<p>Underground construction projects, particularly tunnels, face significant challenges due to the risk of block failure caused by discontinuities in rock masses. Predicting and preventing the collapse of these blocks, especially during excavation and support system design, is crucial. Block theory provides a valuable framework for analyzing such rock instability, focusing on the mobility and mechanical stability of blocks formed by discontinuities. This study applies block theory and reliability methods to the water diversion tunnel of the Hajilerchay reservoir dam. The N11 alignment was identified as optimal, having the smallest volume of critical blocks. EasyKBT software analysis confirmed the presence of three unstable blocks (1010, 0101, 0111), with safety factors of 0.585, 0.619, and 0.319, respectively. Due to uncertainties in rock conditions, limit state functions for these blocks were defined for both symmetric and asymmetric formations. Using first-order reliability method (FORM), second-order reliability method (SORM), and Monte Carlo simulations (MCS) in the RTX software, the reliability index was determined to be 9 for asymmetric blocks and 4.35 for symmetric blocks. Sensitivity analysis revealed that the height of the blocks was the most critical factor influencing stability. These findings emphasize the importance of considering block height and formation type in tunnel design, reinforcing block theory principles for underground construction.</p>