FEM studies on the dynamic behaviour of tunnel-soil-overstructure systems in heterogenous soils

Nowadays, a growing need to solve the problems faced by transportation and utility networks motivates the expand of tunnelling design and construction. A high level of accuracy has been achieved for the static design of underground structures; the same has been not yet completely reached in the seismic field, due to the complex phenomena concerning the soil-tunnel systems' dynamic interaction. During an earthquake, the presence of tunnels may alter the response of the surrounding soil and so of the aboveground structures and, at the same time, vibrations of aboveground structures may modify the dynamic response of tunnels and involving soils. Up to now, studies involving tunnel plus soil plus aboveground buildings are still very rare. Furthermore, usually, dynamic analyses of these coupled systems are performed considering homogeneous soil at the tunnel's depth. The present paper is aimed to evaluate the effects of a tunnel crossing heterogeneous soils on the seismic response of the aboveground building and vice versa, performing FEM analyses of tunnel-soil-aboveground building systems and varying different characteristics of these systems. Starting from a cross-section of the recently built underground network in Catania (Italy), including an aboveground building, and characterized by a strong soil heterogeneity, the tunnel cover, the aboveground building’s position, and the seismic inputs were varied. These parametric analyses were performed adopting an equivalent visco-elastic linear approach for modelling the soil behaviour. Finally, to investigate the effects of different constitutive modelling on the behaviour of the tunnel-soil-aboveground building system, further FEM models were performed, adopting both a linear and a non-linear modelling to describe the behaviour of the soil, that was assumed homogeneous to analyse only the effect of the constitutive modelling. All the FEM models were investigated in the time and frequency domains. Also bending moments and axial forces acting in the tunnel were evaluated and compared with those obtained using the closed-form solutions proposed by Wang (1993) and Penzien (2000) for homogeneous soil deposits.