A present risk from past activities: sinkhole occurrence above underground quarries

Sinkholes are extremely common in Apulia, a low-relief carbonate region of southern Italy that is highly predisposed to this hazard due to the widespread presence of soluble rocks. In addition to the natural setting favoring their development, sinkholes may also be induced by anthropogenic activities and in particular by extensive systems of underground quarries. Many types of rocks have been historically quarried in Apulia, from the Cretaceous limestones to the Quaternary calcarenites, and used in different epochs as building and ornamental materials. In several areas, the rocks with the best petrographic characters are located at depths ranging from a few to some tens of meters. This, combined with the need to save pieces of land for agriculture, caused opening of many quarries underground and development of complex networks of subterranean galleries, especially at the turn between the nineteenth and twentieth centuries. Later on, many quarries were progressively abandoned, partly because of appearance of the first signs of instability, both underground and at the ground surface. With time, memory of the existence and precise location of the quarries was progressively lost, with severe repercussions for safe land use above the excavated areas. Lack of knowledge of the subterranean pattern of galleries, combined with the expansion of builtup areas at the surface, resulted in increasing the vulnerability of the exposed elements at risk. As a matter of fact, a strong increase in the frequency of events had to be recorded in Apulia during the last 5 years, as pointed out by the accounts reported in this paper.