Damage-failure characteristics and nonlinear models of soft-hard composite rock masses under unloading confining pressure

Soft-hard composite rock masses are widely encountered in underground roadway engineering, and their mechanical behavior plays a critical role in surrounding-rock stability. However, their damage and failure mechanisms under excavation-induced unloading paths remain insufficiently understood. In this study, triaxial confining-pressure unloading tests combined with acoustic emission (AE) monitoring were conducted on soft-hard composite rock masses. Their damage and failure characteristics and AE responses under unloading paths were investigated, and nonlinear mechanical models based on axial crack strain were developed for the unloading stage. The results show that the specimens exhibit increasingly pronounced brittle failure as the unloading rate increases. The failure mode transitions from shear to splitting failure, accompanied by an increasing contribution of axial crack strain to the total deformation. AE activity exhibits distinct staged and abrupt characteristics, while the amplitude distribution evolves from a “lowamplitude, broad-distribution” pattern to a “high-amplitude, narrow-distribution” pattern, effectively reflecting crack propagation and precursors to instability. The proposed nonlinear models accurately reproduce the axial strain- differential stress and axial strain-time relationships during unloading. The effects of the model parameters on the predicted curves are consistent with the experimental observations, and the axial strain-time model effectively predicts the temporal evolution of strain increments at low unloading rates. These results reveal the unloading-rate-dependent mechanical response and crack evolution of soft-hard composite rock masses, reproduce their strain-stress and strain- time responses during triaxial confining-pressure unloading, and enhance the understanding of unloading-induced failure mechanisms.