Nuove ipotesi sulla formazione dei piping sinkhole in aree alluvionali: il caso della Piana di Forino (Avellino, Campania)

Abstract

S. Del Prete et al., New genetic hypothesis about piping sinkhole formation in alluvial plain: the case study of Forino
(Avellino, Campania) (IT ISSN 0394-3356, 2008).
This paper deals with sinkholes phenomena occurred in the intramontane basin of Forino village (Campania, Southern Italy). A new
genetic hypothesis about piping sinkhole formation in alluvial plain is proposed.
Aiming at the definition of the underground stratigraphy, some boreholes have been drilled down to the depth of 50 meters. They
showed the presence of an alluvial silty and sandy body laying on a impermeable substratum made up of tuff (Campanian Ignimbrite)
and Miocene flysch deposits. Therefore, in the first 50 m of the depth, the presence of calcareous bedrock and caves can be certainly
excluded.
Eight sinkholes have been observed in the area; some of them are recent, other are older and completely filled in with reworked material.
Some of the boreholes were carried out inside and around the more recent sinkhole (12 June 2005) and they showed it had started forming at about 25 m of depth, affecting sandy and silty alluvial deposits.
The studied sinkholes are lined up with the flow direction of the groundwater located within the alluvial body at the base of the
northern slope of M. Romola. The last one is the sector more susceptible for this phenomena.
The collected data suggest that the sinkholes developed because the underground water circulation, probably with turbulent regime,
produced erosion phenomena in the sandy-silty alluvial deposits, causing void formation. This circulation is only active during rainy day periods and is located at the contact between the impermeable substratum (top of Campanian Ignimbrite end miocenic flysch) and the sandy and silty alluvial deposits. Probably this water table is also fed laterally by groundwater coming from the carbonate massifs surrounding the plain.
As far as the problem of volumes removed (some thousands of m3 ) is concerned, the presence of buried ponors, along the eastern
edge of the endoreic basin, which can transfer in depth the sandy materials, is hypothesised.
The results of this study represents a first step for new researches aimed at recognizing the presence of other underground voids that
could be very near to the topographic surface producing high risk situations.