Natural soils are often modelled as a continuum characterized by the composition of the soil, a particulate material. Yet, in situ, the fabric and structure of soil may govern its behavior. Discrete element modelling is used to simulate the composition of soil as a particulate material and develop fabric quantities. These quantities are presented as average quantities for a volume of particles. It is possible to use DEM to study the evolution of fabric at the particle level. This paper describes a state-of-the-art fabric term, referred to as geometrical stability index, ʎ, which can measure the contacts deviation of each particle from the most stable contacts arrangement during loading. The parameters required to define this new fabric term were attained from a designed algorithm. 2D discrete element method (DEM) biaxial test simulations were performed to validate the effectiveness of the geometrical stability index in defining the local instability. As the sample is loaded, a shear band is formed. The geometric stability index in that band increases relative to the surrounding relatively intact soil. Thus, a brittle failure is associated with an increase in the variation of inter-particle contacts from a stable configuration. The geometric stability index is able to model the development of discontinuities in a particulate material at the particle level. The DEM modelling results demonstrate the correlations between the new fabric term and the progressive of localized failure in densified particulate systems such as over consolidated clay, where the failure is a function of progressive development of local fissure spacing.
Sand is a particulate material but is treated as a continuum solid in some engineering analyses. This approach is proven to be acceptable when dealing with geotechnical structures, provided an adequate factor of safety is applied so that there is no risk of failure. However, the continuum approach does not account for the effect of interparticle forces on the micro–macro behaviour of sand. Sand could be modelled as a particulate material using the discrete element method (DEM), taking into account its discrete nature. This paper shows how the microscopic contact properties between the idealised sand particles influence the macro-mechanical behaviour, highlighting the development of the fabric as the soil approaches failure. Thirty DEM biaxial tests were performed to study the sensitivity of the macro–micro mechanical properties of sand to the inter-particle properties of an idealised sand particle. The conditions of these simulations were the same (e.g., particle size distribution, number of particles, porosity after radius enlargement, boundary conditions, and rate of loading). The sensitivity of the pre-peak, peak, and post-peak behaviour of these simulations to the inter-particle properties of an idealised sand particle was studied. Two extra DEM biaxial tests under different confining pressures were performed to verify the cohesionless nature of the synthetic material used for this study. Since a two-dimensional DEM is used for this study, a detailed approach to interpret the results assuming either a plane strain or a plane stress situation was discussed. This study highlighted the critical inter-particle properties and the range over which these influence macro-mechanical behaviour. The results show that Young’s modulus is mainly dependent on the normal contact stiffness, and peak stress and the angle of internal friction are greatly dependent on the inter-particle coefficient of friction, while Poisson’s ratio and volumetric behaviour of particulate sand are dictated mainly by shear contact stiffness. A set of relationships were established between inter-particle properties and macro-machinal parameters such as Young’s modulus, Poisson’s ratio, and angle of internal friction. The elastoplastic parameters obtained from these tests are qualitatively in agreement with the typical medium and dense sand behaviour.
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