Mechanical behavior and fracture mechanisms of rock-backfill composites under different hole geometries and backfill conditions

Abstract Rock-backfill composites, as key load-bearing structures in deep mining, exhibit mechanical behavior and failure mechanisms that are predominantly controlled by excavation-induced hole geometry and backfill conditions. In this work, a series of uniaxial compression tests were conducted on composite samples with various hole aspect ratios and backfill configurations using a coupled acoustic emission (AE) and digital image correlation (DIC) techniques. The results show that backfilling effectively mitigates local stress concentrations, enhances post-peak residual strength, and promotes the gradual evolution of fracture process. Increasing the hole aspect ratio weakens structural stability and leads to more uniform fracture morphology. AE analysis reveals a staged evolution of fracturing behavior and scale characteristics. The backfill structure shifts the energy release mode from abrupt to segmented, and increasing the backfill degree significantly suppresses high-frequency collapse-type events. Source mechanism analysis indicates that shear failure consistently accounts for a significant proportion, while a transition from tensile-dominated to collapse-dominated fracture occurs between tensile and collapse fractures. DIC observations clearly illustrate the regulatory effect of backfill on crack propagation paths, and displacement discontinuities at heterogeneous interfaces reflect the differential deformation between rock and backfill. These findings provide new insights into the geomechanical behavior of deep excavation-induced composite structures and offer practical guidance for optimized stope design, structural stability assessment, and multi-parameter early-warning strategies in underground engineering. Highlights A series of uniaxial compression tests were performed on rock-backfill composites with varying hole geometries and backfill conditions.Symmetrical backfilling significantly improved post-peak strength and deformation coordination, promoting stable failure.Increasing the height-to-width ratio of holes reduced structural stability and led to simplifi ed crack morphology.AE and DIC jointly revealed the staged energy release and fracture evolution under diff erent backfi ll conditions.Backfill structures transformed energy release modes from abrupt to segmented and suppressed low-frequency collapse-type events.