Risk considerations of cave stability in soft carbonate rocks using 3D geophysical methods

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 man - made and/or natural cavities are sometime assets of historical and archaeological significance. Sometime monuments of historical and archaeological significance are located in areas with high risk of sinkholes. In spite of the importance on the civil protection, the prediction of the sinkhole events is currently an hard issue. This paper provide a new methodological approach on the evaluation of sinkhole hazard in such "soft" carbonate rocks combing geophysical and mine engineering complementary methods. A case study is exposed which concern natural cavity named "Grotta delle Veneri". In this case the approach was: i) 3D Ground-penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) in order to evidence the shape and dimension of karstic cave; ii) seismic refraction tomography in order to study the physical-mechanical characteristic of rock mass that constitute the roof of the cave; iii) the scaled span empirical analysis to instability evaluation of the crown pillar's caves. The research allows to define the geometrical cave shape (span, length and thickness of the cave's roof) which related to the geological features and physics properties of the rocks, determine the instability of the roof's cave and, as a consequence, the sinkhole hazard.