Towards a practice-oriented procedure to account for static and dynamic interaction among three adjacent shallow foundations

Many historic centers in Italy and around the world consist of ancient masonry buildings closely packed together or constructed in clusters for defensive purposes. Alternatively, the recent population growth in modern megalopolises, coupled with the increasing use of reclaimed soils to address land scarcity, has resulted in the development of highly densely urbanized areas placed on soft or very soft soils. In both scenarios, neighboring structures, despite being structurally separate, may interact with one another through the underlying soil. To gain new insights into this phenomenon, the paper presents a comprehensive numerical study. It analyzes the static and dynamic responses of three closely-spaced shallow foundations in both the time and frequency domains using a rigorous 3D continuum approach. The conducted parametric study considers the effects of foundation spacing, oscillation frequency, and soil configuration (homogeneous or layered), quantifying their impact on the soil-foundation impedance matrix. The results of the numerical study reveal distinct responses between the central and lateral foundations of a group of three shallow foundations placed on the same subsoil. A comparison is drawn between the response of the pivotal foundation within a group of three identical footings and its response when paired or isolated. As an extension of an earlier work of the authors (Zeolla et al., 2023), a simplified approach is proposed to address cross-interaction effects in design practice. This approach introduces a set of group factors to modify the impedance matrix of the primary foundation when surrounded by closelyspaced neighbors. The practical approach is applied to estimate the increase in the fundamental period of a masonry terrace house. The cross-interaction effect is deemed non-negligible and warrants consideration in such squat structures, as it can result in significant increases in spectral acceleration along the ascending branch of the design spectrum.