Effect of backfill strength and interface angle on crack evolution and damage law of backfill rock composite under uniaxial compression

• The effect of backfill strength and interface angle on crack evolution and damage law of backfill rock (B-R) composite under uniaxial compression were revealed. • Three typical failure modes of B–R composites were revealed: energy-release failure in the backfill, tensile-induced rock failure, and interfacial slip failure. • The crack evolution processes and associated damage mechanisms under the three failure modes were clarified using DIC and 3D scanning techniques The backfill–rock (B–R) composite is a primary load-bearing structure in underground mining, where the interface angle ( α ) and strength ratio ( S = UCS rock / UCS backfill ) significantly influence its mechanical behavior and crack propagation. This study implements a cross-factorial design ( α= 0°, 30°, 60°, and 90° and S = 8.42, 5.90, and 4.30) under uniaxial compression, synchronizes Digital Image Correlation (DIC) and Acoustic Emission (AE) to monitor crack evolution and damage during loading, revealing the mechanical response and damage characteristics of the composite. The results provide feasible guidance for controlling stope backfill strength and optimizing the stability of underground engineering; however, the study is confined to laboratory scale and specific strength ratios, subsequent validation in stope backfilling practice is required. The detailed research results are as follows: (1) B–R strength is slightly higher than that of the backfill but remains constrained by the backfill’s strength. (2) Crack evolution significantly influences B–R strength: when cracks occur in the backfill (0°–30°), strength is limited by the backfill; at the B-R interface, strength is controlled by the bond between backfill and rock; when cracks occur between both materials, strength is jointly provided by both, leading to the best performance. (3) Crack patterns are influenced by both interface angle and strength ratio, with three distinct failure modes identified: rock failure induced by the backfill, interfacial slip, and dual-peak failure due to interface degradation.