Although rock bolts are widely used in tunnel engineering, their effectiveness remains uncertain under specific geological conditions. This study investigates a single-track railway tunnel excavated in gently dipping stratified mudstone to evaluate the applicability of sidewall rock bolts. Field measurements, including tunnel deformation, initial support reaction, and bolt axial force, were conducted. An equivalent support force of bolts was quantified, and a coupled relationship between bolt axial force and surrounding rock displacement was established based on interfacial mechanics. In addition, a physics-constrained inversion framework was developed to reconstruct the displacement field from measured bolt responses. The results show that tunnel deformation is minimal, with a maximum crown settlement of 4.7 mm and convergence of 3.6 mm. The initial support bears 93.4% of total support capacity, compared with merely 6.6% from rock bolts, revealing that primary support dominates load bearing under small deformation. Further parametric analysis indicates that increasing bolt length beyond 1 m does not further enhance the support performance. These findings suggest that there is potential for optimizing the length of fully grouted sidewall rock bolts in heterogeneous, gently dipping layered mudstone tunnels.
Luo et al. (Mon,) studied this question.