Numerical and analytical modeling of the microstructural behavior of a particulate media-structure interface

The purpose of this research is twofold. First, it is to develop numerical methods that will aid in the understanding of micromechanical interface behavior occurring in particulate media-structure systems. The second purpose is to use the newly developed numerical methods in the formulation of a micromechanics-based constitutive law for granular material-structure interfaces. A better understanding of the micromechanical features that influence interface behavior is vital to develop accurate, faithful models that incorporate the correct physical deformation mechanisms. An enhancement to the discrete element method (DEM), called “clustering,” that more naturally accounts for the effect of particle geometry can aid the understanding of microscopic particle behavior. Clustering is combining several smaller particles of simpler shapes, such as a circle, to model a single particle of more complex shape. The clusters more accurately model the geometry-dependent behavior of the particles such as particle interlock and resistance to rolling. Numerical simulations of granular media-structure interfaces were performed with a simple three-particle cluster to determine the method's effectiveness. The results indicated that the clustered particles underwent less rotation and the particle media exhibited greater shear resistance. Generally the results provided very good qualitative agreement with experimental observations of granular media. More complex clusters, comprised of as many as seven particles were created. Simulations were performed to investigate the effect of particle shape on the behavior of DEM-simulated granular materials. Correlations between increasing void ratio and increasing degree of particle roughness or angularity were found. Correlations between shape factors and shear strength were not seen. One of the most important features of using clusters is the ease with which they are able to model particle damage. A criterion is implemented wherein failure occurs when a certain level of sliding work is attained by a particle within a cluster. Numerical simulations were performed to investigate influence of declustering on a variety of particle shapes and test boundary conditions. Declustering showed that more angular particles resulted in more particle damage. Also, declustering allowed for improved numerical modeling of constant volume tests. A mechanics-based constitutive relationship was developed based upon observations of DEM simulations and experimental results of video-imaging of the interface. The relationship is based on a two-dimensional contact problem assuming non-associative plasticity. The model assumes that “stacks” of particles rotate rigidly as shearing takes place. A coefficient that relates the macroscopic normal stress to the horizontal stress in the interface region is introduced. A comparison of the predictions of the new constitutive law is made with the results of a physical test of photoelastic discs and a discrete element simulation of the physical test. Results from the analytical method agreed very well with the results from the physical and numerical methods, both qualitatively and quantitatively.