This study focused on the mechanical behavior of a deep-buried tunnel constructed in horizontally layered limestone, and investigated the effect of a new combined rockbolt–cable support system on the tunnel response. The Yujingshan Tunnel, excavated through a giant karst cave, was used as a case study. Firstly, a multi-objective optimization model for the rockbolt–cable support was proposed by using fuzzy mathematics and multi-objective comprehensive decision-making principles. Subsequently, the parameters of the surrounding rock were calibrated by comparing the simulation results obtained by the discrete element method (DEM) with the field monitoring data to obtain an optimized support scheme based on the optimization model. Finally, the optimization scheme was applied to the karst cave section, which was divided into the B- and C-shaped sections. The distribution range of the rockbolt–cable support in the C-shaped section was larger than that in the B-shaped section. The field monitoring results, including tunnel crown settlement, horizontal convergence, and axial force of the rockbolt–cable system, were analyzed to assess the effectiveness of the optimization scheme. The maximum crown settlement and horizontal convergence were measured to be 25.9 mm and 35 mm, accounting for 0.1% and 0.2% of the tunnel height and span, respectively. Although the C-shaped section had poorer rock properties than the B-shaped section, the crown settlement and horizontal convergence in the C-shaped section ranged from 46% to 97% of those observed in the B-shaped section. The cable axial force in the B-shaped section was approximately 60% of that in the C-shaped section. The axial force in the crown rockbolt was much smaller than that in the sidewall rockbolt. Field monitoring results demonstrated that the optimized scheme effectively controlled the deformation of the layered surrounding rock, ensuring that it remained within a safe range. These results provide valuable references for the design of support systems in deep-buried tunnels situated in layered rock masses.

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Keywords: Giant karst cave, Multi-objective optimization model, Numerical simulation, Combined rockbolt–cable support, Field monitoring