Multiscale Analysis of Tunnel Behavior in Response to Normal Fault Rupture: Combining PFC3D-FLAC with Centrifuge Experiments

Faults cause serious threats to tunnel safety and fail in capturing the micromechanical procedures, such as force chain evolution, rotation, and particle slippage, which govern macroscopic failure. This study proposes a novel multiscale framework, which incorporates centrifuge testing and discrete-continuum coupling simulations (PFC3D-FLAC). This hybrid approach accurately allows simultaneous investigations of the underlying micromechanical interactions within the soil mass and macroscopic deformation fields. The interaction mechanism between the soil and the tunnel, as well as the mechanical response of the tunnel linings after faulting, is systematically analyzed. The hybrid approach provides more satisfied performance than continuum-based analyses, precisely capturing tunnel and soil settlements that other models underestimate. Results show that the tunnel has a significant depth-dependent shielding effect, decreasing overlying soil settlement by up to 30%. The fault fracture propagates at 88° beneath the tunnels, where the rotation of soil particles and the splitting of the triple force chain – consisting of the compressive strong force chain region, the distributive stress arc region, and the stable support region – redirects stresses towards the footwall. Additionally, when the fault displacement exceeds 1.6 m, the induced tensile strain at the crown rebounds due to the void below the hanging wall side tunnel. This study provides deep insight for the design of tunnels in fault zones, offering critical thresholds for stress mitigation and lining failure.