Mechanical Analysis of Rock Tunnels Reinforced by Grouted Rockbolts Considering the Blasting Damage Effect

Tunnelling using drilling and blasting method inevitably disturbs the surrounding rock and induces a blasting damaged zone around the tunnel. Conventional mechanical analyses assume that the entire ground is damaged, leading to an underestimation of the surrounding rock stability and a conservative support design of the tunnel. This study developed a new analytical model to analyze the displacement, stress, and plastic radius of tunnels reinforced by grouted rockbolts considering the blasting damage effect. The blasting damaged zone was assumed to take a cylindrical shape with a finite radius. First, four different states of an unsupported tunnel based on the plastic zone distribution within the damaged and undamaged zones were distinguished and solved. Subsequently, ten different forms of a bolt-supported tunnel considering the ground state at bolt installation and the relative ranges of the bolt length and plastic zone were established. A method for determining the evolution paths of these possible forms for a specific tunnel project was developed using the critical displacements and longitudinal positions that define the transition between the different states and forms. Finally, an analytical algorithm for the rockbolt–surrounding rock interaction was established by introducing the concept of a fictitious pressure to simulate tunnel advancement. The proposed analytical model was verified by 3D numerical simulation and field monitoring results. Illustrative examples were presented to investigate the effect of the bolt length on tunnel displacement in both hard and soft rocks. Finally, the performance and advantages of the proposed model were investigated by comparing with the conventional analytical model. The proposed method can be used in the rockbolt design of tunnels constructed using the drilling and blasting method.