Dynamic cross-interaction between two closely-spaced shallow foundations

The cross interaction between two closely-spaced shallow foundations has been numerically investigated under static conditions and low-amplitude dynamic loading through a finite difference procedure implemented in the FLAC3D code. The motivation towards this research topic may be found in the increasingly growth of urbanization in modern metropolitan areas worldwide and in the urgent need for seismic requalification of vulnerable historical centres in ancient towns, where buildings are placed very close each other. Even though a disconnection between the superstructures exists, the response of their foundations (and that of the buildings themselves) may be coupled due to the continuity of the soil underneath. Consequently, the overall system response may be different from that predicted with reference to the simple scheme of an isolated footing or building. In the performed numerical study, the soil-foundation impedance functions of a single foundation were firstly compared to well-known literature closed-form solutions to validate the procedure from a numerical viewpoint. The analyses were then repeated with adding another footing identical to the first one and placed at a varying distance from it. The comparison between the response of the master foundation when modelled as isolated and as part of a group shows that the dynamic impedances of the footing in-group may remarkably change with respect to those of the isolated foundation. In particular, the stiffness (real part) increases at a frequency depending on footing-footing distance and the damping ratio increases or decreases depending to the type of imposed oscillation.