Study on the triaxial unloading failure characteristics and acoustic emission features of weakly cemented mudstone

As coal mining gradually shifts to the western regions, the problem of floor heave in weakly cemented mudstone tunnels has gradually become a bottleneck affecting the safe and efficient production of coal mines. The floor heave behavior of weakly cemented mudstone is fundamentally caused by the unloading damage of mudstone brought about by tunnel excavation. Fully understanding the failure characteristics of weakly cemented mudstone unloading under different stress conditions is conducive to improving the dilemma of floor heave in weakly cemented mudstone tunnels. Based on this, triaxial unloading acoustic emission mechanical tests were conducted on weakly cemented mudstone specimens under different stress conditions (84.5 MPa, 92.17 MPa, 105 MPa, 118 MPa), obtaining stress-strain and acoustic emission response curves, which well characterized the strength, deformation, damage, macro instability, and crack propagation characteristics of mudstone under unloading effects. The research results show: (1) In triaxial unloading tests, there exists a critical value of deviatoric stress, which is 64.26-69.61% of the triaxial strength, and this critical point has a significant influence on rock failure mode, internal crack development, and rock failure conditions. (2) When the deviatoric stress is less than 94.7 MPa, the elastic modulus of weakly cemented mudstone gradually increases with the decrease of confining pressure; when the deviatoric stress is greater than 102.59 MPa, the elastic modulus gradually decreases with the decrease of confining pressure. The Poisson's ratio increases gradually with the decrease of confining pressure. The peak strength during unloading process shows a nonlinear relationship with confining pressure. (3) In triaxial unloading tests, there are fewer acoustic emission signals compared to signals during triaxial loading failure, and the failure mode of mudstone is diverse, exhibiting significant randomness and abruptness. The conclusions obtained can provide a certain theoretical basis for the formulation of subsequent preventive and control measures for weakly cemented mudstone engineering.