Seismic analysis of deep tunnels in near fault conditions:a case study in Southern Italy

The importance of underground structures in transportation and utility networks makes their vulnerability to earthquakes a sensitive issue. Underground facilities are usually less vulnerable to earthquakes compared to above-ground structures, but the associated risk may be relevant, since even a low level of damage may affect the serviceability of a wide network. Seismic analysis of tunnels close to seismogenic faults is a complex problem, which is often neglected at the design stage for the lack of specific codes or guidelines for the design of underground structures in seismic conditions and also because, as mentioned above, underground structures are considered less vulnerable to earthquake loading. This paper investigates the seismic response of deep tunnels focusing on the required steps for a proper design under both static and dynamic loading. The study aims at contributing to improve the methods currently used for the seismic analysis of underground structures. At this purpose, the seismic response of a deep tunnel in Southern Italy has been investigated along the transversal direction. The infrastructure is part of the railway switch line connecting Caserta to Foggia in the Southern Apennines which is one of the most active seismic regions in Italy. The seismic response in the transversal direction has been analysed by using the pseudo-static approach as well as through advanced numerical modeling using the spectral element method coupled with a kinematic approach for finite fault simulations. The pseudo-static approach has been implemented using a closed-form analytical solution. The results obtained from advanced numerical modeling and the pseudo-static method have been compared to assess their validity and limitations.