Evaluation of soft limestone degradation in the Sassi UNESCO site (Matera, Southern Italy): Loss of material measurement and classification

Soft limestone degradation is the main problem in the old district of Matera (Sassi), a rupestrian settlement excavated and built in the Pleistocene Gravina Calcarenite, which has been recognized as a UNESCO heritage site since 1993. The Gravina Calcarenite outcrops in a large area in the Apulian foreland, onlapping the Cretaceous Altamura limestone. It can be divided in two main typologies, the first one characterized by the predominance of bioclastic components and the second one mainly consisting of lithoclastic grains. All Sassi are built and excavated in this soft stone that gives a unique aspect to the town. Unfortunately, the decay of calcarenite materials is very fast and the calcarenite constructions are now in a bad state of conservation. In this work, we present a new method for the classification and the quantitative estimation of material loss in degraded natural stones by using Structure from Motion (SfM) and image analysis techniques. In detail, degradation patterns were described on 250 façades of Matera Sassi by using the International Council on Monument and Site - International Scientific Committee for Stone - ICOMOS-ISCS atlas. The peculiar decay morphologies have been related to the extent of material loss, especially with alveolization patterns that develops on the 76% of the façades. The relationships between the identified decay patterns and the lithological features were discussed, highlighting the higher susceptibility of bioclastic calcarenite respect to the lithoclastic type. Following this analysis, five stages of decay related to loss of material were determined and quantified. The proposed approach represents a new and easy method for evaluating the degradation stage of ancient buildings and for better assessing the long-term survey and the maintenance of stone monuments.