Lessons learned from occurrence of sinkholes related to man-made cavities in a town of southern Italy

The presence of man-made cavities below the historical parts of towns is a common feature in large portions of Italy. Different typologies of anthropogenic cavities have been excavated in different epochs for many purposes, including research and collection of potable water, establishment of underground working sites for olive oil production, worship sites, etc. Underground quarries are probably the most diffuse typology of subterranean cavities, especially the largest ones. Originally located at the outskirts of towns, quarries are increasingly found in built up areas due to urban expansion that has characterized the last century. This paper describes the recent occurrence of sinkholes related to underground quarries in the town of Altamura, in the Murge plateau of inland Apulia, where since 2006 a number of sinkholes have formed above subterranean calcarenite quarries, the local rock mostly used for building purposes. These quarries developed below ground because the calcarenite is generally located covered by clays (ranging in thickness from a few to 15 meters). Their abandonment, and the progressive weathering of the rock, has caused failures in the underground quarries. Eventually, such instabilities propagated upward until reaching the surface, and producing sinkholes.