Pore-scale Modeling On Hydro-mechanical Coupling Effects Of Geotechnical Materials

The interaction between groundwater and deep rock masses is quite common in the projects of large-scale resources exploitation, water conservancy project construction, carbon sequestration and radioactive waste underground storage at present. A groundwater level fluctuation in deep rock masses may cause a flow of pore water and fissure water, which subsequently results in a stress redistribution in deep rock masses. An excess pore water pressure is easy to generate when saturated rock and earth mass suffer exterior loads.The excess pore water pressure is a serious threat to the security and stability of geotechnical engineering. The hydro-mechanical coupling has always been an important subject in geotechnical engineering. However, understanding this problem only from a single macro-scale already cannot satisfy the demands of engineering practice and basic science research.In this study, we first discuss the characterization method of particle shape from meso-level. A shape factor, sphericity is defined and the relation between the shape parameter and the macroscopic mechanical properties of the rock is illustrated. For the bonding particle model in discrete element method, a porosity calculation method considering the bond thickness is proposed. The fluid coupled method in PFC is developed. A reseach on crack propagation induced by fluid injection is performed using this developed fluid coupled method. And then a particles-fluid coupled method considering pore fluid pressure is put forward. This method is used to simulate the triaxial drained and undrained compression tests of saturated soil. The main findings of the research are:(1) A shape factor, sphericity is defined to quantify the particle shape. Sphericity is the ratio of surface area of the sphere which has same volume as the particle and particle surface area. The sphericity is an applicable shape factor to measure particles shapes. The sphericity describes the proximity of a particle to a sphere. It directly influences the interlocking ability of particles. How the particle shape affects the macroscopic mechanical properties of rock is explained using 4 kind of representative particles in numerical simulations. The results show that the crack initiation stress, crack damage stress, and peak stress of rock are affected by particle shapes. Specifically, three stress indices decrease with increasing of sphericity. The increasing sphericity also leads to smaller elastic modulus and larger poisson ratio. The decreasing sphericity causes particle interlocking of different degrees which restrains slip and rotation. Consequently, cohesion and internal friction angle rise.(2) A mechanical servo module is introduced into the fixed coarse-grid fluid flow scheme in PFC to model the triaxial seepage coupling test. The variation of permeability in the complete stress-strain procedure is studied. Considering the difference of porosity between the discrete element simulation and the real rock sample, we put forward a new porosity calculation method. This method considers the bond thickness and shows the consistent porosity with real rock sample. Considering the bonding failure in the process of crack propagation, the relationship between the porosity and crack number is established. After obtaining the porosity in the complete stress-strain procedure, we deduced the variation of permeability using Kozeny-Carman equation. The results show that the permeability curve deduced by new porosity calculation method agrees well with the curve from other scholars'experiments.(3) The fluid coupled method in PFC is improved by applying the fluid shear stress to particles and considering the volume change of fluid network. The crack propagation process induced by fluid injection is studied by a double injection holes sample. We illustrate the effect of injection hole spacing on crack pattern, crack orientation, topology of fluid network and topology of crack network. The shortest flow path between two injection holes is predicted using the Dijkstra algorithm. When two injection holes get close, the interaction of micro-crack is stronger and the connectivity of crack network is poorer. The cracks starting from two injection holes tend to direct coalescence and the sample shows more micro-cracks when the spacing of injection hole is small. While the spacing of injection hole increases, the sample shows less micro-cracks and the connectivity of crack network is better. The direction of the crack is basically perpendicular to the compression force chain. The tension force chain is concentrated on the zone of crack tip.(4) A particle-fluid coupled discrete element model is proposed based on the previous research findings. The excess water pressure induced by stress and the water pressure acting on particles by fluid is derived in detail. The model considers the mechanism of pore pressure generation and volume change induced by deformation. The size of measurement sphere is optimized to improve the computational efficiency.(5) A numerical scheme for the particles-fluid coupled discrete element model is achieved based on three-dimensional particle flow code. With the aid of undrained triaxial compression test for Beijing clayey silt, key model parameters are calibrated. Deviatoric stress and pore pressure from coupled method are compared with that of constant method and experiments. It turns out that results calculated by fluid coupled-DEM method are consistent with that by constant volume methods and experiments. A series of numerical tests for drained and undrained triaxial compression tests of saturated soil under different confining pressures are conducted using the calibrated model parameters. It is clearly seen that deviatoric stress increases with increase of confining pressure. In undrained test, the results show that stress ratio decreases with increasing confining pressure, while the pore pressure increases with rise of confining pressure.