An inter-particle capillary force model for coarse-grained soils – implementation in DEM and experimental validation on ground cavities

The interaction between soil and water is crucial in geotechnical engineering, and it is essential to understand the mechanics of unsaturated soils. The discrete element method (DEM) is a powerful tool for modeling the behavior of granular media. The conventional DEM approach, however, may be insufficient to capture unsaturated soil behaviors due to the lack of capillary forces. This contribution provides a review of capillary force models and derives a new model that incorporates inter-particle suction and surface tension in granular soils, with three input parameters: degree of saturation, surface tension, and the rupture distance of the liquid film. The newly proposed model allows straightforward implementation with fewer input parameters compared to other capillary force models in the literature. To validate the accuracy of the proposed model, the stability of a glass bead sample having a subsurface cavity is investigated using both DEM and equivalent laboratory experiments. A good qualitative agreement is observed between the numerical and experimental data, which can be effectively used to perform numerical simulations of coarse-grained soils for underground cavity development. In contrast to the smaller-diameter sample, the 2 mm mean-diameter sample could not sustain a stable cavity in both model tests and numerical simulations. Although the matric suction is low for coarsely grained soil, the capillary force is a prerequisite for solving the cavity stability problem.