Experimental Study On Fracture Behavior And Destabilization Precursor Determination Of Fissured Rock

With the development of mineral resources and infrastructure constructions,the breakdown instability of engineering rock mass has been become more and more outstanding which often threatens the human life and property safety.Therefore,to understand and control the phenomena of fracture and instability of engineering rock mass is one of the most concerned problems for scholars at home and abroad.In this paper,we comprehensively used laboratory test,theoretical analysis and numerical calculation and other methods.The basic mechanical characteristics and avalanche dynamic activities were systematically studied and revealed the deformation and failure mechanism of fractured rock.In addition,the applications of mechanical parameters,crack propagation characteristics and avalanche activity laws in the precursory information of overall instability and failure of fractured rock were analyzed in detail.The main achievements were as follows:(1)The strength deformation characteristics and brittleness characterization method of fractured rock were obtained.Based on the influence factors of engineering rock stability,the influence laws of fracture geometry parameters,fracture filling condition,freeze-thaw condition,loading rate and confining pressure on mechanical behavior of fractured rock were revealed.According to the characteristics of pre-peak energy dissipation and the shape of post-peak stress-strain curve,a quantitative evaluation model of brittleness characteristics was considering with dissipated strain energy and total input strain energy of pre-peak,the strength and the strain of peak,as well as the residual strength and strain.The reliability of the model was verified by the compression experimental data.(2)The mechanisms of crack initiation and propagation in the deformation and fracture process of fractured rock were revealed.Using CT scanning and 3D reconstruction technology to obtain the influence laws of the fracture filling condition,fracture dip angle and confining pressure on the final fracture mode of fractured rock.And with the high-speed photography technology,the evolution process of crack propagation in fractured rock was described.The maximum circumferential stress criterion and radial shear stress criterion were modified by introducing the three components of T-stress in nonsingular terms and the shearing transmitting factor.It had found that the initiation mechanisms of wing crack and oblique secondary crack were related to the fissure dip angle,fissure length,critical distance of crack tip,friction coefficient of crack surface,shear stiffness on the crack surface,elastic modulus and Poisson's ratio of rock material,and other related.Combined with the experimental results of uniaxial and triaxial compression,the theoretical and experimental values of crack initiation angle were compared thoroughly which proved the rationality of the crack initiation mechanical model.Through the parameter sensitivity analysis,it was clear that the critical size of crack tip and the friction coefficient of crack surface had the most significant influence on the initiation angle of wing crack.(3)The avalanche mechanism of micro fracture dynamic activity in fractured rock was displayed.On the basis of AE energy release process of fractured rock which obtained the evolution law of AE signal parameters.It was clarified that the compression failure of small-scale rock materials follows the laws such as Gutenberg-Richter law,Omori-Utsu's law,Productivity law and unified scaling law,which proved that avalanches have similarities on different scales and satisfy the scale-free power-law distribution in time.The influence laws of the existence of the fissure,fissure dip angle,filling condition of the fissure and confining pressure on the power-law exponent of acoustic emission energy were revealed,and the effect degree of fissure dip angle on aftershock frequency attenuation coefficient,Productivity index and characteristics of waiting time distribution was clarified.A quantitative characterization method for acoustic emission activity level was proposed,and the relationship between crack propagation mode and statistical characteristics of acoustic emission energy had been established.(4)The methods to determine the precursor information of fracture instability of fractured rock were proposed.The distribution characteristics of stress drop were obtained by using statistical theory of probability density,and the energy damage evolution mechanisms of fractured rock were revealed.From the perspective of the crack propagation characteristics and the change laws of mechanical parameters,it was considered that the phenomenon of stress drop and sudden increase of dissipated strain energy could be regarded as precursor information of overall failure of fractured rock.Starting from Gutenberg-Richter law,a magnitude index of acoustic emission activity based on the logarithm of acoustic emission energy was proposed,the evolution processes of b value of acoustic emission and the change characteristics of power law exponent of acoustic emission energy were clarified.From the perspective of avalanche effects of acoustic emission,it was pointed out that significant decrease of b value of acoustic emission and decrease of power-law exponent of acoustic emission energy could be used as precursor information of overall failure of fractured rock.