A geotechnical insight into the soil-foundation system of the Two Towers of Bologna, Italy

Garisenda and Asinelli, also known as the Two Towers, are the best preserved medieval towers in the city of Bologna (northern Italy). They are worldwide famous symbols of the past power and wealth of the city which in the 12th century counted more than 75 masonry towers and tower-houses. In order to preserve these invaluable monuments, a deep understanding of their original structural configuration, foundation and subsoil included, must be carried out prior to any possible strengthening intervention design. Indeed, the foundations of historic towers and the surrounding soil often hide major hazards for the preservation of these monuments and a deep knowledge of their interaction is imperative. The paper outlines the investigation steps applied to the soil-foundation system of the Two Towers of Bologna: from the hydrogeological context of the area to the analysis of the main historical events that changed their structural configurations. Furthermore, the analysis and the interpretation of the foundation soil data, obtained from relevant in situ and laboratory tests, showed the presence of a 100 m deep succession of alluvial silty clays and clayey silts, relatively homogeneous in regards of the particle size distribution, but with a considerable scatter of their mechanical properties. Such variability has been interpreted through a sedimentological study of the extracted cores that provided the identification of local desiccated horizons and microstructured soils by carbonation (together constituting a paleosol). The resulting geological-stratigraphic profile represents a useful tool for the definition of a reliable and more defined geotechnical model. This study could therefore shed a light on the possible reasons of the substantially different structural configuration shown with time by the Two Towers of Bologna, despite their numerous similarities