Comparative Time-Frequency Analysis of the Seismic Response of Underwater Rail and Mountain Road Tunnels

Extended Abstract The evaluation of the seismic response expected at road and rail tunnels is a critical factor in ensuring the structural integrity and safety of such critical infrastructures. However, these underground infrastructures are far less investigated than their above-ground counterparts (e.g. bridges and viaducts). Furthermore, it is also much less common to find case studies with fully dynamic monitoring systems; even less in active seismic areas. The dynamic behaviour of man-made tunnels differs substantially according to several factors such as their design and, most importantly, the different geological conditions. Indeed, tunnels excavated at shallow depths in soft soils are generally expected to be more vulnerable to earthquake loads than those bored through dense soil or hard rock. That derives from the kinematic loading induced by the surrounding materials, with their different stiffnesses and amplification effects, by the depth, and other factors. Nevertheless, few direct comparisons, based on experimental recordings, are available in the current scientific literature; for instance, Cui & Ma [1] tested tunnel portal sections located in the soft-hard rock junctions with laboratory shaking table tests; Tsinidis et al. [2] summarised the main empirical findings for different tunnel typologies and soil characteristics; and Cilingir et al. studied the effects of depth on the seismic response of square [3] and circular [4] tunnels – but only with scaled-down laboratory experiments and numerical simulations. To shed light on these aspects, the seismic responses of one underwater rail tunnel and two nearby road mountain tunnels to the same seismic event have been investigated. More specifically, the recordings of the Mw=4.4 Berkeley Earthquake of 04 January 2018 aftermaths on the Bay Area Rapid Transit