Experimental Study On High Temperature Mechanical Properties Of Beishan Granite

Deep-buried geological disposal is a complex deep underground project being studied by existing nuclear countries.The construction of high-level radioactive waste repository needs to ensure its long-term stability and safety within tens of thousands of years or even longer service life.The heat generated by the decay of high-level radioactive waste in the seal layer will significantly affect the mechanical properties of engineering rock mass,therefore,in-depth study of rock strength and deformation characteristics under high temperature is the basis of deep underground engineering construction and stability evaluation.Deep rock mass is affected by excavation,its mechanical properties are not only affected by temperature,but also closely related to the stress state and fragmentation degree of rock mass.At present,the analysis method of thermal damage rock is not comprehensive enough,and the influence of many factors is not considered comprehensively.In view of this,this paper takes Beishan granite as the research object,carries out compression failure and acoustic emission tests of thermal damaged rock samples under different sizes and confining pressures,studies the size effect of thermal damaged rock and the evolution law of strength deformation parameters,and constructs three different constitutive models to reflect the deformation and failure characteristics of thermal damaged rock,so as to provide help for subsequent engineering construction.The main research results are as follows:(1)Aiming at whether the size effect of thermal damaged rock is different from that of traditional size model,through uniaxial compression tests of Beishan granite with different aspect ratios at different real-time high temperatures,the effects of thermal damage and size effect on strength and deformation parameters,acoustic emission characteristics and fracture characteristics of specimens under uniaxial stress are analyzed,and the strength attenuation considering temperature and size effect is constructed,and its damage evolution characteristics are revealed.The results show that thermal damage and sample size have significant effects on the mechanical properties of Beishan granite.The critical brittle-ductile transition temperature of rock increases with the increase of aspect ratio,and the critical slenderness ratio corresponding to the typical material properties tends to be smaller with the increasing temperature.(2)Considering the influence of temperature,confining pressure and plastic development history on the deformation characteristics of deep surrounding rock during the construction of repository,the macro-physical and mechanical properties of thermal damaged Beishan granite samples and the acoustic emission characteristics during the failure process were studied by conducting conventional triaxial compression tests under different confining pressures.Based on the stress-strain curve,the evolution models of cohesion,internal friction angle and dilatancy angle of rock after damage and yield are established,which take into account the effects of temperature,confining pressure or plastic shear strain.The results show that the deterioration degree of rock increases with the increase of heat treatment temperature after T=200℃,and the existence of confining pressure effect inhibits the crack propagation.The increase of temperature and confining pressure will enhance the plastic deformation ability of rock.With the increase of confining pressure to a certain value,the increase range becomes smaller and smaller.Based on Mohr-Coulomb criterion,the evolution model constructed by MATLAB tool has high fitting accuracy,which can better describe the evolution law of strength and deformation parameters of thermal damaged rock after damage and yield.(3)In view of the sensitivity of acoustic emission monitoring technology to crack development,a damage constitutive model based on acoustic emission cumulative ringing counts is established by using acoustic emission time series parameters.Considering the obvious influence of residual strength on the characteristics of stress-strain curves,a statistical damage model considering residual strength is proposed based on Weibull function and D-P criterion.Based on the elastic-plastic coupling characteristics and strain softening characteristics of Beishan granite after damage yield point,a strain softening model of Beishan granite with thermal damage considering elastic-plastic coupling is proposed.The results show that the model curve fits well with the test results.(4)Based on Visual Studio 2010 software development platform,the thermo-elastic-plastic coupled strain softening model is compiled into.DLL file,and embedded into FLAC3 D 5.0.The correctness of the model is verified by simulation triaxial compression test,and the rationality of the self-defined constitutive model is verified by simulation of circular tunnel excavation.