Study On Mechanical Model And Fluid-Solid Coupling Mechanism For Brittle Rocks

As the geotechnical engineering around the world becames deeper, where the geological and stress environment has become more complex, the study of rock mechanics under the condition of high stress and high seepage pressure has been put on the agenda. In particular, how to simulate the mechanical response of rock has been one of the key issues. The failure of surrounding rock mass under high geo-stress and the occurrence of water burst under high seepage pressure are the two main problems met during the construction of diversion tunnel in Jinping II hydroelectric station. In order to solve these problems, the mechanical model and fluid-solid coupling mechanism under high seepage pressure of marble are both studied here. In sum, the main work and conclusion including:1. In order to study the elato-plastic character, subsequent yield character and dilation of brittle rocks, cyclic-loading experiments, both before and after the peak strength of two kinds of marble (T2y6 and T2b) and a kind of sandstone, are carried out. A plastic internal variable is defined, which takes the influence of confining pressure on yield process into consideration. Based on the results of cyclic-loading experiments and the plastic internal variable, the evolution process of elastic, strength and dilation parameters with plastic strain is studied.2. The relation of strength under unloading and loading conditions is studied with respect to the rate of loading. Yield approach rate (YAR) is proposed to denote the stress varying rate under loading and unloading conditions. Then, the characters of YAR under loading and unloading conditions are analyzed. The simulation by EPCA2D (Elasto-Plastic Cellular Automaton) and the unloading tests of marble indicate that if the initial unloading stress is in the elastic state, the strength under loading and unloading conditions is equal when the unloading rate is 0.2-0.3 of the loading rate.3. Based on the results of experiments on two kinds of marble (T2y6 and T2b) and a kind of sandstone, an elasto-plastic coupling mechanical model is proposed. The model takes the variety of elastic parameters, strength parameters and dilation parameter into consideration and can reflect the influence of stress state on the yield process. Simulation results of the traxial compression experiments and excavation process in Mine-by tunnel show that the mechanical model proposed here can capture the deformation and yield characters of brittle rock under different stress path. Especially, the deformation and yield characters in confining pressure unloading tests are also well described by the model.4. Combined with the elasto-plastic coupling mechanical model, the basic frame of fluid-solid coupling analyze under plastic condition is established. As different researchers disputed on the continuum equation of fluid, two methods are used to derive the continuum equation. The continuum equations deduced form the two methods are found to be equivalent. In addition, the equations are compared to the continuum equation derived by other researchers based on mass conservation law, and they are found to be coincident. Then, the right form of fluid continuum equation is proved.5. Physical mechanism and influencing factors on the effective stress coefficient for rock/soil-like porous materials are investigated, based on which equivalent connectivity index is proposed to denote the meso-scaled structure of these materials. Then a general expression describing the relation between effective stress coefficient tensor and equivalent connectivity tensor of pore is proposed, and the expression can be applied to isotropic media and also to anisotropic materials. Based on the effective stress tests of coal and sandstone under elastic deformation, and the effective stress tests of marbles and sandstone under plastic deformation, the evolution law of effective stress coefficient with strain is obtained. The simulation results agree very well with the test results.Unified equation of Darcy flow and non-Darcy flow is deduced. Then, the criterion of non-Darcy flow is put forward based on the contrast relationship of bond shear stress in laminar flow and additional shear stress in turbulent flow.6. Finally, with intelligent inversion theory, the elato-plastic coupling constitute model is used to simulate the excavation process of the 2# experiment tunnel in the assisted tunnel of Jinping II diversion tunnel. Evolution process of deformation, disturbed zone and stress are got according to the excavation step. The position and extent of failure zone is in good agreement with the field.Using the fluid-solid coupling method proposed here, the excavation process of the Jinpingâ…¡diversion tunnel is simulated which takes the high fluid pressure into consideration. The result shows that the high fluid and seepage pressure has effect on the stability of the wall rock. Based on the simulation results, suggestions are raised for the prevention and management of water burst.