Field observations and studies of real earthquakes have highlighted the impact of underground subway tunnels on ground behavior during seismic events. In addition to these observations, both analytical and numerical studies have further validated this phenomenon. However, the limitations of existing analytical and numerical methods underscore the increasing need for field-based approaches, especially given the complex geometries of underground environments and the heterogeneous, anisotropic nature of subsurface profiles.
With this goal, a comprehensive field (based on ambient noise measurements) and numerical study using a finite element method was carried out to investigate the effect of twin tunnels on the site seismic response, using Shiraz's under-construction subway tunnels.
This article compares the site seismic response before and after the excavation of a single tunnel, while the effects of two adjacent tunnels will be addressed in a separate paper. The horizontal-to-vertical spectral ratio (H/V) results from field measurements at all stations reveal a distinct frequency peak below 1 Hz before tunnel excavation, which aligns with the geological conditions indicating the presence of thick alluvial deposits in the study area. The excavation of the tunnel has led to the elimination of the frequency peak directly above it. Additionally, subsequent numerical analyses show that subway tunnels contribute to the de-amplification of the ground seismic response above them, particularly for low-period input waves.