DEM modelling of the axial behaviour of underground pipelines buried in unsaturated coarse-grained soils

The Discrete Element Method (DEM) is a useful tool for investigating the microscopic mechanisms of axial soil-pipe interaction (ASPI). Previous studies have not simulated the influence of matric suction on ASPI, mainly due to high computational costs and challenges related to particle size scaling effects on capillary forces. This study introduces a new 3D DEM approach to simulate ASPI under unsaturated conditions. Computational efficiency is enhanced through periodic boundary walls, the plane symmetry assumption, and the particle refinement method. Pipes are modelled with overlapping spheres, facilitating the capillary force calculation between the pipe and soil. A capillary model based on the Laplace-Young equation is implemented to compute capillary forces in the pendular regime. A new similarity criterion based on scaling laws is derived and validated, ensuring consistent suction effects when scaling numerical soil particle size by equating scaling factors for surface tension and particle size. The new code is validated using direct shear tests and physical model tests. Simulation results indicate that the ratio of axial resistance for a pipe in clayey sand at 70 kPa suction to that at 0 kPa is 2.71, demonstrating the significant matric suction impact. Two main microscopic mechanisms responsible for this impact are identified. First, capillary forces between soil and pipe increase at least 20% Bishop's interface contact pressure. Second, stronger dilatancy at the unsaturated soil-pipe interface causes more significant outward movement of soil particles, which is partially constrained by the surrounding soils, resulting in a 3.86-fold increase in net interface contact pressures.