Assessing the causes of damages to the Osservanza Church in Bologna (Italy)

The Osservanza church in Bologna (Italy), whose construction is dated back 1406, has under-gone several restorations in the last 40 years, as a consequence of a severe progression of the crack pattern affecting its walls and, to a lesser extent, the structure of the adjacent convent. The paper describes the results of geotechnical investigations, carried out on the soil-foundation system of the church, and those of monitoring systems, installed to control the crack pattern evolution of the walls of the church and the creep movements of the surrounding slopes. The preliminary study of the historical events that affected the structure of the church, from its construction to its present configuration, also provided significant pieces of information to define the programme of the geotechnical investigations. The soil profile, deduced from in situ tests, consists of a top stratum of anthropic fill, followed by a succession of high plasticity clayey silts and silty clays, from stiff to very stiff. The oedometer tests carried out on the collected undisturbed soil samples highlighted the high swelling potential of the investigated soil. Such evidences explained the pronounced seasonal variation of the amplitude of the crack systems affecting the masonry walls of the church, as shown by the monitoring system. In addition, the crack pattern was proved to be locally ascribed to differential settlements caused by differences on founding levels of the columns of the church, as well as to partial underpinning work.