Abstract: Grouting is widely used to improve the mechanical stability and hydraulic sealing capacity of fractured rock masses in deep underground engineering. However, quantitative criteria for selecting the filling thickness are still lacking, and the mechanisms by which grouting effectiveness varies under different stress conditions remain insufficiently understood. In particular, the coupled effects of filling thickness and confining pressure on the strength, permeability evolution, and failure pattern of grouted fractured rock have not been systematically clarified. To address this issue, this study investigates the hydro-mechanical behavior and failure mechanism of rock specimens with 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 response and permeability during loading. After failure, industrial CT scanning was performed to identify the internal crack distribution and propagation paths. In addition, a Particle Flow Code (PFC) model was calibrated against the experimental results and used to analyze mesoscopic crack evolution and energy dissipation. The results show that filling thickness and confining pressure play different but interacting roles in controlling the hydro-mechanical response of the specimens. Increasing the filling thickness causes a slight reduction in peak strength, but significantly decreases the initial permeability, indicating that a thicker infill more effectively blocks the dominant seepage channels along the fracture plane. In contrast, increasing confining pressure markedly enhances the load-bearing capacity of the specimens, suppresses fracture propagation, and reduces permeability by promoting the closure of pre-existing voids and seepage pathways. The permeability evolution during loading exhibits a clear stage-dependent pattern. Permeability decreases during the initial compaction stage because primary pores and microcracks are gradually compressed and closed. A local rebound occurs before peak stress, which is associated with the initiation of new cracks and local interface damage. After peak stress, permeability increases sharply as through-going cracks develop and connect into dominant seepage channels. These results indicate that the hydraulic response of filled fractured rock is governed by the competition among crack closure, interface degradation, and crack coalescence. CT observations and PFC simulations consistently show that failure mode strongly depends on the filling thickness. Specimens with a thinner filling layer are dominated by dispersed shear-band failure, whereas those with a thicker filling layer tend to develop crack clusters near the rock-grout interface, with fractures preferentially propagating along the infilled plane. This suggests that increasing filling thickness promotes a transition from matrix-dominated failure to interface-controlled damage. Energy analysis further shows that the reduction in interfacial bonding energy is closely correlated with the decrease in peak strength under thicker filling conditions, whereas higher confining pressure improves energy storage capacity and delays unstable crack propagation. Overall, this study integrates triaxial hydro-mechanical testing, CT-based crack reconstruction, and PFC simulation to reveal how filling thickness and confining pressure jointly control the bearing-seepage response of grouted fractured rock. The findings provide a mechanistic basis for optimizing grout filling thickness and improving grouting design in deep underground rock engineering.