| Soil is a multiphase geomaterial that consists of mineral particles, pore fluid and gas. The complex microstructures and mesofabrics of soil that are formed by soil particles at different scales, and the interactions between soil components with different phase cause different physical mechanism and mechanical response of soil to appear at various scales, which presents dramatic hierarchical coupling and trans-scale evolving effect of the deformation and strength properties of soil. In the present work, both theoretical analysis and experimental study are used to investigate the multi-scale coupling mechanical properties of soil. The theoretical analysis mainly elaborates the multiformity and coupling of the mechanical response of soil at different scales and establishes a hierarchical and multi-scale constitutive model to reproduce and predict the multi-scale coupling mechanical properties of soil; while the experimental study mainly relates the soil particle characteristics to the shear strength of soil and calculates the micro and meso-model parameters quantitatively. Based on the forementioned work, the feasibility and validity of the proposed model are evaluated, and the physical meanings of the model parameters are analyzed and discussed. Whereafter, the particle size effect of soil is interpreted in detail by the proposed multi-scale research framework, combining with the conventional strength theory of soil.The main research contents and conclusions are as followed:(1) According to the interactions between various forces generated by soil particles at different scales, a new concept, defined as the ratio between micro-forces and gravity(RMG), is proposed for the division of soil particle category. Micro-forces, such as van der Waals and Coulomb forces, are calculated quantitatively. The relationship between these forces and gravity are established, which indicates that, when the particle size is less than 10 micrometers, the interaction between micro-forces that mainly reflects the cohesive effect is dramatic, as the particle size increases, the gravity impact that mainly reflects the frictional effect emerges. Therefore, the RMG is capable of providing physically sound evidence for the classification of soil particles.(2) Based on the physical effects of cohesion and friction generated by the interaction between soil particles at various scales, a soil cell element that is capable of reflecting the particle characteristics of soil is constructed, and a soil cell element model that is capable of reproducing and predicting the multi-scale coupling mechanical properties of soil is established by using the density of coordinated micro-cracks and strain gradient to describe the micro and mesostructures and using the energy balance principle and geometric deformation compatibility conditions to relate the physical mechanisms and mechanical responses of soil at different scales. The feasibility and validity of the soil cell element model are preliminarily verified by comparing the soil yield stress calculated by the proposed model in concert with the experiment results. Based on the soil cell element model, a hierarchical and multi-scale incremental constitutive model is derived and is then used to analyze the plastic strain distribution in an unconfined compression soil sample. The results show that, the proposed model is capable of describing the particle size effect and multi-scale coupling mechanism of the mechanical properties of soil.(3) A series of soil cell element samples with different reinforcement particle combinations and matrix liquidity index are prepared for direct shear tests and triaxial compression tests. The results indicate that, when the reinforcement particle volume fraction is less than 0.271, the soil shear strength in triaxial compression tests increases as the reinforcement particle volume fraction increases and decreases as the size of these particles increases, while the shear strength in direct shear tests also increases as the reinforcement particle volume fraction increases but keeps unchanged as the size of these particles varies; when the reinforcement particle volume fraction is larger than 0.318, the soil shear strength in both direct shear tests and triaxial compression tests remains unchanged as the reinforcement particle volume fraction varies, and a critical volume fraction of these particles appears. The results of theoretical and experimental study on the soil cell element model indicate that, the phase transformation of the meso soil fabric caused by the increase of the reinforcement particle volume fraction is the root cause that leads to the critical volume fraction of the reinforcement particle.(4) According to the results of theoretical and experimental study on the soil cell element model, an interpretation of the physical mechanism of the particle size effect of soil is to be drawn: the incompatible deformation between the matrix and reinforcement particle causes the coordinated micro-cracks and strain gradient to appear at the micro and meso-scale,respectively. On this regard, the density of coordinated micro-cracks and strain gradient increase the dissipation or storage of strain energy in a unit volume of soil, which enhances the deformation ability of soil and results in higher shear strength at the macro-scale.(5) On the basis of the soil cell element model, a soil yield stress formula that is capable of reflecting the particle size effect of soil is derived by taking the rotation gradient of the soil cell element into consideration, and a multi-scale Mohr-Coulomb(MSMC) strength criterion is established by incorporating the conventional soil mechanics. According to the experiment results, the yield surfaces of the MSMC strength criterion and its conventional counterpart are obtained and analyzed. The results show that, the MSMC strength criterion can reproduce and predict the particle size effect and multi-scale coupling mechanism of the mechanical properties of soil, and can conveniently be combined with the conventional soil mechanics. |
PDF Full Text Request