Three Dimensional Discrete Element Simulations Of Mechanical Behavior And Critical State Failure Of Granular Media

As a typical granular medium, sand processes inherent anisotropy and thus exhibits complex mechanical responses when subjected to various external loadings. The research on the mechanical characteristics and critical state of sand has always been an important subject in soil mechanics profession. The macroscopic behavior of sand is largely influenced by its internal structure which is referred to how the particles are arranged and packed in the space, while the internal structure will continuously evolve as the loading proceeds. Therefore, to understand the mechanical behavior of granular materials and then perform micromechanical mechanism based constitutive modeling, the microscopic analysis of granular materials is a premise and of great importance.The research described in this dissertation is aimed to investigate the evolution of internal structure of sand under triaxial stress paths and examine the macroscopic and microscopic mechanical characteristics of sand at the critical state. A series of triaxial compression and extension tests were carried out on the granular assemblies using three dimensional discrete element method. The numerical samples were generated using two different methods, namely the expansion method and deposition method, which will result in the samples with different initial fabrics. The fabric tensor was used to quantify the anisotropy of the granular soil and the influences of densities, confining pressures, initial fabric anisotropy, loading mode and drainage condition are investigated on the mechanical behavior and critical state. The existence and uniqueness of critical state line are also presented.The main conclusions resulting from this study are as follows:1) All the samples may reach the critical state when they are sheared to large strains with sa>60%, which is not feasible in most physical tests in laboratory Both the densities and confining pressure have significant influence on the behavior of the samples, while the critical state is hardly affected, under otherwise identical conditions;2) The initial fabric anisotropy induced by the sample preparation methods have a great impact on the dilatancy of granular soil, which becomes more pronounced when the samples are more anisotropic or the direction of fabric anisotropy and loading direction are coincided. The initial fabric affects the mechanical behavior of sand in the very beginning of shearing, but this influence diminishes gradually as the shear continues and vanishes completely at the critical state;3) The critical fabric tensor of sand has a unique measure, whose direction is consistent with the loading direction and its norm is loading mode dependent;4) The loading conditions also have influence on the macroscopic mechanical behaviors and microscopic fabric evolution of samples. The critical fabric of samples in the triaxial compression and extension tests have reciprocal relationship to their corresponding critical stress ratio;5) The outcome of the study proves a unique critical state line in the void ratio-mean stress space regardless of the sample’s densities, confining pressures, initial fabric anisotropy, loading mode and drainage conditions, which is consistant with the recent development of anisotropic critical state theory.