Study on the energy evolution mechanism and periodic micro-fracture of the BR under triaxial compression tests

In underground metal mining, the complex environment and in-situ stress control requirements impose increasingly strict standards on backfill mining methods. Owing to its advantages of safety, high efficiency, economy, and environmental friendliness, backfill mining has become an inevitable choice for green mining. This study focuses on the two-step stope post-backfill method, where the stability of the backfill-encased-rock (BR) composite structure-formed by backfill wrapping rock-plays a crucial role in overall stope stability.Taking the BR composite structure as the research object, with single rock and backfill as controls, this study conducted triaxial compression tests combined with acoustic emission (AE), computed tomography (CT) technology, and theoretical analysis. It revealed the stage characteristics of the stress-strain curve during BR's deformation and failure, as well as the energy evolution and crack propagation laws in each compression stage. The results indicate that BR's stress-strain curve undergoes six stages, while its AE evolution is divided into five stages. As confining pressure increases, BR's peak strength rises and energy release shows regular changes; CT scanning clearly presents its porosity variation and crack propagation rules.This study clarifies BR's failure law under high stress: its peak strength is 20%-30% lower than pure rock, and residual strength is 65%-80% higher than pure backfill. These findings reflect the synergistic mechanical effect between backfill and rock under triaxial compression, and provide important guidance for stope structure stability research in practical mining engineering.