Research on the Stability and Support Effect of a Tunnel Excavated by a Mechanical Drilling Method: Insights from a PFC3D-FLAC3D Coupling Simulation

This study employs a three-dimensional numerical simulation based on the discrete–finite element coupled method to investigate the mechanical excavation of a tunnel and its influence on the support structures. The discrete element method accurately reproduces the mechanical cutting procedure during excavation, and the finite element model covers the majority volume of the model for reflecting the response of far-field rock. The main conclusions drawn from this research are as follows: (1) Under true triaxial loading conditions, the influence of the inclined interface between the rock strata on the tunnel’s displacement and stress field is relatively low, and the uniform displacement and stress field are formed around the tunnel. (2) The detailed mechanical excavation and rock-breaking process is simulated, and a secondary crack layer (shear failure dominates) with a thickness of about 30 cm formed on the surrounding rock of the tunnel; secondary tensile and shear cracks present different distributions and orientations, which are caused by the mechanical drilling and cutting processes. (3) Although over 60% of the lengths of the anchor rods in the tunnel-side walls reach the yield strength of the Q235 steel rod, the anchor rod system is relatively safe (lower than the tensile strength) and plays a positive role on rock stability; however, the anchor rods in the tunnel roof are safer because the low deformation (about 50% compared to the rods in the side walls), and only a minority of the anchor rods exceed the yield stress.