Comparative Study Of Nonlinear Flow Characteristics In Shear And Tensile Fractures Of Granite Considering The Evolution Of Contact State

Shear and tensile failure are two typical forms of rock failure,which are very common in geothermal mining,tunnel excavation and other engineering activities.Due to the different mechanism,there are significant differences on the mechanical properties and flow characteristics between shear and tensile fractures.Therefore,this paper carried out a nitrogen gas flow test for the shear and tensile fractures of granite under effective confining stress varying from 1 to 40 MPa.On the one hand,this paper compares and analyzes the nonlinear relationship between volumetric flow and stress gradient in the fracture.The Forchheimer equation is used to fit the experimental data.The linear coefficient and the nonlinear coefficient are analyzed with the effective confining stress.The Reynolds number and the friction factor were further calculated to compare the flow patterns in the shear and tensile fractures.On the other hand,the fracture surface scanning data was imported into the steady-state seepage simulation program,and with combination of the experimental data,the evolution process of the fracture contact state during loading process was calculated.The evolution of the contact area,the void volume and the lacunarity with the effective confining stress was calculated.Then the morphology of shear and tensile fractures were compared and analyzed.Finally,the evolution of the contact state and its influence on the flow inside the fracture was investigated by introducing the lacunarity into the friction factor model,and by this way the nonlinear equation between the volume flow and stress gradient was better developed by importing into the lacunarity.In total the following conclusions were drawn in this dissertation.1)Through the experimental study of gas flow under different stress and pressure gradients,the seepage law of the tensile and shear fractures of the granite,the critical Reynolds number and the nonlinear flow friction characteristics are obtained.The experimental results show that the fluid flow in shear and tensile fracturs shows obvious nonlinearity,and the conductivity of shear fractures is obviously stronger than that of tensile fractures.2)Combining the experimental results and numerical simulation,the evolution rules of the fracture parameters such as contact area,void volume and the lacunarity of granite are obtained,and the influence mechanism of different seam-forming mechanisms on the seepage characteristics of farctures is revealed.The results show that the normal displacement and contact area increase with the increase of effective confining pressure,and the void volume decreases with the increase of effective confining pressure.The lacunarity decreases with the increase of effective confining pressure.3)The nonlinear flow model of granite fracture considering the stress loading and th evolution of contact state is established and applied to the numerical simulation of engineering scale.The effects of different seam forming mechanisms on the geothermal drainage flow of dry hot rock are compared and analyzed.The simulation results show that the yield of shear fractures is significantly higher than that of tensile fractures.