A three-dimensional back-analysis of the collapse of an underground cavity in soft rocks

The assessment of stability of man-made underground caves, excavated in the past and later on abandoned, represents a serious challenge for land and urban planning operations, especially for the areas of possible interaction of the caves with overlying structures and infrastructures. Several areas of Southern Italy are characterized by the presence of abandoned underground quarries for the extraction of soft calcarenite rocks, which now threatens the overlying environment due to the risk of collapse and the consequent generation of sinkholes. This work presents a back-analysis of a sinkhole occurred in 2011 in the town of Marsala, caused by the collapse of an underground quarry, as a representative case study of these phenomena. Based on the available geometrical and geological dataset as well as the field observations of the phenomenon, in this article the assumptions and the results about the genesis of the 2011 sinkhole, as derived from a three-dimensional finite element backanalysis aimed at reconstructing the stress-strain evolution that brought to the ground failure collapse, is discussed. In particular, the 3-D numerical analyses have been performed in order to identify the factors responsible of the genetic mechanism of the sinkhole. The finite element analysis has been carried out by accounting for the geotechnical characterization of the Marsala calcarenites derived from both specific laboratory tests performed on samples taken from the site and literature data available on the same rock material; the numerical results have been then validated by means of the comparison with field observations and also compared with those achieved through a 2-D model of the same case study.