Abstract: Grouting is widely used to enhance the mechanical stability and hydraulic sealing performance of fractured rock masses in deep underground engineering. However, quantitative criteria for determining the optimal filling thickness remain lacking, and the mechanisms governing grouting effectiveness under varying stress conditions are not yet fully understood. In particular, the coupled influences of filling thickness and confining pressure on the strength, permeability evolution, and failure characteristics of grouted fractured rock have not been systematically elucidated. To address this knowledge gap, this study investigates the hydro-mechanical behavior and failure mechanisms of fractured rock specimens containing grout-filled fractures through a combined experimental and numerical approach. Rock specimens containing rough fractures were collected from an engineering site and prepared with two representative filling thicknesses. Triaxial hydro-mechanical tests were conducted under three confining pressure levels to examine the coupled evolution of stress-strain behavior and permeability during loading. Following failure, industrial CT scanning was performed to characterize internal crack distributions and propagation pathways. In addition, a Particle Flow Code (PFC) model was calibrated against the experimental results and employed to investigate mesoscopic crack evolution and energy dissipation.The results show that filling thickness and confining pressure exert distinct yet interdependent influences on the hydro-mechanical response of the specimens. Increasing the filling thickness causes a slight reduction in peak strength but substantially decreases the initial permeability, indicating that a thicker infill more effectively obstructs the dominant seepage pathways along the fracture plane. In contrast, increasing confining pressure markedly enhances load-bearing capacity, suppresses fracture propagation, and reduces permeability by facilitating the closure of preexisting voids and flow pathways. Permeability evolution during loading exhibits a pronounced stage-dependent behavior. During the initial compaction stage, permeability decreases as primary pores and microcracks are progressively compressed and closed. A localized rebound occurs prior to peak stress and is associated with crack initiation and localized interface deterioration. Following peak stress, permeability increases sharply as through-going fractures develop and coalesce into dominant seepage channels. These results indicate that the hydraulic response of grout-filled fractured rock is governed by the competing effects of crack closure, interface degradation, and crack coalescence. CT observations and PFC simulations consistently show that the failure mode is highly sensitive to filling thickness. Specimens with a thinner filling layer are characterized primarily by dispersed shear-band failure, whereas those with a thicker filling layer exhibit concentrated crack clusters near the rock-grout interface, with fractures preferentially propagating along the infilled plane. These findings suggest that increasing filling thickness promotes a transition from matrix-dominated failure to interface-controlled damage. Energy analysis further shows that reductions in interfacial bonding energy are closely associated with decreases in peak strength under thicker filling conditions, whereas higher confining pressure enhances energy-storage capacity and delays the onset of unstable crack propagation. Overall, this study integrates triaxial hydro-mechanical testing, CT-based crack reconstruction, and PFC simulation to elucidate how filling thickness and confining pressure jointly govern the coupled bearing-seepage behavior of grouted fractured rock. The findings provide a mechanistic foundation for optimizing grout filling thickness and improving grouting design in deep underground rock engineering.