Study of the Fracture Evolution Mechanism of Surrounding Rock-Tailings Cemented Backfill Composite Specimens Based on Acoustic Emission and Digital Image Technology

In order to explore the influence of structural parameters and fractional filling on the mechanical properties of surrounding rock-tailing layered cemented backfill (RLB) composite specimens, the energy evolution law and fracture mechanism of RLB were revealed. In this study, RLB specimens with different inner diameters (15, 20, 25, 30, and 35 mm) were prepared. Uniaxial compression tests were conducted, and acoustic emission signals and surface deformation processes during fracture were monitored. nuclear magnetic resonance technology was used to characterize the microscopic pore structure of the RLB before loading. Results show that, although the porosity decreases with the increase in the inner diameter of the backfill, the volume fraction of the weakly cemented tailings backfill (with lower strength) dominates the failure process of the RLB, causing the uniaxial compressive strength and elastic modulus of the RLB to decrease as the inner diameter increases. When the inner radius ( r ) is 25 mm, the area fraction of the backfill reaches 25%, causing the load-transfer path to shift from primary load-bearing by surrounding rock to interface-backfill dominating damage. When the inner diameter exceeds 25 mm, the rate of decrease tends to slow down. During the loading process, microcracks within the RLB continuously expand and coalesce to form macroscopic cracks, leading to a significant increase in acoustic emission (AE) ring-down counts, with the b - value simultaneously decreasing. The high-density regions of the average frequency-rise angle (AF-RA) distribution for specimens with different inner diameters are concentrated near the AF region, indicating that the failure is characterized by a tensile-shear mixed mode, with tensile failure being dominant. As the backfill inner diameter increases, surface cracking in the specimens intensifies, and the spalling mode gradually transitions from granular to blocky. In terms of energy evolution, k e ( U e / U ) exhibits a decrease-increase-decrease trend, while k d ( U d / U ) shows the opposite trend. Additionally, as the inner diameter increases, the rate of U d growth in the postpeak energy release stage becomes faster, with k d continuing to rise and k e decreasing at an accelerated rate. The experimental results provide a theoretical basis for the design and optimization of backfill engineering parameters in underground mined-out zones.