Modeling uplift failure of pipes buried in sand using material point method

Uplift of underground pipelines is frequently encountered in urban tunneling and landslide-prone areas, seriously affecting their structural integrity and serviceability. However, the uplift resistance of pipelines and the failure modes of surrounding soils under permanent ground deformation have not been fully understood. In numerical simulations, the element distortion phenomenon significantly restricts the application of mesh-based numerical methods in addressing such issues. In this paper, the material point method (MPM) is used to investigate the upward movement of pipes buried in dense sand and the mobilization of uplift resistance. Two typical constitutive models are used, and their model parameters are estimated using state index, whose values are dependent on relative density and confining pressure. The post-peak softening characteristics of soil have also been considered and a series of numerical simulations are conducted. The results from physical model tests in the literature are used to verify the numerical model. It is found that the experimental and numerical results agree well with each other in terms of force-displacement relationships and soil deformation patterns. The effects of burial depths on the uplift failure mechanism of pipes are investigated in detail. The numerical results show that for shallow burial conditions, the inclination of shear bands is approximately identical to the maximum dilation angle of sand at the peak resistance and then decreases when the pipe undergoes larger uplift displacement, leading to a reduction of uplift resistance. For deeply buried pipes, the inclination of shear bands does not change in the post-peak phase, but a failure pattern with a series of progressive shear zones gradually forms, which is analogous to the bearing failure modes of foundations. At large uplift displacements, significant flow-around phenomena are observed for various burial depths of the pipe. Finally, the distribution profiles of earth pressure acting on the pipe surface are explored. It is found that the normal pressure concentration area mainly ranges from 0 degrees-45 degrees from the pipe crown. In comparison, the tangential contact pressure primarily concentrates on the pipe shoulder within 15 degrees-45 degrees from the crown. Finally, the peak uplift forces and corresponding failure modes within the maximum embedment ratio of 50 are investigated to provide practical guidance for the design of underground pipelines.