Study on evolution law of interface cracks in tunnels and grouting reinforcement mechanism based on particle flow simulation

In deep tunnels, interface cracks between surrounding rock and lining degrade the mechanical performance of composite supporting structures. Using particle flow code (PFC), this paper establishes a validated discrete-element numerical model for lining-surrounding rock with pre-set interface cracks. The influences of crack position angle α (0°, 45°, 90°) and crack range angle β (15°–75°) on structural peak strength and failure patterns are investigated. The meso-scale grouting reinforcement mechanism and coupling effect between confining pressure and grouting are revealed. Results show that interface cracks produce obvious anisotropic weakening effects. At a fixed crack position, as the crack extent angle β increases from 15° to 75°, the peak load decreases by 8.8% for the spandrel position (α = 45°)—the most unfavorable stress state—and by approximately 10.8% for the crown/invert position ( α = 0°). Grouting improves bearing capacity by 9.28%–26.94%, and the maximum growth rate (26.94%) occurs for the weakest case of α = 45°, β = 75°. The reinforcement essence lies in force-chain bridging and stress-ring reconstruction. Increasing confining pressure drives failure transition from brittle tensile splitting to ductile shear failure. Under high confining pressure (1.5 MPa), grouted samples barely generate macroscopic cracks, demonstrating positive coupling between confining pressure and grouting. This study provides theoretical support for grouting-reinforcement design of deep defective tunnel linings.