Assessing the Seismic Performance of Underground Infrastructures to Near-Field Earthquakes

Assessing the seismic response in road and rail tunnels is crucial for ensuring their structural integrity and safety as vital infrastructures. However, these underground spaces receive far less scrutiny compared to their above-ground counterparts such as bridges and viaducts. Moreover, comprehensive case studies with fully dynamic monitoring systems, especially in active seismic zones, are uncommon. The dynamic behaviour of man-made tunnels varies significantly based on factors like design and geological conditions, particularly the surrounding soil or rock characteristics. Tunnels excavated in shallow depths within soft soils are generally considered more susceptible to seismic forces compared to those bored through dense soil or hard rock. However, direct comparisons based on experimental data are limited in the current scientific literature. To this aim, the seismic responses of one underwater rail tunnel and two nearby mountain road tunnels to the same near-field seismic event are examined. Specifically, data from the Mw=4.4 Berkeley Earthquake on the Bay Area Rapid Transit's Transbay Tube and Caldecott tunnel system's Bore 3 and 4 are analysed. This represents a rather unique case for near-fault strong motions, which are the ones expected to be most dangerous for civil structures and infrastructures. Moreover, this set of target infrastructures includes different boundary conditions (soft soil and hard mountain rock), cross-sectional shapes, year of construction, and other characteristics. This enables a detailed investigation of these contributing factors. Finally, Arias Intensity (AI) and significant duration (𝐷𝑠595) are proposed as potential explanations for the different seismic susceptibility of these different tunnels