Small sinkhole-like features in alluvial plains: the example of Paganico (Lucca Plain, Italy)

Paganico is a little village located in the southeastern portion of the Lucca Plain (northwestern Tuscany, Italy). Since the seventies, a few square kilometers’ area around Paganico has been involved in the opening of small cavities (sinkhole-like) on the land surface. At the beginning they were very small and sporadic. Later on the phenomena were characterized by a significant extension, increasing in frequency and size (up to 2m in diameter and depth), causing inconvenience to local people, agricultural operations and occasionally a little damage (cracks to buildings, fencing walls and outside floorings). The cavities prevalently occur at the end of the dry season, during or immediately after the first intense rainy events, that is, between the end of summer and early autumn. Even so, the predisposition and triggering causes at present are not completely clarified. Therefore this study is aimed at individuating the triggering and evolution mechanism of the Paganico sinkhole-like features, particularly referring to the stratigraphic, hydrogeological and geotechnical features of the involved materials. Another important issue made clear with this research is represented by the overexploitation of the local aquifer, characterized by particular hydro-structural conditions. In fact, the Paganico underground shows three horizons with different lithologic, hydrogeological and geotechnical properties: a superficial silty-sandy horizon (2–3m thick), which is particularly involved in collapses; an intermediate siltyclayey horizon (2–4m thick); and a lower and thick gravelpebbly horizon, characterized by important water resources and heavy pumping. Since the seventies, such water pumping rose considerably due to the local demographic (well field),industrial (paper manufacture) and agricultural development. From a hydrogeological point of view, this area is consequently characterized by two water tables: a temporary one, housed in the superficial silty-sandy horizon (perched aquifer), and a second one, confined, associated with the lower gravel-pebbly horizon (aquifer). The perched aquifer and confined aquifer are separated by an impermeable silty-clayey horizon. According to the observations resulting from this study, the latter probably tends to fracture by desiccation during the dry season, originating water exchange between the two water tables during the first important autumnal rainfall, depending on the pumping conditions, which lower the piezometric surface of the confined aquifer. Cracking would also characterize the superficial horizon. Thus, the water exchange would produce erosional phenomena in the superficial material, with removal of the fine fraction and collapse. This process could be at the origin of the cavities opening.