Mechanical response of tunnels under sequentially combined seismic loading and fault dislocation: insights from FDM-DEM coupled approach

The loading sequence of seismic and fault dislocation, together with the post-peak strain-softening of surrounding rock, directly affects the damage behavior and condition of fault-crossing tunnels. To address these issues, an FDM-DEM coupled approach is proposed to simulate the seismic and dislocation response of tunnels. The validity and superiority of the proposed coupled model over the conventional FDM model are verified. Parametric analyses considering different seismic-dislocation loading sequences are conducted. Results indicate that: (1) compared with the traditional FDM model, the coupled model captures post-peak strain softening of fault-zone rock and discontinuous lining deformation, and reproduces coseismic slip induced by strain softening; (2) Under both dislocation-seismic (D-E) and seismic-dislocation (E-D) sequences, damage spreads from the hanging wall to the foot wall with increasing fault displacement. The E-D sequence yields a wider range of minor damage than the D-E sequence, while the extent of extensive damage exhibits a crossing trend with fault displacement, indicating a shift in the dominant failure mechanism. Under the D-E sequence, smaller displacements cause cumulative global damage, while larger displacements induce notable defects and drive rapid longitudinal extension of extensive damage. Under the E-D sequence, seismic motion induces stiffness degradation, allowing local failure to be triggered even by small fault displacements, while the impact of large displacements is mitigated by pre-existing damage. (3) Under the two loading sequences, the location and extent of plastic zone development differ. Ground motion spectral and pulse characteristics do not change the qualitative damage distribution but affect its extent and magnitude.