Study On The Mechanical Behavior And Damage Mechanism Of Granite After High Temperature Treatment

With the development of nuclear power in china,it is necessary to dispose of the high level nuclear waste(HLW).According to the law of the People's Republic of China on the prevention and control of radioactive pollution,HLW should be disposed by deep geological.Granite is potentially an excellent medium for HLW disposal,showing low native permeability and high integrity.A lot of heat was produced in the decay process of nuclear waste,which make the surrounding rock of the nuclear waste repository under the coupling of thermo-hydro-mechanical(THM),and it is complicated for damage fracture mechanism.Therefore,investigating the mechanical and permeability characteristics of thermal treated granite is significant for the safe and stable operation of HLW repository.This work was supported by the Fundamental Research Funds for the Central Universities(2015XKZD05).The research object is granite with a crystalline and blocky structure.Laboratory experiment,theoretical and numerical modeling were involved in this study.The following general results can be derived:(1)By adopting triaxial compression and cyclic loading-unloading compression tests,the effects of temperature and confining pressure on stress-strain curves,strength,deformation and failure modes can be obtained.Then,the failure processes and effects of cyclic loading-unloading compression on failure modes were analyzed,and the variation of elastic modulus,Poisson's ratio,plastic strain and dissipated energy with cyclic number were investigated.Finally,the reasonable assumption was raised to explain the variation of physical and mechanical parameters with temperature and confining pressure.The penetration test for the thermal treated specimens under different confining pressure were adopted,and the variation of permeability with temperature and confining pressure was analyzed.The relationship between seepage characteristics and micro-crack geometry characteristic was constructed,and the average aperture,fraction of connected and initial compressibility of micro-crack under different temperature and confining pressure can be obtained.Then,the variation of porosity,formation factor and thermal conductive with temperature and confining pressure can be obtained.Finally,low frequency nuclear magnetic resonance(NMR)was used to certify the observed results.The new gain based model(GBM)was constructed based on cluster model,and it was used to simulate the mechanical behavior of granite specimens under different confining pressure.Then,the micro-failure process of granite specimens were analyzed.Considering the coefficient of linear thermal expansion of different mineral in granite,the mechanical behavior of thermal treated granite under triaxial compression were simulated,and the effects of temperature and confining pressure on strength and deformation parameters were analyzed.Then,the effects of thermal-induced microcrack on failure processes for the thermal treated specimens were explored.Considering the differences between permeability characteristics of intra-granular,inter-granular and micro-crack,the new fluid-solid coupling algorithm was obtained,and then it was used to simulate the permeability of thermal treated specimens under different confining pressure.The permeability processes for thermal treated specimens were analyzed,and the effects of temperature and confining pressure on permeability were also investigated in micro-scale.