An investigation into pipelines subjected to lateral soil loading

Paulin, Michael J. (1998) An investigation into pipelines subjected to lateral soil loading. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (24MB)

Abstract

With the increased use of pipelines for carrying gas, oil, water, and electrical cables, the response of the pipeline to soil movements in the vicinity of the pipeline needs to be understood. These movements may be due to adjacent earth works, landslides, thaw settlement of permafrost, frost heave or a variety of other causes. These soil movements set up stresses within the pipeline and, depending upon the magnitude of these stresses and the nature of the pipeline, may cause damage to or failure of the line. Several consequences may be associated with pipeline failure and include loss of life, damage to the environment, and economic costs. The stresses which are generated by the soil movements are dependent upon a number of parameters, which include the nature of the soil, the properties of the pipeline, and the geometry of the pipeline/soil/backfill system. -- The state-of-practice (SOP) for pipeline design for areas where soil may move relative to the pipeline involves discretizing the pipeline into elastic-plastic segments which are connected to sets of springs/sliders which simulate the soil. As the springs replace the soil, their force-displacement characteristics should correspond to the actual soil response if a meaningful analysis is to be conducted. Much of the theory behind the interaction parameters used in the SOP are derived from theories developed for other geotechnical applications such as pile/soil or anchor plate/soil interaction; there is little or no verification of the mechanisms or the magnitude of forces which arise during pipeline displacement. This thesis presents a research program conducted to examine one aspect of pipeline/soil interaction; that of lateral pipeline/soil interaction. -- The objectives of the research program outlined in this thesis were to: (1) conduct physical model testing of lateral pipeline/soil interaction in cohesive soil to ascertain the effects of trench width, burial depth, interaction rate, backfill properties, and stress history of the soil on the interaction using the centrifuge technique to maintain similitude between model and full-scale; (2) determine the characteristics of normalized force-displacement curves or interaction factors so that they can be used generically; (3) assess the displacement patterns and failure mechanisms of the soil around a pipeline; and (4) generate conclusions and recommendations regarding current and proposed methods of analysing lateral pipeline/soil interaction through comparison with experimental results. -- This thesis demonstrates that: (1) the trench width had little or no effect on an undrained interaction; (2) the undrained load on a pipeline increased with increasing burial depth; and (3) the pipeline displacement rate (and thus drainage conditions) had a significant effect on the loads transferred to the pipeline by the soil (for this particular soil/backfill system). The pipeline displacement rate effect is significant because the current state-of-practice for cohesive media is based on an undrained interaction between the pipeline and the soil which can significantly underestimate the ultimate load transferred to the pipeline. Also, the displacements required to reach these ultimate loads are significantly underestimated in existing guidelines for the soil/backfill system considered. Results from existing and proposed analysis methods to predict pipeline/soil interaction curves are encouraging. -- The undrained force-displacement response could be reasonably predicted and ultimate loads can be predicted within ±20% using existing methods of analysis. Experimentally derived methods of undrained analysis provided reasonable fit to the experimental data; an average of within 10% of the ultimate lateral load might be expected. Other potential methods based on undrained anchor/soil and pile/soil interaction resulted in predictions of ultimate loads within 20% of those measured experimentally. Pile/soil interaction p-y curves were found to provide reasonable predictions to the experimental data. Bearing capacity solutions showed potential to bound the actual ultimate load. Passive earth pressure solutions were found to yield reasonable undrained prediction for cases where the pipeline was shallow (H/D<2). -- Predicted drained (c-Ф) ultimate loads on the pipelines were consistently underestimated using existing methods. Drained analysis methods from the experimental data could be expected to predict the force-displacement response of the experimental data to within ±20%. Drained anchor/soil interaction analysis methods underestimated the experimental data. Drained pile/soil interaction methods provided reasonable fits to the data in cases. Cohesionless soil p-y curves were found to overestimate the actual experimental force-displacement response. Cohesionless soil bearing capacity solutions were found to overestimate while passive earth pressure solutions tended to underestimate the ultimate lateral loads.

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