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

The backfill–rock (B–R) composite is a primary load-bearing structure in underground mining, where the interface angle and strength ratio between the backfill and rock significantly influence its mechanical behavior and crack propagation. This study conducted uniaxial compression tests using four interface angles (0°, 30°, 60°, and 90°) and three strength ratios (S = 8.42, 5.90, and 4.30) based on a cross-factorial experimental design. Digital Image Correlation (DIC) and Acoustic Emission (AE) techniques were used to monitor crack evolution and damage during loading, revealing the mechanical response and damage characteristics of the composite. The results show that (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.