Methodological approach on the evaluation of sinkhole hazard in "soft" carbonate rocks.

According to the Protection Civil Department database, Apulia is the fourth region in Italy affected by sinkholes, due to collapse of natural or mane-made cavities. The southern part of the region (the Salento peninsula) had hosted in the last twenty years at least fifteen events of sinkholes, the greatest part of which occurred inside "soft" carbonate rocks (calcarenites). The more potential catastrophic sinkhole occurred at Gallipoli on 29 March 2007, when a collapse had created a 12 x 18 m sinkhole which intersected two 3 floors buildings. Some adjacent constructions might been involved by a enlargement of the sinking and 140 people were evacuated. The underground area was geologically explored and the results were exposed to the crisis unit constitute to manage the first civil protection action, empathizing the widespread of "soft" carbonate rocks crevices due to opening of tectonic fractures, the presence of crushing fractures at the base of the columns holding the vaults of the quarries and the worrying thin thickness of the ceiling. Against these evidences, filling operations of the sinkholes were decided after the crisis unit meeting. The 1 April 2007, during the laying of cement and stones, a further collapse had approximately triplicate the sinkhole crater area but fortunately it did not cause any victims. In spite of the importance on the civil protection, the prediction of the sinkhole events is currently an hard issue. This paper provide a methodological approach on the evaluation of sinkhole hazard in such "soft" carbonate rocks combing geological, geophysical and mine engineering complementary methods, such as: geological analysis of outcrops and boreholes, aerophotogrammetric interpretation of aerial photos, electrical resistivity tomography (ERT), ground penetrating radar (GPR), seismic tomography, and instability evaluation of the crown pillar's caves. A number of cases study are exposed which concern both natural and mane-made cavities. Moreover, sinkholes previously occurred (a kind of back analyses) and caves affected by effective or probably ceiling instability are considered. Finally, two of the case regard underground archaeological sites that are the "Grotta della Poesia" and the "Grotta delle Veneri". The research allows to define the geometrical caves shape (span, length and thickness of the crown pillars) which related to the geological features and physics properties of the rocks, determine the instability of the ceiling caves and, as a consequence, the sinkhole hazard.