Influence of alignment geometry on the global seismic response of tunnel systems

Conventional seismic analyses of tunnels often focus on local seismic actions at specific tunnel sections, overlooking broader effects of asynchronous seismic motion along the wider tunnel alignment which can produce significant amplification of internal forces. This study comparatively investigates the influence of four common tunnel alignment geometries—straight, gradual-L-shaped, tight-L-shaped, and sigmoid—on the global seismic response of long tunnels under synchronous and asynchronous seismic conditions. Nonlinear 3D numerical simulations using the Beam-on-Non-linear-Winkler-Foundation (BNWF) approach were employed considering shallow tunnels running in soil (moraine) and deeper tunnels in rock. The findings reveal that alignment geometry significantly influences the distribution of seismically-induced actions, particularly at locations of stiffness contrast such as tunnel-station connections. Shallow tunnels in soil demonstrated amplified transverse actions at the station-tunnel junctions due to ground-structure stiffness contrast, while deeper tunnels in rock showed more uniform distribution. Non-straight alignments exhibited increased bending moments and shear forces under asynchronous loading conditions as their orientation changed compared to the direction of wave propagation, particularly at shallow depths. The results provide new insight into the mechanisms through which tunnel alignment geometry influences seismic demand under spatially variable ground motion and may assist in the preliminary assessment of alignment-dependent seismic vulnerability in long tunnel systems.