Macro-meso fatigue fracture and instability of rock-backfill composite structure under increasing-amplitude cyclic loading

The rock-backfill composite structure (RBCS) material in mine stopes is often subjected to stress disturbance. In order to reveal the fracture and instability mechanism of the RBCS exposed to disturbed stresses, a series of multistage cyclic loading tests were conducted on RBCS specimens with various cement-tailing ratios (c/t). The results indicate that deformation characteristics, stiffness degradation, damage propagation and failure mode of the RBCS were greatly influenced by the c/t. The volume dilatancy increases as well as the secant modulus (Es) increases and then decreases with decreasing c/t, i.e., from 1:4 to 1:12. For a sample with c/t of 1:4 and 1:8, the damage displays two-phase increasing pattern. However, for a sample with c/t of 1:10 and 1:12, inverted Sshaped damage accumulation pattern is found. As the c/t decreased, failure mode of RBCS specimens transformed from mixed tensile-shear failure to dominant tensile failure. Post-test CT imaging reveals obvious fracture evolution pattern that was characterized by shear fracture within the backfill, a circular tensile fracture along the rock-backfill interface, and tensile-shear fracture within the surrounding rock. It is noteworthy that complicated crack network is prone to forming for RBCS specimens with the c/t of 1:4 and 1:8. It is found that the use of "flexible CTB" with relatively low c/t is beneficial to improve the overall stability of the mine stopes.