<p>Spalling and rockburst events induced by discontinuous joint planes adjacent to deep tunnels present a critical and non-negligible challenge in underground engineering practice. To investigate the joint-induced failure mechanism, a series of biaxial compression experiments were conducted on red sandstone specimens with D-shaped tunnels containing an adjacent pre-existing joint. The study systematically reveals the influence of jointed inclination and lateral stress on the failure behavior. The results indicate that with increasing jointed inclination and lateral stress, the peak vertical stress of the specimens initially increases and then decreases, demonstrating pronounced intermediate principal stress effects and joint effects. The entire spalling process within the tunnel was captured from multiple perspectives using a visual monitoring platform and can be categorized into four distinct stages. Due to the inducing effects of jointed cracks, the jointed sidewall fails earlier but with lower failure severity. Conversely, the intact sidewall experiences delayed failure accompanied by higher bursting intensity. In addition, the joint inclination altered the crack coalescence behavior between the joints and tunnel sidewalls, thereby influencing the geometry and orientation of the V-shaped pit and determining the initial failure location within the tunnel (e.g., corner or springing). Furthermore, increasing lateral stress enhanced the compaction, crack initiation threshold, and energy storage capacity of the surrounding rock, which led to reduced deformation of the tunnel sidewalls but a more severe failure extent. Correspondingly, the dominant failure mode transitioned from wing-shaped to lateral cracking, indicating a transition from tensile to shear mechanisms. Furthermore, the signals and the tensile–shear fracture mechanisms of the cracks were monitored and quantified through the integrated use of AE and DIC technologies. In addition, the study summarizes the joint-induced failure modes of the tunnel under different lateral stresses and jointed inclinations, establishing the relationship between the cracking fracture and internal tunnel damage. The laboratory phenomena in this study align with actual engineering failure cases, confirming the accuracy and reliability of the experimental model. This study provides a scientific reference for a deeper understanding of the joint-induced failure mechanisms in deep tunnels.</p>