Study on the mechanical characteristics and load-transfer mechanism of single-layer linings in highway tunnels considering the influence of support parameters

Single-layer linings are increasingly considered for highway tunnels because they can simplify construction and promote the cooperative bearing of surrounding rock and support components. However, the load-transfer path among shotcrete, bolts, steel ribs and surrounding rock remains insufficiently understood. Based on the Jinping Tunnel of the G4216 Yibin-Panzhihua Expressway, this study combines field monitoring and three-dimensional numerical simulation to investigate the mechanical response of a single-layer shotcrete-bolt support system. Field measurements of surrounding-rock contact pressure, shotcrete stress and steel-rib stress were used together with simulations of crown settlement, horizontal convergence, bolt axial stress and shotcrete deformation. The model was validated by field deformation data, with relative errors of approximately 1.87% for crown settlement and 0.03% for horizontal convergence. The results show that hollow grouting bolts reduce crown settlement by about 9.03% compared with cartridge bolts and lead to a more balanced support stress state. Shotcrete thickness strongly affects vertical deformation control; increasing the thickness from 15 to 30 cm reduced crown settlement by 28.12%, while a 20–25 cm range provides a better balance between support effect and economy. Two-cycle shotcrete forming reduced crown settlement by 19.83% and maximum shotcrete deformation by 28.22% compared with one-cycle forming. Field stress monitoring further indicated that the crown and shoulder were the main load-transfer zones, while hollow grouting bolts reduced local stress concentration in shotcrete and steel ribs. These findings clarify the cooperative load-transfer mechanism of single-layer linings and provide references for support-parameter optimization in similar Grade III basalt highway tunnels.