Study On The Mechanical And Shear-Flow Characteristics Of Fractured Rock Masses After High Temperature Treatment

The influence of temperature on the physical,mechanical and microscopic characteristics of granite was investigated by means of laboratory test and numerical simulation.On the basis,the evolution laws of seepage field of fractured rock masses during shear after high temperature treatment were analyzed.The research findings provide valuable references for potentiation of hydro-shearing in the engineered geothermal systems,improving the permeability of geothermal reservoir and promoting the exploitation of deep geothermal resources.The main contents and findings are as follows.The uniaxial compression test,variable-angle shear test,acoustic emission signal monitoring and the measurement of physical parameters including mass,size and Pwave velocity were carried out on granite samples treated by temperatures ranging from25 ? to 900 ?.The results show that the mechanical behavior of granite weakens and the failure mode of granites gradually transits from brittleness to ductility with the increasing temperature.Concretly,as the temperature increases,the uniaxial compressive strength,elasticity modulus,shearing strength and cohesion exhibit decreasing trends,whereas peak strain and frictional angle gradually increase.Specifically,the mechanical properties of granites have a significant variation in the range of 500 ? ? 600 ? because the quartz changes from an ?-phase crystal lattice structure to a ?-phase crystal lattice structure and the fracture structure rapidly develops.The maximum count of acoustic emission decreases whlie the duration increases with an increasing temperature.In addition,the waters of granite escape,the volumes expand and porosity increases resulting in the decrements of density and P-wave velocity as temperature increases.To investigate the relationship between the multiscale fracture structures of granites and temperature,the scanning electron microscope(SEM),computerized tomography(CT)and mercury injection porosimetry(MIP)were used to analyze the fracture structure of granites after thermal treatment.From the SEM images,it is obviously known that high temperature induces generation and propagation of the micro-cracks and the aperture of fractures increases with an increasing temperature.The CT images exhibit that the micro-cracks gradually develop and form a network.As temperature increases,the fracture network becomes more irregular and more homogeneous.From the results of MIP,the porosity of granite increases and fracture structure gradually becomes homogeneous with the increment of temperature.The 3D topography of fracture surfaces is obtained by splitting test and 3DSS scanning system.Based on the parameters from mechanical experiments and Indraratna model,the shear sliding of rock fractures was simulated.Then,the evolution laws of seepage field of fractured rocks during shearing under high temperature condition were obtained using ANSYS FLUENT software.The results show that the JRC of fracture surface increases with an increasing temperature.High temperature induces the increment of fracture aperture and the decrement of contact area.Conversely,the fracture aperture decreases and contact area increases with an increasing shear displacement.As temperature rises,the rate of flow and equivalent seepage coefficient of fractured rocks decrease and critical hydraulic gradient increases.On the contrary,the rate of flow and equivalent seepage coefficient increase and critical hydraulic gradient decreases as shear displacement increases.In terms of the summary of physical,mechanical,microcosmic and shear-flow characteristics of the granites after high-temperature exposure,it is known that high temperature induces the waters of granites escaping and fracture structure developing,leading to the weakened mechanical properties and decreased permeability of fractured rocks during shearing.Furthermore,500 ??600 ? is a critical temperature range where all the characteristics of granites suddenly change.There are 74 figures,18 tables and 129 references in this paper.