From survey to analysis of the damage mechanisms in stone walls: diagnostic investigations on a bastion of the Venetian fortress in Bergamo

The impressive city walls, built in the 16th century by the Republic of Venice around the city of Bergamo, are composed by many different functional and architectonic elements: 4 principal gates, 3 viaducts, 12 curtains, 11 bastions and 5 platforms, forming a complex defensive system that from 2017 belong to UNESCO World Heritage List. The physical and environmental conditions of a system developing over a perimeter of 5 km and more than 70,000 m2 of wall surface expose this huge work to continuous deterioration events. This is caused by the poor quality of facings and the lack of maintenance, particularly by the growth of infesting vegetation. In the case of the northwestern part of the fortress that originally hosted the defensive garrison, known as Forte di San Marco, decades of carelessness caused the diffused deterioration of wall facing. In this context, the northern corner of the Valverde bulwark shows huge cracking and ejection of some stone ashlars. Together with the local University, the support of volunteers of Orobicambiente and the founding of Fondazione Cariplo, the Municipality of Bergamo carried on a deep diagnostic campaign focused on the Valverde bastion. By assessing the characteristics of the materials employed for building the wall and the causes of the crack pattern, the aim was to ensure both the monument conservation and the safety of the visitors. Starting with the geometric survey, the investigations ranged from the historical and archaeological analysis of the stone facing to the chemical-physical analysis of the materials and from the mechanical tests on specimens of stone and mortar to the interpretation of instability phenomena involving the northern corner of the bulwark. The integrated survey procedure and on-site examinations notably made it possible to understand the main causes of the damage and will be discussed here.