Intrinsic and induced vulnerability of the Osservanza Church in Bologna (Italy)

Assessing the vulnerability of built heritage requires a multidisciplinary approach, in which the interaction between buildings and soil may change during the lifespan of the structures, due to the deformation of the soil or a change in its properties. Such modifications may occur naturally over time or as a consequence of environmental variations, affecting the conservation state of the historical building. In this context, this paper summarizes a comprehensive effort to assess the causes of the severe damages to the Osservanza Church in Bologna (Italy). The original structure was built in 1406, and, afterward, it was significantly modified several times during its lifespan. In the last 40 years, the church and its convent have undergone several restorations, as a consequence of a pronounced progression of the crack pattern affecting the structure. Then, a preliminary study of the historical events that affected the structure and a subsequent geotechnical investigation campaign were carried out. This latter included continuous coring boreholes and electric CPT, supplemented with laboratory tests on undisturbed samples. The foundation system of the church was also directly inspected through trial pits that surprisingly revealed significant differences on foundation levels of the columns of the church, as well as partial underpinning work. A monitoring system was also installed to control the crack pattern evolution of the masonry walls, the water table fluctuations and the creep movements of the surrounding slopes. Geotechnical investigations provided the local soil profile, that can be described as a succession of high plasticity clayey silts and silty clays, from stiff to very stiff. At the same time, the results of oedometer tests revealed the medium-high swelling potential of these materials when subjected to volume change with seasonal fluctuation in moisture content. The structural inhomogeneity discovered on the foundation system may be considered as the primary source of the relevant differential settlements occurred between different parts of the church. In addition, the seasonal variations of the water content and of the suction regime within the foundation soil may be recognized as a source of the fluctuating crack patterns, that could be also amplified by extremely slow slope movements in the long term, as those detected after the installation of two inclinometers. Mitigation and conservation measures are discussed within such a framework, that are certainly open to some scrutiny and can be improved by further investigation.