| Geotechnical materials, such as soil, sand, concrete, slurry, and synthetical geotechnical materials, are encountered widely in nature and engineering. All of them can be regarded as a collection of discrete particles. The mechanical behavior of granular materials is dependent on the particulate properties including size, shape, physical characteristic and distribution of particles.The mechanical properties of geo-materials are discrete and flowing. Continuum theory is known to be inefficient in dealing with granular materials due to its disability to characterize the dispersing of the physical properties. However, DEM can avoid the limitations and plays an important role in the numerical modelling of geotechnical engineering. It can be used to analyze the macro-mechanical behavior by investigating the microscopic parameters of the particle structure and the correlation between the micro- and macro- quantities can be obtained. Compared with physical experiments, the advantage of DEM is that different specimens with random particle size distribution, diverse particle shapes, surface characteristics can be generated easily using DEM.The emphases of this thesis focus on three aspects. 1) A theoretical model for critical state in terms of the valence (number of edges per void cell) is presented based on the DEM analysis of the microscopic shear dilatancy. 2) Three contact states in unsaturated soil are considered and the interparticle adhesive force induced by capillary suction is incorporated into the contact model. 3) According to the strain localization phenomena observed in granular materials, the regularize mechanism is introduced to the macroscopic description of porous media. The material stability and the prediction of the internal length scale are studied. Specific details are as follows:A series of biaxial compression test are carried out to investigate the effects of particle shape, surface characteristic, consolidation stress and initial void ratio on the micro- and macro-mechanical responses. The transition of the internal contact force is reproduced. The localization of slip deformation in dry granular materials and the evolution of vortex structure are presented.Based on the dilatancy of granular materials, a new micromechanical model for the critical state analysis of granular materials from the viewpoint of valence is presented and the relationship between the average valence and the plastic strain is obtained. According to the numerical results from DEM simulations, it can be concluded that the parameters in the theoretical model, including the critical valence and the plastic shear strain, are dependent on the particle shape, surface friction, the consolidation stress and the initial void ratio of the granular assembly.All the possible interactions between neighboring particles in unsaturated soil, which is composed of solid, liquid, gas phases and meniscus, are analyzed. The interpartical adhesive force induced by the capillary bridge is integrated into a discrete element contact model through the appropriate solution of the Laplace-Young equation. It is found from the numerical results that the capillary suction enhances the strength of the unsaturated soil, improves the internal stability of granular materials and reduces the sliding contacts during deformation.To deal with the strain localization problem of unsaturated soil, the gradient dependent model and rate dependent model are introduced in the continuum frame. In the continuum frame, the granular materials are regarded as porous media. When the strain localization occurs, the solution of the governing equation is ill-posed and the regularized constitutive model is necessary to be developed. The detailed discussion about the stability problem and the internal length scale are carried out. The wave number domain with real wave speed in K-k plane is given to explain the reason of mash dependence of the numerical resultsAt the end of the dissertation, the main conclusions are summarized and the futher work is suggested. |
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