Seismic response analysis of the Two Towers subsoil in Bologna (Italy)

The paper summarises the main findings of a parametric numerical investigation focusing on the seismic response of the foundation soil below the Two Towers, the historic medieval symbol of Bologna (Italy), dated back to the XII century. The fine-grained strata were recently characterised by an experimental field campaign down to 40 m below the ground level (b.g.l.), providing a shear wave velocity profile not exceeding 400 m/s. The seismic geotechnical model was then extrapolated down to either 150 m or 230 m b.g.l., where the presence of a rigid bedrock was assumed on the basis of geological considerations, with modest effects on the final outcome though. Analyses were run in the frequency domain with the code MARTA, using an equivalent linear visco-elastic law for the soil, and in the time domain with the finite element code PLAXIS, using instead the hardening soil model with small strain stiffness. The seven selected seismic inputs were scaled to a peak ground acceleration equal to 0.191 g, in order to be representative of the earthquake reference hazard scenario of the site for a return period of 712 years. Results at the tower embedded foundation depth (i.e. 5 m b.g.l.) clearly indicate that, while the peak ground acceleration is approximately coincident with the input, a significant seismic amplification can be observed at the fundamental periods of the Two Towers (about 1.35 s for the Garisenda and 3.00 s for the Asinelli). Larger values of spectral accelerations and maximum shear strains, also exceeding 0.1% in the shallower layers, are typically predicted by PLAXIS, thus highlighting the importance of adopting a more advanced constitutive formulation in the context of strong earthquake excitations.