In order to study the influence of the cement–tailing ratio and curing age on the mechanical properties of cemented backfill with unclassified tailings and improve the accuracy of the backfill monitoring method, the mechanical characteristics of the failure process of cemented backfill with unclassified tailings at different curing ages were studied using the acoustic emission and infrasound method. The results show that the peak strength and modulus of elasticity of backfill are positively correlated with the curing age. During the failure process of cemented backfill with unclassified tailings, the acoustic emission ringing count shows a “stabilizing–rising–falling–rising” trend, and the infrasonic ringing count will have a “multiple peak” phenomenon. The ib value of the acoustic emission signal generally increases with the extension of the curing age. The ib value of the infrasonic signal generally has a negative correlation with the increase of the curing age. When the ib value of the acoustic emission and infrasonic wave all start to decline in a jumping manner, this indicates that the backfill is about to be damaged. The dominant frequency ratio of acoustic emission signals (F_AE) is distributed between 0–4 during the failure process, and the total number of F_AE shows the phenomenon of “first decreasing and then increasing” with the curing age. The dominant frequency ratio of infrasonic signals (F_s) is distributed between 0–6. During the failure process of cemented backfill with unclassified tailings, the quantity ratio of class A and B of acoustic emission signals shows a gradually increasing trend, and the quantity ratio of class A and B of infrasonic signals shows a “decreasing–increasing–decreasing” trend, and the quantity ratio of class A and B of acoustic signals shows a sharp increase at the stage of entering the crack’s unstable growth stage.

The instability and collapse of surrounding rock in mine goaf areas often lead to the destabilization of geological structures, surface subsidence, and mining safety accidents. To investigate the evolutionary mechanisms and precursor characteristics of rock instability and failure processes, uniaxial loading and cyclic loading–unloading tests were conducted on red sandstone using a rock mechanics loading system. These experiments aimed to explore the mechanical behavior of the rock and the development process of internal fractures. The characteristics of infrasonic signals generated during red sandstone fracturing and the laws governing damage evolution were analyzed with an infrasonic acquisition system. The research results indicate that the infrasonic signal activity generated by rock under loading conditions can be characterized by three distinct stages, namely the relative stability period, the active period, and the pre-failure precursor period. Prior to peak strength, a substantial number of infrasonic signals are generated in rocks with significant activity; this characteristic is independent of the loading path but dependent on the stress magnitude. The variation in cumulative infrasonic energy reflects the accumulation of damage in rock specimens during the loading process, and as damage accumulates, the stress–strain curve exhibits hysteresis effects and nonlinear increases, accompanied by a rapid rise in infrasonic energy. By analyzing the characteristics of infrasonic parameters and characterizing the damage and its evolutionary features in red sandstone based on infrasonic energy, the internal crack damage evolution process in rocks can be effectively characterized. This approach provides theoretical foundations and technical support for early warning and monitoring prior to rock failure.