using pipe diameters of 1.50 and 3.00 inches. The model pipes were also filled with ballast but the pipe specific gravity was not varied. The laboratory investigation was conducted to extend the results of the field work in a controlled environment using a different soil (kaolin clay). The parameters varied in the laboratory investigation included depth of embedment, pipe diameter and rate of displacement. The soil type, water content and shear strength were not varied.

Based on both sets of studies, a modified theoretical model was developed to calculate the maximum values of resistance based upon the theory of plasticity using the limit analysis techniques.

A relationship between the depth-to-diameter ratio of the pipeline and the maximum resistance developed against lateral movement in the soil is presented. The latter term is represented by the dimensionless factor Ncl, which is similar to a bearing capacity factor. Conclusions are drawn with respect to the effect of the diameter of the pipe, pipe weight, and the rate of displacement on the soil resistance developed. Finally, recommendations are made for further work in this field.

INTRODUCTION

The pipe-soil interaction problem has classically been investigated with a view towards determining vertical movements related to pipeline stability. In addition, however, lateral stability of pipelines has become a concern in areas suspected to be susceptable to sub-aqueous slope failures. Definitive experimental studies have been conducted concerning lateral stability of pipelines in sand (1), but little such data have been collected for clays. Of the work that has been published regarding lateral resistance of pipes in soft clays, little attention has been paid to the effect of embedment depths greater than one pipe diameter. Knowledge concerning lateral stability at greater depths is desirable because it has become common practice to place the pipeline in a trench cut into the seabed to decrease the effect of hydrodynamic drag and for added protection against anchor drag.

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