Combined effects of different dislocation and seismic temporal sequencing on tunnels crossing active faults

As the demand for large-scale underground projects in western China grows, long tunnels inevitably cross active faults, exposing them to the threats of fault dislocation and high-intensity earthquakes. Previous studies mainly treated these hazards separately or through simple linear superposition, overlooking the complex stress path dependencies and sequential mechanisms inherent in real geological hazard chains. To establish a solid theoretical foundation for the safety assessment of cross-fault tunnels in high-seismicity regions, this study explores the structural dynamic responses and cumulative damage mechanisms of a typical tunnel crossing a strike-slip fault under two extreme temporal sequences: a dislocation-earthquake sequence and an earthquake-dislocation sequence.

To simulate a horseshoe-shaped tunnel crossing an active strike-slip fault with a 70°dip angle, a detailed three-dimensional numerical model was developed using FLAC3D finite difference software. The 455 m wide fault area was carefully subdivided into a 270 m wide fault zone, a 20 m wide central fracture zone, and a 165 m wide fault influence zone. The Mohr-Coulomb strain-softening constitutive model was used to accurately represent plastic strain accumulation. During dynamic time-history analysis, a 4.0 m quasi-static horizontal displacement was applied to simulate the dislocation phase. For the seismic phase, a Ricker wavelet was converted into equivalent stress loads on a static viscous boundary. To validate the numerical approach, a 1:50 scaled physical model test was conducted using a split-box shear device and specialized analogous materials. Subsequently, a comprehensive parametric sensitivity analysis examined how fault dip angles (60°, 70°, and 80°), fracture zone widths (10-30 m), and eccentric fracture zone placements (left, middle, and right) affected the lining internal forces.

The experimental and numerical results aligned well, successfully reproducing typical "Sˮ-shaped bending and high-strain concentration in the fracture zone. The finding showed: 1) The two temporal sequences produced fundamentally different failure mechanisms. In the dislocation-earthquake sequence, the initial dislocation caused significant plastic damage. The subsequent earthquake triggered stress redistribution and a severe damage superposition effect, leading to strain increases of 323.27% and 425.49% at the tunnel crown and invert, respectively, with the maximum invert strain reaching 70 146.7 με. 2) The earthquake-dislocation sequence was mainly dominated by the fault dislocation. The initial earthquake caused only 0.15 με of elastic strain. but the fault dislocation was responsible for the structural failure, raising the crown strain to 62 547.2 με and the maximum compressive stress to –8.29 MPa. 3) The fault dip angle was a key factor controlling the lining internal forces. Under the dislocation-earthquake scenario, increasing the dip angle from 60° to 80° reduced the peak bending moment from 266.39 MN·m to 128.91 MN·m—a 51.61% decrease. 4) Fracture zone width and location were secondary influences but significantly affected local internal force distribution.

Evaluating fault dislocation and seismic loads separately or through simple linear superposition significantly underestimates structural damage risks. Fault dislocation drives the "S"-shaped deformation and high-strain failure. The timing of earthquake intervention greatly influences stress evolution: the dislocation-earthquake sequence causes stress redistribution after damage, while the earthquake-dislocation sequence intensifies stress concentration induced by the dislocation.