<p>This study investigates the time-dependent damage behavior and delayed rockburst mechanisms in deep tunnels excavated in hard rock formations. Different from conventional studies that focus on standard tunnel cross-sections, this research investigates the effects of horseshoe-shaped cross-sections and varying aspect ratios on long-term tunnel stability under high geostress conditions, emphasizing the degradation of mechanical properties, nonlinear dynamic instability, and the role of cross-sectional geometry in time-dependent failure mechanisms. A series of multi-stage uniaxial compression tests were conducted on granite samples obtained from a high geostress region in Southwestern China. These samples were designed with horseshoe-shaped cross-sections and varying aspect ratios (the ratio of the short axis h to the long axis d of the opening) to simulate real-world tunnel conditions. To examine the influence of structural planes, prefabricated cracks were introduced into selected samples, enabling a comprehensive analysis of crack propagation and failure modes under increasing loads. The experimental results demonstrate that increasing the aspect ratio of the tunnel cross-section enhances the rock’s creep strength, alters damage patterns, and reduces the frequency of dynamic instability events. Higher aspect ratios promote shear failure over tensile failure, extend quiet periods in the damage evolution process, and delay the onset of significant instability. Additionally, the presence and orientation of structural planes have a substantial impact on the failure process. Structural planes inclined at 45° to the principal stress direction induce more severe flexural deformations and crack propagation, increasing the risk of shear-bending failures. In contrast, vertical prefabricated cracks exert minimal influence on the rock’s mechanical behavior. Complex structural plane conditions result in dominant shear-bending failure modes, posing significant risks to the stability of tunnel crowns and increasing the likelihood of collapse. Microseismic monitoring data reinforce these findings by showing that the failure and instability of the surrounding rock exhibit pronounced time-dependent characteristics, particularly in the incubation of delayed rockburst. The progressive damage observed in the granite samples mirrors real-world microseismic events in tunnels across Southwestern China, further validating the experimental model. These findings provide valuable insights for optimizing deep tunnel design and support strategies. By accounting for tunnel geometry and structural discontinuities, this study provides critical insights for mitigating rockburst risks and improving the long-term stability of deep tunnels.</p>