3D modelling of construction sequences and subsoil heterogeneity effects on the seismic response of shallow tunnels in complex topographical settings

For the seismic risk assessment of underground infrastructures, the construction of reliable numerical models may become challenging, especially in the case of old tunnels for which information about the geometry of the structure, the subsoil conditions and the excavation procedure are often unavailable. To assess the influence of the tunnel construction sequences and subsoil conditions on the tunnel lining force distribution, the paper describes a nonlinear 3D numerical analysis of the dynamic response of a shallow tunnel crossing a topographically complex area. The study is inspired by the case of the old Alvaro tunnel, located at the toe of a natural slope along the Basentana State Road (Matera, Italy). Two realistic tunnel construction sequences have been considered in the 3D models, implementing homogeneous and heterogenous subsoil conditions. The results show a clear correlation of the lining forces distribution with the cover depth variation and their overall increase after the seismic action related to the subsoil heterogeneity. The study highlights the potentiality of the 3D approach in estimating traction forces along the tunnel, that could practically inform the design of a monitoring system for the assessment of the tunnel serviceability limit state both during operating conditions and after an earthquake.