1st Croatian Conference on Earthquake Engineering 1CroCEE 2021

Nowadays, underground structures play a crucial role in transportation and utility networks in urban areas. Their static design has achieved a high level of accuracy. Instead, tunnels' efficient seismic design is not yet completely gotten, even due to the complex soil-tunnel interaction phenomena that occur during an earthquake. So, the few code regulations and guidelines about the seismic design of tunnels and the numerous parameters that affect the soil-tunnel systems' dynamic response have led the geotechnical research community to devote great attention to the study of the tunnel response under ground shaking.
But, usually, dynamic analyses of coupled soil-tunnel systems are performed considering homogeneous soil at the tunnel's depth. Analytical solutions do not take specifically into account the possible heterogeneity of the soil crossed by the tunnel. To take into account this heterogeneity is necessary to move towards numerical approaches.
The present paper investigates the role of soil heterogeneity and soil-tunnel interface conditions on tunnel seismic behaviour. Different numerical FEM parametric analyses were carried out, considering great differences in soil stiffness at the tunnel's depth. In particular, starting from a real case-history regarding the Catania (Italy) underground network, a cross-section characterized by a strong heterogeneity in terms of soil stiffness was firstly analysed. Then, the degree of heterogeneity was varied; furthermore, the soil-tunnel interface conditions were modified to comprise a large number of case studies.
The achieved results were reported in terms of tunnel seismic bending moments and axial forces. The numerical results were also compared with those obtained using the closed-form solutions proposed by Wang (1993) and Penzien (2000) for homogeneous soil deposits.