Study On The Failure Law And Acoustic Emission Characteristics Of Columnar Jointed Rock Mass Under Triaxial Unloading

In the process of engineering construction in the area with columnar jointed rock mass,the excavation of the rock mass will cause the stress release of the columnar jointed rock mass at the working face.This makes the internal cracks of the rock mass expand and penetrate,which reduces the bearing capacity of the rock mass,and is prone to relaxation and cracking,resulting in collapse events and threatening construction safety.Earlier studies on columnar-jointed rock mass were mostly studies on mechanical properties under loading conditions,and few studies on its mechanical properties under unloading conditions.In this paper,columnar jointed rock mass specimens with different joint inclination angles are fabricated by rock-like materials.Then,the triaxial unloading confining pressure test under different initial confining pressure and unloading confining pressure rate was carried out to study its mechanical characteristics,energy evolution characteristics and acoustic emission characteristics under different conditions.Concluded as follow:(1)Based on the laboratory test results of the columnar jointed rock mass specimens,the mechanical characteristics,deformation characteristics and failure modes of the columnar jointed rock mass under the condition of unloading confining pressure are analyzed.The results show that with the increase of the initial confining pressure,the peak strength and elastic modulus of the columnar jointed rock mass gradually increase.With the increase of the inclination angle of the joint,the peak strength and elastic modulus of the specimen both show a "U" shape that first decreases and then increases.The higher the unloading confining pressure rate,the lower the bearing capacity of the columnar jointed rock mass specimen,and the failure mode of the specimen is closer to the failure characteristics of the uniaxial compression test.The Mohr-Coulomb strength criterion,the Mogi-Coulomb strength criterion and the Hoek-Brown strength criterion can all describe the relationship between the principal stress of the columnar-jointed rock mass specimens,and the Mohr-Coulomb strength criterion has the best fitting effect.(2)According to the principle of energy calculation,explore the energy evolution law of columnar jointed rock mass under different conditions,the results show: With the increase of the initial confining pressure,the energy values of the columnar jointed rock specimens show an increasing trend when the peak strength is reached,the proportion of elastic energy gradually decreases,and the proportion of dissipative energy gradually increases.With the increase of the joint inclination angle,the BIM value of the columnar jointed rock mass specimen increases first and then decreases with the increase of the joint inclination angle,it shows that the brittle characteristics of the specimens are more obvious under the condition of0° and 90° joint inclination angle.With the increase of the confining pressure unloading rate,the energy values in the specimen all showed a decreasing trend,and the elastic energy peak appeared earlier.(3)Based on the evolution characteristics of acoustic emission in the triaxial unloading confining pressure test of columnar jointed rock mass under different conditions,the analysis results are as follows: With the increase of the initial confining pressure of unloading,the maximum value of the AE ringing count rate and the maximum value of the AE amplitude of the columnar jointed rock mass specimen both show an increasing trend,and the hysteresis effect of the maximum value appears more obvious.With the increase of the joint inclination angle,the AE ringing count rate and the maximum AE amplitude show a "U"-shaped trend that first decreases and then increases.With the increase of the unloading confining pressure rate,the maximum value of the acoustic emission ringing count rate of the columnar jointed rock mass specimen during the test shows an increasing trend,and the maximum value appears earlier.Taking the Hurst index reaching 0.8 as the precursory characteristic of the failure of the specimen,the precursory prediction of the instability failure of the columnar-jointed rock mass under the condition of triaxial confining pressure relief is realized.