Mechanical properties and acoustic emission evolution of coal-rock combination under unidirectional unloading condition

For the rock burst disaster problems caused by unloading disturbance of deep roadway excavation, the study on the deformation and fracture mechanism of roadway surrounding rock under different unloading conditions was carried out. Taking the coal-rock combination as the object of research, the true triaxial testing machine for rock mechanics and acoustic emission monitoring system was used to carry out the unidirectional unloading test of constant axial pressure and unloading lateral pressure, explore the mechanical properties of coal-rock combination under the influence of unloading initial confining pressure and unloading rate, invert the propagation laws of different types cracks in the coal-rock combination through the acoustic emission signal characteristics, and quantify the damage of coal-rock combination during loading and unloading with the ringing count. The results showed that coal-rock combination had obvious failure characteristics of tensile and shear composite under the condition of unidirectional unloading, macroscopically the coal was dominated by shear failure and the rock was dominated by tensile failure; the increase of the unloading initial confining pressure led to aggravating the failure of coal-rock combination, and the increase of unloading rate promoted the expansion of tensile cracks along the stress unloading direction of rock samples; the change of RA–AF of acoustic emission parameter accurately described the proportion of different types cracks in the coal-rock combination, coal-rock combination mostly expanded in the form of shear cracks during the loading and unloading process, the proportion of shear cracks decreased with the increase of the unloading initial lateral stress, and the increase of unloading rate promoted the increase of the proportion of tensile cracks in the stress unloading stage, the proportion of shear cracks decreased; it was found that unloading initial lateral stress and unloading rate were the main factors affecting the damage development of coal-rock combination in unloading stage through studying the damage calibrated by acoustic emission ringing count during the loading stage; after unloading initial lateral stress exceeded 20 MPa, it basically had no effect on the damage of coal-rock combination in unloading stage. The research results could provide reference for the occurrence mechanism and disaster prevention of rock burst in deep roadway excavation.