3D Numerical Investigation of the Seismic Site Effects in a Natural Slope with a Complex Topographical Setting Interacting with a Shallow Tunnel

In seismic risk assessment, an important aspect is the prediction of seismic site effects, which may be performed through either two- (2D) or three-dimensional (3D) numerical approaches, depending on the topographic and stratigraphic conditions, as well as the possible presence of structures and infrastructures interacting with the area of interest. The paper illustrates a 3D numerical investigation of the seismic amplification phenomena occurring in a natural slope interacting with a shallow road tunnel, aimed at evaluating the effect of the slope-tunnel interaction on the seismic site response. The work is inspired by the case study of the Alvaro tunnel along the Basentana State Road, located at the toe of the Costa del Canneto slope (Basilicata region), characterised by a complex topographical setting. The simulations are performed with reference to two 3D nonlinear finite element (FE) models, one including and one excluding the shallow tunnel. The soil behavior is simulated through the nonlinear elasto-plastic constitutive model HSsmall, opportunely calibrated to reproduce the cyclic response of the slope soils. The results, expressed through maps of amplification factors computed over specific period intervals (i.e. 0.1‒0.5 and 0.7‒1.1 s) for both horizontal directions, reveal a period-dependence of the seismic response, due to the interaction between seismic waves and the heterogeneous and topographically complex deposit. The tunnel influences the seismic amplification field, particularly in the long-period range, which is also dependent on the direction of the propagation of the seismic motion, reflecting the anisotropic nature of seismic site effects in topographically irregular settings.