Integrated Empirical–Analytical–Numerical Assessment of Tunnel Stability in Flysch: A Case Study of the Zenica Tunnel

This study investigates road tunnel stability in heterogeneous flysch formations using the Zenica Tunnel as a case study. A hybrid research framework integrating empirical classification, analytical modeling, and numerical simulation was applied. The approach combines the Rock Mass Rating (RMR) system, the Convergence–Confinement Method (CCM), and nonlinear two-dimensional finite element (FEM) analyses. Statistical evaluation of the results reveals a strong exponential relationship between the stability factor Ns and measured tunnel convergence, with coefficients of determination (R2) between 0.89 and 0.96. Particular attention was given to sections classified as Category V rock mass. The analysis indicates that when RMR values fall below 25, the stability factor Ns exceeds the critical value of 5, marking the onset of pronounced squeezing behavior. The results show that analytical methods provide conservative estimates of tunnel stability, while numerical modeling enables improved calibration of support system stiffness. The proposed integrated methodology contributes to more reliable stability assessment and support design in road tunnels excavated in complex flysch formations.