Knowing the underground, as the first step for hazard management: an experience in southern Italy, in the aftermath of a catastrophic collapse

Several types of geological hazards are related to the underground, the most typical being represented by sinkholes, a subtle and dangerous hazard which generally occurs with very little precursory signs, thus putting at high risk the vulnerable elements present nearby. Italy is worldwide known for its long history, and the beauty of many historical town centres. Where the local geological conditions allowed, more than one civilization used the underground, to dig and excavate in the soft rocks (volcanic rocks, calcarenites, etc.) artificial cavities to be used for a variety of purposes. As a matter of fact, artificial cavities, once abandoned, become sites of likely degradation of the rock mass, suffering a slow but continuous decaying process, potentially bringing to decreasing the physical properties of the rock mass, due to water infiltration, weathering processes, etc. At present, many of these cavities lie below the urbanized areas, quite often without the present population is aware of them, which might be at the origin of situations at risk. In January 2014, a collapse due to instabilities in artificial cavities developed at the historical centre of Ginosa (a small town in southern Italy), forcing the local authorities to close a large sector of the historical part of the town. In the aftermath of this event, we worked to evaluate the susceptibility to other possible collapses, as a consequence of bad instability conditions in the existing network of artificial cavities. At this aim, about 100 cavities were surveyed, documented and controlled in two months and a half. The susceptibility to collapse was evaluated in accordance to a specific procedure (which is also applicable to natural caves) aimed at contributing to mitigate the risk from this geohazard. The procedure develops from the identification and geographical location of the cavities, and then proceeds with the speleological survey, before characterising the caves in terms of geological-structural data (highlighting all the existing discontinuities in the rock mass, of both stratigraphic and tectonic origin), and of all the features related to occurrence and development of instability processes. Laboratory tests and monitoring are also mentioned as further possible steps of the analysis. Eventually, the procedure results in a zonation depicting the sectors most prone to development of sinkholes. In this contribution we present the outcomes of our work at Ginosa, within the framework of the emergency phase management, highlighting the possible use of the method, at the same time also describing the difficulties encountered in developing such a study.