Study On Load-Bearing Characteristics Of Surrounding Rock Of Hydraulic Tunnels Under High Geotemperature And Hydraulic Pressure Conditions Using Coupled THMD Numerical Model

At present, with the construction of many deep long hydraulic tunnels in the west of China, the number of high geothermal geological disasters is growing. Hydraulic tunnels, under high temperature, undertake combination load of temperature stress, internal water stress and in-situ stress. The surrounding rock is in a complex stress conditions. However, the current hydraulic tunnel design specification does not contain this situation. It will cause potential safety hazard for the design and operation of hydraulic tunnels. So, this thesis does the research for safe design and operation, and obtains the following results:(1) Aiming at revealing the mechanical behavior characteristics of surrounding rock of hydraulic tunnels under high geotemperature and hydraulic pressure conditions, this paper proposes a coupled thermo-hydro-mechanical-damage (THMD) model which considers strength parameters(Modulus of elasticity E, Cohesive c、Friction Angle φ、Tensile strength σt) of hard rock to be deterioration involving damage evolution based on the multi-field coupling theory with the permeability coefficient and heat transfer coefficient varying with rock damage. The implementation of the THMD numerical simulation by the FLAC3D software is given. The coupled model is proved to be feasible by the comparative validation of a physical modeling test. Compared with other existing models, it has better applicability to predict rock mechanics behavior and bearing capacity evaluation of high pressure tunnel surrounding for high temperature hydraulic.(2) The THMD model is employed to deduce the evolution process of multi-field coupling of hydraulic tunnels under high geotemperature and hydraulic pressure. The results show that, after the water-filling operation, multi-field coupling effects are very significant in the hydraulic tunnels under high geotemperature and hydraulic pressure. Especially, the complex tensile stress generated by combined load of high geotemperature gradient and hydraulic pressure will contribute to adverse impacts on the safety performance of bearing capacity of surrounding rock. The evolution of damage, seepage and temperature field is consistent, and the damage value of cracks is larger than other places, the distribution of osmotic pressure and temperature are the same with the damage value.(3) Using the THMD coupled model to research the bearing characteristics of surrounding rock of hydraulic runnels under high geotemperature and hydraulic pressure conditions, the results show that:①the damage degree and crack depth of surrounding rock will be larger as the temperature gradient, thermal expansion coefficient, Modulus of elasticity and hydraulic pressure tend to be larger, besides, the influence of the temperature gradient and linear expansion coefficient is quite. ②If the lateral pressure coefficient tends to 1, it comes to be more cracks around the tunnel with random directions. Otherwise when the lateral pressure coefficient is less than 1/3, relatively fewer cracks appear around the tunnel and the directions of the cracks are mainly parallel to the direction of the maximum initial geostess. ③In such problem, the general bolt-shotcrete support can not effectively improve the tensile strength of the surrounding rock, so it has a lower reinforcement effect for hydraulic tunnels, we need further research to find effective support measures.