Analysis Of The Microscopic Mechanical Property Of Sandy Soils Based On The Energy Dissipation Mechanism

As a classical granular material, micro-mechanical behaviors of particles directly decide the mechanical properties of sandy soils at the macroscopic level including constitutive relationships, shear-induced localization and anisotropy, etc. After using a series of plane shearing test at quasi-static state, the present study aims to provide deep insight into the micromechanical behavior of sand particles including movement, contact consitituvie model, crushability and self-organizition, etc. Therefore, the multi-scale mechanical properties of sandy soils can be particularly studied, and the relationship was explained between the micro-mechanical and the macro-mechanical behaviors after the numerical simulations by Particle Flow Code in two dimensions (PFC D).In order to study the particle anti-rotation effects, a novel clump technique by overlapping a series of elemental disc was used to generate irregular particle shape; while a conventional rheology-type rolling resistance model was developed to simulate the particle surface texture, respectively. Meanwhile, a modified algorithm in generating crushable agglomerates was introduced to simulate the crushing behavior of particles under shearing conditions. At the macroscopic level, the mesh-free method was adopted to calculate local strain tensors in the granular system. To bridge the mechanical behaviors between microscopic and macroscopic levels, mechanisms of the energy allocation was analyzed based on the particle behaviors, which can deeply explain micro-mechanical properties, such as anti-rotation effects, crushability and self-organized structures.The results show that irregular shapes and high rolling resistance significantly enhance the shear strength due to their different anti-rotation effects. For irregular shape particles, the interlocking force highly mobilizes the inter-particle friction and limits the development of shear banding, which makes great contributions to the increasing of strength. With respect to disc particles with the rolling resistance, the anti-rotation moment remarkably increases the strain energy storage due to elastic rotation component of the inter-particle interaction in different surface textures, which will frustrate the failure and peak state under shearing. Regardless of different anti-rotation effects, different patterns of macroscopic mechanical behaviors were captured, including shear-induced localization and anisotropy.It can be noted that particle crushing exists during all the shearing process, while the crushing rate decreases with the development of shear strain. From the energy analysis, the breakage dissipation of particles takes a negligible amount of the total input energy. The storage of the elastic strain energy increases with the increasing of the friction surface ratio and the local density induced by the particle crushing, which is related to the strain hardening at critical state. On the other hand, the process of particle crushing can be explained as the transitional development from the original grading to the ultimate grading. During the process of particle crushing, the particle size distributions were noted as fractal dimensions and the fractal coefficient is valued as about1.3by considering the particle crushing criterion proposed in this thesis.Finally, self-organized behaviours of particles were found based on the topological identification of the contact network in the granular system. At the meso-scale level, the self-organized structures of particles can be defined as sub-domains in the topological theory. By introducing two structural stability indexes, i.e., λd and Aλ, it is found that3-cycle is the most stable element in the granular system, which can provide dual supports to force chain transmission, including the lateral supports and rotation frustration mechanism. After detailed investigation of the topological evolution, the development of the shear banding and shear-induce dilation can be explained as the localization of the unstable high-order structures transformed from the stable low-order structures, which implies the shearing failure of the granular system. Furthermore, the shear-induced anisotropy evolution for different n-cycle structures was studied in details. To sum up, the mechanical properties of meso-structures play a significant role of the transition from a fully discrete state at the microscopic level to a continuous state at the macroscopic level.In conclusion, the microscopic mechanical behaviors of sandy soils are the foundation for the explanation of the mechanical property at the macroscopic level. To bridge the relationship of mechanical property between these different scales, the mechanism of energy allocation can be used to analyze the development of shearing and crushing behaviors. The results can explain the fundamental mechanism of the yield criterion and constitutive models of sandy soils in the practice of geotechnical engineering.