Damage And Failure Characteristics Of Soft-Hard Composite Rock Mass With Prefabricated Joints Under Triaxial Unloading Confining Pressure

In the process of tunnel and underground engineering construction,excavation unloading is easy to produce surrounding rock damage,rock burst and other disasters.Composite rock mass is very common in rock engineering,and the mechanical deformation characteristics under loading and unloading conditions are different from that of single rock mass.In the actual engineering rock mass,there are various defects such as joints,cracks and weak structural planes,which have an important influence on the mechanical properties of rock mass.In this paper,cement,river sand and water are used to prepare soft and hard rock materials,and mica sheets are inserted to make standard cylindrical specimens of jointed soft rock and jointed rock mass with different dip angles.The conventional triaxial compression and triaxial unloading confining pressure acoustic emission tests were carried out to explore the mechanical mechanism of unloading confining pressure damage and failure of jointed rock mass.A mechanical model that can reasonably describe the mechanical behavior of jointed rock mass in unloading confining pressure stage was proposed.The unloading damage evolution law of jointed rock sample was revealed from the perspective of energy and acoustic emission,and the influence of joint inclination angle on the characteristics of jointed rock sample was discussed.Through this study,some progress has been made in the following aspects :(1)Based on the laboratory test results,the deformation and failure characteristics of jointed soft rock and jointed composite rock mass are analyzed,and the variation law of mechanical parameters of jointed rock samples under loading and unloading is studied.The results show that the strength of jointed rock samples decreases under unloading confining pressure,and the brittle failure characteristics are more obvious.With the increase of initial confining pressure of unloading,the peak strength and peak strain of jointed rock samples gradually increase.The higher unloading confining pressure rate,the larger the internal friction angle and the smaller the cohesion of the jointed rock sample;The Mohr-Coulomb strength criterion and Hoek-Brown strength criterion can well describe the transformation between the principal stress difference of jointed rock samples,but the Mohr-Coulomb strength criterion has better fitting effect.With the increase of joint dip angle,the strength of jointed rock samples changes in the ‘ U ' shape in the loading and unloading tests,and the minimum value is obtained between 30° and 60°.(2)According to the energy calculation formula,the energy evolution law of jointed soft rock and jointed composite rock mass under loading and unloading conditions is analyzed.The results show that with the increase of initial unloading confining pressure,the internal elastic energy of jointed rock mass increases gradually,and the more energy released during fracture,which makes the failure of jointed rock mass more severe under high confining pressure.With the increase of unloading confining pressure rate,the energy conversion rate of jointed rock samples is accelerated during unloading stage,and the failure of jointed rock samples is more severe.The energy index values of jointed rock samples at the peak point change in the form of ‘ U ' with the increase of joint inclination angle,but when the joint inclination angle is between 30° and 60°,the elastic energy accounts for a large proportion,the energy conversion is more intense,and the brittle characteristics are more obvious when the failure occurs.(3)The evolution law of acoustic emission characteristics during deformation and failure of jointed soft rock and jointed composite rock mass under different loading and unloading conditions is analyzed.The results show that the peak value of acoustic emission ringing count rate increases with the increase of initial confining pressure,indicating that more energy is released when the jointed rock sample is destroyed with the increase of initial confining pressure.With the increase of unloading confining pressure rate,the ' lag ' time of the peak value of acoustic emission parameters of jointed rock samples decreases gradually,and the peak value of ringing count rate increases gradually,indicating that the failure of jointed rock samples is earlier and more severe with the increase of unloading confining pressure rate.With the increase of joint dip angle,the ' lag ' time of acoustic emission parameters peak of jointed rock samples is generally ' U ' type,but the peak of acoustic emission parameters of jointed rock samples with different dip angles may also appear before the peak stress point.(4)The composite rock mass is conceptualized into two parts : crack body and matrix,which are described by natural strain and engineering strain respectively.According to the calculation formula of crack strain,the relationship model of axial strain-principal stress difference and axial strain-time of jointed rock mass in unloading confining pressure stage is established,and the applicability and rationality of the model are verified and the model parameters are discussed.The theoretical value of the model is in good agreement with the experimental value,which can better describe the nonlinear characteristics of stress-strain and strain-time in the unloading confining pressure stage of jointed rock mass.