L’ultimo sprofondamento a Marsala: l’evento del 21 novembre 2013

Abstract

The Marsala area, at 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. 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) effects of the discontinuities in the rock mass with the 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 sectors 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. 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. 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.