Pipeline Lateral Stability in Soft Clay
Authors: Wantland Gary M. , O&apos;Neill Michael W. , Kalajian Edward H. , Reese Lymon 
Published: December 2007

This paper concerns the lateral stability of submerged pipelines in soft clay. The material presented in this pipelines in soft clay. The material presented in this paper is derived from a laboratory and field investigation paper is derived from a laboratory and field investigation using model pipelines. A relationship is presented between the depth/diameter ratio of the pipeline and the maximum resistance developed against lateral movement in the soil. 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.

Introduction

The problem of determining the stability of pipelines in soil classically has been investigated with a view toward determining vertical movements. In addition, however, lateral stability of pipelines has become a concern in areas suspected to be susceptible to subaqueous slope failures. Definitive experimental studies have been conducted concerning lateral stability of pipelines in sand, but few such data have been collected for clays. Of that work published for soft clays, little attention has been paid to the effect of embedment depths greater than one paid to the effect of embedment depths greater than one pipe diameter. pipe diameter. Lyons conducted small- and large-scale model studies in sand and clays. Tests conducted with clays were typically at depths of pipeline embedment of less than one diameter. Lyons observed that lateral resistance in clays increased with increasing pipe submerged weight. Karal approached the problem using the upper bound approach of the theory of plasticity. Lyons study was modeled using parametric solutions. Karal's theoretical solution compared favorably with Lyons measurements.The study reported in this paper concerns itself with the effect of the following variables on the resistance developed during lateral displacements of a pipe:(1)pipe weight,(2)pipe diameter,(3)embedment depth, pipe weight,(2)pipe diameter,(3)embedment depth,(4)loading rate. and(5)soil type.

Experimental studies were conducted in the field in a montmorillonitic clay in Harris County, TX, near Galveston Bay, and the work was extended in the laboratory with kaolin clay. The thrust of this work was to provide relationships between lateral resistance and the factors just listed.

Theoretical Considerations

The problem of lateral resistance due to pipe movement is in principle similar to that of the bearing capacity of a foundation. However, because of the geometry of the pipe/soil interface and the position of the free boundary, it is somewhat more complex. The classical theory of plasticity has been adapted by other investigators to solve problems of this nature in soil mechanics, both onshore and offshore. The limit analysis technique was used, along with the concept of perfect plasticity, to model the behavior of the soil perfect plasticity, to model the behavior of the soil for this study.The least upper bound solution for load-carrying capacity of a strip footing expresses the bearing capacity as a function of soil shear strength. If consideration is restricted to short-term loading, the least upper bound solution for bearing capacity is

(1)

where c is the undrained cohesion, bf is the footing width, and P is the resistance to displacement per foot of length.If the soil is assumed to be weightless and incompressible and the effects of pipe curvature and geometric nonlinearities are neglected, the failure mechanism assumed in the development of Eq. 1 can be applied to the lateral capacity of a pipe moving horizontally in an open trench, as shown in Fig. 1. Thus, P ideally should be maximized as a function of embedment D at D/d=2 and with P max = 5.14cd.

JPT

P. 217