Integrated approaches to evaluate the susceptibility to natural and artificial sinkholes in hard and soft carbonate rocks of the Apulian karst (southern Italy)

Apulia region (southern Italy) represents the foreland of the southern Italian Apennines. It is mostly made up of a 6-7 km-thick succession of Mesozoic shallow-water limestones and dolostones, locally covered by thin and discontinuous Tertiary and Quaternary carbonate and clastic deposits. Mesozoic rocks are well-bedded and form a hard bedrock affected by several families of tectonic discontinuities; poorly deformed Tertiary and Quaternary deposits unconformably overlie the Mesozoic rocks. Due to the long subaerial exposure of the Mesozoic succession, the carbonate bedrock recorded the development in the subsurface of a dense network of karst cavities, mostly controlled by tectonic discontinuities. As a result, the Apulia region presents a strong susceptibility to natural sinkholes. To this hazard of natural origin, the possibility of occurrence of other problems related to the high number of man-made cavities has to be added in the region. As a matter of fact, a great variety of different typologies of artificial cavities is present, including underground quarries, worship sites, oil mills, civilian settlements, etc. These cavities were excavated mostly in the Plio-Pleistocene soft calcarenites. As a consequence of the huge urban expansion recorded in the last century in Apulia, several natural and anthropogenic cavities lie nowadays below densely populated neighborhoods or roads with heavy traffic. These conditions represent the main geomorphological hazard for the human society in Apulia, which requires a careful evaluation, aimed at protecting and safeguarding the human life, and at providing the necessary information for a correct land use planning and management. It is worth to be noted that the Federazione Speleologica Pugliese (the association that includes all the caving clubs in the region) has already surveyed and mapped in Apulia, respectively, 2200 and 1200 natural and artificial cavities. In recent years, a worrying increase in the number of sinkhole events has been registered in Apulia that reached a peak in 2009 and 2010. Following such circumstances, several research activities were started by the Institute of Research for Hydrological Protection of the National Research Council (CNR-IRPI) and the Basin Authority of Apulia. The main activities have been: i) the collection of information on natural and anthropogenic sinkholes in Apulia, ii) the construction of a detailed chronological catalogue of sinkholes, iii) the implementation of numerical analyses for modelling the instability processes, and iv) monitoring activities of selected natural and artificial cavities. Choice of the latter was dictated by their location below densely populated urban areas. Two distinct approaches were established to take into account the different petrographic, structural and geotechnical features of the hard and soft carbonates. The approach dealing with hard carbonate rocks (for natural karst cavities) is based on speleological and geometrical surveys of the caves and on an integrated geological and geomechanical characterization of the carbonate rock mass, aimed at individuating the main critical aspects of the karst caves in terms of likely effects on the society. On the other hand, the approach to verify the stability of soft rocks where artificial cavities have been excavated is mostly dependent upon the peculiar petrographic and geomechanical characteristics of the calcarenite rock mass, typically massive and unaffected by tectonic discontinuities. As a consequence, the traditional analytical methods of rock mass classification fail in these materials, since the rock strength of soft calcarenites is mostly dependent upon sediment texture, porosity type and distribution and degree of cementation. The fluid circulation into the rock mass is also important because the removal of the rock matrix may induce a rapid deterioration of the mechanical behaviour of the rock mass. The approach to the calcarenite is mostly based on the characterization of petrographic and geotechnical parameters by means of direct sampling from the rock walls and in situ surveys (wells, trenches, etc.). Through implementation of the two described approaches, our goal is to reconstruct accurate geometrical, geological and geotechnical models for both natural caves and artificial cavities. Starting from these models, it will be possible to plan specific monitoring activities in order to understand the development of underground instability, and the related evolution through the rock mass, possibly threatening the urban areas and infrastructures above.