The last sinkhole at Marsala: the 21 November, 2013,event

Marsala area, in the western end of Sicily, has been historically interested by quarry activities, both at surface and underground. The carved rocks are Lower Pleistocene calcarenites, defined as “Calcarenite di Marsala” and referred to the Marsala syntheme. These calcarenites are composed of three main lithofacies with vertical and lateral passages: a)coarse to fine yellow bio- and lithoclastic calcarenites, rich in macrofossils; b) sands; and c) gray sandy clays. According to previous authors, the calcarenitic lithofacies can be divided into three lithotypes: i) coarse calcarenites and calcirudites strata, from 10 to 100 cm thick; ii) fine to coarse calcarenites with thickened grains; iii) medium to coarse calcarenites in irregular strata with intercalations of thin silt beds. The subterranean quarries, now abandoned, show increasing instability signs becoming in time a risk factor for several causes, among which: a) breakdowns due to poor (weak) strength of rock and to large size of voids; b) progressive weathering of rock; c) relationship between the horst rock discontinuities and pillars and/or walls of the underground quarries. These factors contributed to enlargement of the subterranean voids and to their upward propagation, thus triggering several sinkholes. The fast urbanization of the city masked many subterranean quarries causing the loss of memory of their location. In the last decades, numerous sinkholes occurred both in urban areas and in areas designated for agricultural use, creating extensive damage to buildings and infrastructures. The latest sinkhole episode occurred in the Amabilina area, at the eastern suburbs of Marsala. Here, in the late afternoon of November 21st, 2013 a sinkhole of impressive dimensions formed affecting an agricultural area, where until a few minutes before the owner was working. The area is located in the neighborhood of small rural buildings, housing and factories. The depression shows an elliptical perimeter (100×70 m) and a depth of at least 15 m. At the bottom, some rooms up to 5 m high of an underground quarry, are visible. Since the first investigation, it was clear that the sinkhole was caused by pillars failure and the upward propagation of the voids. This last factor increased as soon as the void reached the portion of the calcarenitic lithofacies rich in fossils. From the evidences collected a few days after the event, it was possible to reconstruct the time sequence in the formation of the sinkhole. The collapse started due to the propagation of voids and a first failure of some pillars, and was subsequently followed by a second event, which caused a widening of the depression, due to the redistribution of the stress resulting after early failures.