Mechanical Behavior and Modeling of Polypropylene Fiber-Reinforced Cemented Tailings Interface with Granite Under Shear Loading: Effects of Roughness and Curing Time

Cemented paste backfill (CPB) is widely adopted in underground mines, where the shear resistance of the CPB–rock interface critically governs the integrity of backfill–rock systems. This study investigates the effects of polypropylene fiber reinforcement, surface roughness (Joint Roughness Coefficient, JRC = 0 and 1.76), and curing time (1, 3, and 7 days) on the shear strength and deformation characteristics of CPB–rock interfaces. Direct shear tests were performed under normal stresses of 50, 100, and 150 kPa, with synchronous measurements of shear and vertical displacements. Results show that increasing roughness markedly strengthens the interface, with the peak shear stress rising by up to 45% due to enhanced mechanical interlocking and dilation. In contrast, adding 0.5 vol.% PP fibers slightly reduces peak shear capacity but consistently improves post-peak deformability, indicating a transition from brittle interfacial fracture to a more ductile, progressive failure mode. A three-stage mechanical model was established to describe the shear stress–displacement relationship, incorporating elastic, bond degradation, and frictional sliding phases. The model parameters, including the shear stiffness (Ks), bond degradation coefficient (η), and residual strength (τr), were calibrated using the experimental data. Mohr–Coulomb analysis further quantifies the curing-dependent evolution of interfacial strength parameters, highlighting a marked increase in cohesion from 1 to 7 days alongside roughness-governed peak strengthening. This research provides insights into the optimization of the CPB–rock interface design for enhanced geomechanical performance in underground applications.