Experimental investigation on mechanical properties of backfill-rock composite structure under in-situ mining and filling stress loading

The backfill-rock composite structure (BRCS) is pivotal to the stability of underground mining areas. However, during mining and backfilling cycles, it is subjected to coupled dynamic-static loading. In order to systematically investigate the mechanical properties of the BRCS under in-situ mining and filling stress loading, true triaxial dynamic-static tests were conducted. First, the effects of depth, cement-tailings ratio (C/T), and interfacial angle (IA) on composite strength, deformation characteristics, and failure modes were systematically investigated. Subsequently, the evolution of acoustic emission (AE) signal parameters during failure of the BRCS was analyzed. Finally, a damage constitutive model was established based on AE energy analysis. Test results indicate that with increasing depth, C/T ratio, and IA, the BRCS 's peak strength and elastic modulus exhibit an upward trend, while strain during the loading-unloading disturbance stages correspondingly increases. At a C/T of 1:8, specimens primarily exhibited rock-dominated load-carrying capacity with distinct brittle failure. Conversely, at a C/T of 1:4, specimens demonstrated coupled backfill–rock load-carrying capacity, exhibiting ductile failure in shallow regions and transitioning to brittle failure in deeper zones. AE signals were concentrated during loading-unloading disturbance, plastic yielding, and failure stages. The dominant failure mode was tensile-shear composite fracture, with the proportion of shear cracks gradually increasing with depth. The damage evolution process of the BRCS can be divided into three stages: initial damage, accelerated damage, and ultimate failure. This study provides important theoretical basis and practical guidance for optimizing the C/T and enhancing stability assessment in backfilled mine design.