Vibration responses and stability assessment of anchored extremely fractured rock mass based on modal analysis

This study is devoted to revealing the vibration responses of anchored fractured rock structure subjected to external excitation disturbances. By utilizing a self-constructed vibration testing platform, a series of vibration tests were conducted regarding various anchoring parameters (bolt spacing D, pre-tightening torque P_t and types of bottom support) and lateral forces F_l. Modal analysis was employed to propose modal parameters including natural frequency ω_r and damping ratio ξ_r, extracted from the vibration signals, as sensitive indicators for assessing structural stability. The test results reveal that a smaller D and higher P_t can effectively enhance the reinforcement effect provided by bolts, as indicated by an increase in ω_r and a decrease in ξ_r in self-stabilized state of the anchored structure. As F_l increases, the restraining effect supported by constrained frame intensifies, the increasing ω_r and decreasing ξ_r can be observed during lateral loading. When the anchored structure is subjected to a large F_l, the weak support areas between adjacent steel belts are susceptible to concentrate as zones of rock collapse, resulting in the formation of circular cavities and a corresponding sharp reduction in F_l, alongside a pronounced decrease in ω_r and a significant increase in ξ_r. Compared to the steel belts, the steel mesh acting as bottom support structure has more advantages in providing effective restraint to achieve self-stability of the anchored structure, as the steel mesh is able to distribute the internal stress more uniformly and reduce localized stress concentrations between rock blocks. As the mining face advances, the stress concentration and redistribution caused by mining activities lead to an increase in fragmentation degree of the roadway roof and a decrease in ω_r. The findings of this study provide a scientific basis for understanding the relationship between vibration signals and the stability of anchored fractured surrounding rocks in roadway.