Study Of Crack Damage And Fracture Law Of Fractured Rock Mass In Different Mining Depths

Underground rock mass will produce defects of different scales under the action of long and complex geological activities,which are divided into small-scale defects including joints,cracks and structural planes.And large-scale defects include faults,folds,weak interlayers,etc.Due to the different internal defect structures,lithology and stress environments of rock masses at different depths,the forms of fracture failure are also different.In geotechnical engineering such as roadway excavation and underground cavern construction,excavation effect often leads to unloading and instability of surrounding rock,which poses a great threat to the stability of surrounding rock and the safety of life and property of field personnel.Therefore,studying the failure and fracture mechanism of jointed rock mass has certain theoretical and engineering significance for preventing rock burst and controlling the stability of roadway surrounding rock.In this paper,the engineering background is the failure of isolated coal rock with fracture development,the law of rock burst in coal rock with different mining depths and the excavation disturbance of coal rock with different mining depths.The sandstone of different depths of roadway in hard rock mine of Bayangaole Coal Mine in Inner Mongolia is taken as the research object,and the penetrating cracks are prefabricated in the intact sandstone.The mechanical behavior,macro and micro fracture morphology and acoustic emission law of fractured sandstone with different mining depths under different loading modes were explored by means of laboratory test,theoretical analysis and numerical simulation.The main research contents and conclusions are as follows:(1)In view of the above three engineering backgrounds,the uniaxial compression test of fracture specimens,the triaxial compression test of fracture specimens with different mining depths and the unloading confining pressure test of fracture specimens with different mining depths are generalized respectively to study the fracture morphology,mechanical characteristics and acoustic emission law of fractured sandstone with different mining depths.It is found that the fracture of uniaxial compression specimen is mainly tensile wing crack.The wing crack gradually disappears with the increase of confining pressure in triaxial compression.The extension length of wing crack increases with the increase of unloading confining pressure rate.The crack initiation and peak strength of uniaxial compression wing crack specimens increase with the increase of prefabricated crack dip angle.The peak strain of 30 ° inclination is the highest,the crack initiation strain of 45 ° inclination is the highest,and the strength and deformation of 60 ° inclination are the highest.There are many stress drops in the post-peak stage of 200 m specimen under triaxial compression.The stress drops of 400 m,700 m and 900 m specimens appear in the yield stage and continue to the end of the test.Since the unloading point,the volume strain and hoop strain appear obvious strain platform.With the increase of mining depth and unloading confining pressure rate,the volume strain platform is shorter and the hoop strain platform is closer to the failure confining pressure.There are two modes of acoustic emission ringing under triaxial compression,and the acoustic emission energy is relatively close.With the increase of mining depth,the acoustic emission ringing changes from single peak to multiple peaks,and the acoustic emission energy changes from multiple peaks to single peak.The faster the unloading rate,the higher the acoustic emission ringing and energy peak.(2)Aiming at the difference of fracture morphology of fractured sandstone in uniaxial and triaxial compression,the stress function and stress field equation of crack tip are deduced based on the theory of fracture mechanics.The Γ function of uniaxial and triaxial compression is given,and the parameter analysis is carried out according to the effective shear stress of the fracture surface,which shows that the confining pressure has an inhibitory effect on the initiation and propagation of wing cracks.According to the maximum circumferential stress criterion,the relationship between the cracking angle,the initiation stress and the prefabricated crack is calculated.By comparing the experimental data with the theoretical value,it is found that the agreement is good.(3)Aiming at the fracture morphology of fractured sandstone in unloading confining pressure test,the PFC2 D particle flow numerical simulation is used to reproduce the test results.The fracture mechanism of fractured sandstone under unloading confining pressure is analyzed from the aspects of crack propagation morphology,particle displacement field,force chain distribution and meso-crack number and distribution.It is found that the macro fracture is mainly wing crack and anti-wing crack.With the increase of unloading confining pressure plastic cloth,the crack initiation position of low mining depth specimen moves to the middle of prefabricated crack.In the direction of crack propagation,the particle displacement is disorderly and the vortex distribution appears.The force chain concentrated area mostly appears in the crack propagation direction,and the force chain sparse area mostly appears near the middle of the prefabricated crack.The microscopic cracks are concentrated in the direction of crack propagation,and the microscopic tensile cracks are far more than the microscopic shear cracks.With the increase of mining depth,the total number of meso-shear cracks increases first and then decreases,and the meso-tensile cracks show a fluctuating change rule.With the increase of unloading confining pressure rate,the change law of microscopic shear and tensile cracks is the same as that of low mining depth,and the high mining depth decreases.