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

Abstract. Paganico is a little village located in the south-eastern portion of the Lucca Plain (North Western Tuscany, Italy). Since Seventies a few square kilometres area around Paganico has been involved in opening of small cavities (sinkhole-like features – in this work named micro-sinkholes for simplification) on the land surface. At the beginning they were very small and sporadic. Then (in Eighties), the phenomenon has been characterized by a significant extension, increasing in frequency and size (up to 2 m in diameter and depth), causing inconvenience to local people, agricultural operation and sometimes damage (cracks to buildings, walls, 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 autumn. Even so the predisposition and triggering causes are not at present completely clarified. Therefore this study is aimed at individuating the triggering and evolution mechanism of the Paganico micro-sinkholes, 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 involving 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 (2–3 m thick), which is particularly involved in collapses; an intermediate silty-clayey horizon (2–4 m thick); a lower and thick gravel-pebbly horizon, interested by important water resources and heavy pumping. Since Seventies, such water pumping considerably rose, due to the local demographic (well-field), industrial (paper manufacture) and agricultural development. From an hydrogeological point of view, this area is consequently characterized by two water tables: a temporary one, housed in the superficial silty-sandy horizon (aquitard), and a second one, confined, associated to the lower gravel-pebbly horizon (aquifer). Such water tables 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 interest also 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 base of the micro-sinkholes opening.