Microscale Investigations of Hydromechanical Failures of Granular Soils

Abstract

The Discrete Element Method (DEM) has proven useful to capture the micro and macro behaviour of soils. The complex micromechanical characteristics associated with hydromechanical failure of soils, such as internal instability and fluidisation, can be replicated with DEM. This research is divided into two parts, i.e., (i) microscale analysis of internal instability of cohesionless soils by using DEM under isotropic stress conditions and during shearing, and (ii) micromechanical analysis of fluidisation of granular soils by coupling DEM with the Lattice-Boltzmann Method (LBM).

Micromechanical analysis of the internal instability of cohesionless soils under isotropic stress state was carried out using DEM. The coordination number and the stress reduction factor were used to estimate the potential for internal instability of granular soils, and the clear boundaries between the samples that were internally stable and those that were unstable were delineated. Thereafter, the dense samples were sheared under drained conditions following a constant mean stress path to study the influence of shear deformation on internal instability. The simulation results showed that a dense sample could transition from internally stable to unstable soil as it dilates during shear.

Furthermore, microscale investigations on soil fluidisation were carried out using the DEM in combination with the LBM. The development of local hydraulic gradients, the distribution of contacts, and the associated fabric changes were examined. The microscale findings suggest that a critical hydromechanical state that induces fluid-like instability of a granular assembly can be described by a substantial and sudden increase in grain slippage combined with a decrease in interparticle contacts. Inspired by these results, a novel criterion is proposed to characterise the transformation of granular soil from a hydromechanically stable to a fluid-like state based on the constraint ratio, representing the relative slippage between the particles and the loss of contacts between the particles within the granular mass. The constraint ratio of unity corresponds to zero effective stress, representing the critical hydromechanical state.