Pipeline-backfill-trench interaction during large lateral displacements

Kianian, Morteza (2020) Pipeline-backfill-trench interaction during large lateral displacements. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Subsea pipelines, particularly in shallow areas, are usually buried inside trenches backfilled with pre-excavated material as cost-effective protection against the environmental, constructional, and operational loads. The design of buried pipelines against potential lateral displacements is a challenging task that is usually simplified by assuming a uniform soil surrounding the pipeline. However, the remolded backfill and its lower stiffness compared with the native ground can significantly affect the failure mechanisms around the moving pipe and the mobilized lateral soil resistance. Having a trench backfilled with a material softer than the native seabed soil will lead to a complicated pipe-soil interaction problem which has not been entirely explored in the literature. In this study, the lateral pipeline-backfill-trench interaction and the resultant soil failure mechanisms were investigated by centrifuge models (in partially drained conditions) and also numerical simulations (in undrained conditions). Particle image velocimetry (PIV) analysis was conducted to capture the interactive soil displacements and failure mechanisms during centrifuge tests. It was observed that the interactive effects of pipeline, backfill, and trench precede their individual shear strengths. The advanced numerical simulations were developed by using the Coupled Eulerian-Lagrangian (CEL) approach with two different Eulerian materials behaving in undrained conditions. The numerical simulations in undrained conditions show a good agreement with the previously conducted centrifuge tests in terms of lateral load-displacement response and failure mechanisms. The investigated parameters are pipe roughness, pipe weight, pipe initial embedment into the trench-bed, backfill strength properties, soil strain-softening, native soil tension cut-off, and burial depth. The effects of influential parameters are comprehensively examined using the developed numerical model, and the results show good agreement with some previously conducted centrifuge tests. The study revealed the significance of the pipeline-trenchbed interaction in the mobilization of the lateral soil resistance and several other mechanisms not yet addressed in the literature. As a result, several new research avenues were identified, and the ground was prepared for proposing solutions to improve the prediction of the lateral response of buried pipelines in the near future.

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