Paganico is a little village located in the south-eastern portion of the Lucca plain (north-western Tuscany, Italy). Since the sixties, it is interested by a rather unusual phenomenon: the opening of little holes and collapses in the ground surface. At the beginning, such phenomena were sporadic, but since the eighties their frequency and intensity increased. At present, many zones of the Paganico area are interested by holes and collapses. Their size sometimes reaches 2 m in diameter and depth. The opening of the holes mainly occurs at the end of the dry season, during or immediately after the first important and intense rainfall events, which normally characterize, in this area, the end of the summer and the beginning of the autumn. At present, the collapse phenomena are producing many problems to the population and agricultural activity of the area, but their causes are not so clear. For such reasons, this research provides a contribute in order to comprehend the triggering mechanism and evolution of the holes, with relation to the stratigraphic, hydrogeological and geotechnical features of the materials involved. It seems clear that the holes opening is directly linked to the strong pumping through an aquifer with particular hydro-structural conditions, as pointed out by the geological, hydrogeological and geotechnical surveys carried out in this area. The Paganico underground is in fact characterized by three horizons with different lithologic, hydrogeological and geotechnical features: a superficial silt-sandy horizon (1-3 m thick), which is particularly interested by the collapses; an intermediate silt-clayey horizon (1-3 m thick); a lower and thick gravel-pebbly horizon, interested by an important water circulation and heavy pumping. Since the seventies, such water pumping has considerably rose, due to the local demographic (wellfield), and, above all, industrial development (paper manufacture). Therefore, this area is characterized by the presence of two water tables: a temporary one, located in the superficial silt-sandy horizon, and a second one, located in the lower gravel-pebbly horizon. Such water tables are separated by the less permeable silt-clayey horizon. 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 events. The formation of fractures would interest also the superficial horizon. Thus, the water exchange would produce erosive phenomena in the superficial material, with removal of the fine fraction. This process could be at the base of the holes opening.

A contribution to the comprehension of the Paganico (Lucca plain, Tuscany)

GIANNECCHINI, ROBERTO;PUCCINELLI, ALBERTO;LO PRESTI, DIEGO CARLO;SQUEGLIA, NUNZIANTE;D'AMATO AVANZI, GIACOMO ALFREDO
2009

Abstract

Paganico is a little village located in the south-eastern portion of the Lucca plain (north-western Tuscany, Italy). Since the sixties, it is interested by a rather unusual phenomenon: the opening of little holes and collapses in the ground surface. At the beginning, such phenomena were sporadic, but since the eighties their frequency and intensity increased. At present, many zones of the Paganico area are interested by holes and collapses. Their size sometimes reaches 2 m in diameter and depth. The opening of the holes mainly occurs at the end of the dry season, during or immediately after the first important and intense rainfall events, which normally characterize, in this area, the end of the summer and the beginning of the autumn. At present, the collapse phenomena are producing many problems to the population and agricultural activity of the area, but their causes are not so clear. For such reasons, this research provides a contribute in order to comprehend the triggering mechanism and evolution of the holes, with relation to the stratigraphic, hydrogeological and geotechnical features of the materials involved. It seems clear that the holes opening is directly linked to the strong pumping through an aquifer with particular hydro-structural conditions, as pointed out by the geological, hydrogeological and geotechnical surveys carried out in this area. The Paganico underground is in fact characterized by three horizons with different lithologic, hydrogeological and geotechnical features: a superficial silt-sandy horizon (1-3 m thick), which is particularly interested by the collapses; an intermediate silt-clayey horizon (1-3 m thick); a lower and thick gravel-pebbly horizon, interested by an important water circulation and heavy pumping. Since the seventies, such water pumping has considerably rose, due to the local demographic (wellfield), and, above all, industrial development (paper manufacture). Therefore, this area is characterized by the presence of two water tables: a temporary one, located in the superficial silt-sandy horizon, and a second one, located in the lower gravel-pebbly horizon. Such water tables are separated by the less permeable silt-clayey horizon. 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 events. The formation of fractures would interest also the superficial horizon. Thus, the water exchange would produce erosive phenomena in the superficial material, with removal of the fine fraction. This process could be at the base of the holes opening.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/207972
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