Investigation of axial pullout behaviour of small diameter medium-density polyethylene pipes in sand

Reza, Auchib (2023) Investigation of axial pullout behaviour of small diameter medium-density polyethylene pipes in sand. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Buried polyethylene pipes are increasingly used for gas distribution systems due to various advantages, including low cost, being lightweight, the ease of installation, corrosion resistance, and considerable flexibility. Permanent ground deformations due to ground subsidence, earthquakes, landslides, and slope movements can jeopardize the pipeline’s structural integrity. It is often not possible to avoid areas exposed to ground movements for pipeline routing. Although different technologies are currently available to monitor ground movements, such as GPS surveying at discrete points (survey hubs), slope inclinometer, LiDAR image analysis, and satellite image analysis, a reliable tool to correlate the monitored displacements to the condition of the buried pipe is required for assessing the pipeline distress (i.e., wall strains) due to the forces from the moving ground. The objective of this thesis is to develop techniques for predicting pipe performance for pipelines exposed to axial ground movements. Small diameter (i.e., 42.2- and 60.3-mm diameter) medium-density polyethylene (MDPE) pipes, commonly used in gas distribution systems, were considered. Pipelines are generally laid in the ground with the backfill soil well-compacted to secure ground support. In areas prone to ground movements, the pipelines are sometimes installed in a trench and then backfilled with loose to medium-dense sand to reduce the loads during relative ground movement. The behaviours of MDPE pipes in loose and dense sand were investigated using full-scale axial pullout tests. The tests were conducted under three different loading rates using the test facility developed at Memorial University of Newfoundland. Test results revealed that the load transfer mechanism for MPDE pipe depends on the pipe’s extensibility and the loading rate. Three-dimensional (3D) finite-element (FE) analysis is employed to interpret the test results, demonstrating that a rate–dependent interface friction angle could be used to account for the ii loading rate effect. Based on the study, simplified methods are proposed to calculate the mobilized frictional lengths and pipe wall strains from the relative ground movement in the pipe’s axial direction. For the pipe buried in loose sands, the maximum pullout resistance could be successfully predicted based on the normal force as the mean overburden pressure at the pipe’s springline and a rate–dependent interface friction angle. The conventional FE modelling with Mohr-Coulomb’s plasticity successfully simulated the test results. However, the compaction-induced lateral earth pressure and shear-induced soil dilation were found to contribute to the pipe responses for pipes in dense sand that could not be simulated using the conventional methods of FE analysis. Simplified approaches were proposed to account for the effect of soil dilation and calculate the mobilized frictional lengths, pullout resistances, and pipe wall strains for known relative ground displacements in dense sand. The developed method reasonably predicted the pipe responses measured during the tests. For a more rigorous analysis of pipes, 3D FE modelling techniques were developed for the pipes in dense sand. The effect of compaction-induced earth pressure was simulated during analysis using an equivalent temperature load. The method successfully simulated the test results. The study revealed that the effect of shearing-induced soil dilation depends on the magnitude of the earth pressure and could be insignificant for MDPE pipes. The compaction-induced lateral earth pressure was found to be significant for shallowly buried pipes. Based on the results of investigations, a method is proposed to include compaction-induced earth pressure for calculating the maximum spring force for pipeline evaluation using the beam-on-spring type FE modelling techniques recommended in the industry design guidelines.

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