Three-Dimensional Finite-Element Modeling of Polyethylene Pipes in Dense Sand Subjected to a Lateral Force

Buried medium-density polyethylene pipes are conveniently used in gas distribution systems, particularly in areas subjected to ground movements, due to their flexible property to accommodate large deformation. The pipelines experiencing large ground movements require assessments for the fitness-for-services. Conventionally, the beam-on-spring idealization is used to evaluate pipelines exposed to ground movements. However, it is challenging for the beam-on-spring idealization to identify the spring parameters for representing soil-pipe interaction appropriately, which may vary from problem to problem. In the current study, three-dimensional (3D) finite element (FE) modeling was employed to understand soil-pipe interaction near the connection of a branch to a main pipe subjected to lateral movement. The FE model was developed through validation with full-scale test results. The study revealed that the conventional elastic-perfectly plastic model with a stress-dependent modulus of elasticity for soil could be reasonably used to simulate pipe-soil interaction observed during the tests. The FE analysis effectively simulated the mechanisms observed during the tests. Similar to the observations in the tests, the analysis calculated a lower pulling force yet higher strains for shallow buried pipes than for deeply buried pipes, confirming lower resistance to bending of the shallow buried pipes. The calculated contact pressures were nonuniform along the pipe length, indicating nonuniform axial and lateral soil resistances to the pipe. Thus, the spring forces recommended in the design guidelines should be revisited to account for the variation of contact pressure to model the pipe behavior using the conventional beam-on-spring analysis. The outcomes of this research would apply to the integrity assessment of MDPE pipes in areas prone to ground movements. Pipes in a distribution network can experience additional stresses/strains due to ground movements, which should be evaluated for fitness-for-service assessment. This paper presents an assessment of pipe wall strains near a connection of a network branch. The understanding from this research would be useful for assessing pipes in the field under a similar ground movement scenario.