The Sinkhole Projects: results achieved

In Italy, natural sinkhole phenomena which are not strictly related to karst types are frequent (downfall dolinas) due to the strong, semi-permeable sedimentary soils covering the karstificable bedrock (deep piping sinkhole or sinkhole s.s.).
These phenomena are more common than what was originally believed. They occurred in our country a long time ago, when the inducing genetic causes and mechanisms were still unknown. Historical sources confirm that catastrohpic sinkholes were known since the Roman age. With a centenary frequence, they have affected the same areas where the first phenomena had been either artificially or naturally cancelled. APAT registered and studied about 800 cases of natural sinkholes in plain areas. These have been identified as sinkhole s.s phenomena. On-site surveys and detailed analyses were carried out on the first 320 cases. After the on-site surveys, not all studied cases resulted to be sinkhole s.s. Some of the registered phenomena were classified as anthropic sinkholes, volcanoes, evorsion and suffosion phenomena (by bank route) or purely karstic phenomena.
Analysed phenomena were then grouped into 173 risk areas over the whole national territory.
Risk areas are more concentrated on the Tyrrhenian side and particularly in the regions of Lazio, Abruzzo, Campania and Toscana. Due to their particular geological structure, the Adriatic side and the Alpine and Dolomitic arc are not affected by these types of sinkholes.In Northern Italy (where on-site surveys have still not been carried out and where the census is still in course) conditions are different. In the plains of the Veneto and Emilia-Romagna regions, especially in the Po valley where the river Po flows into the Adige, there are several small sub-circular lakes. Their formation may be caused by evorsion processes (erosional phenomena related to vertical turbulence) of sedimentary bodies characterized by a moderate thickness of sandy material and/or siphoning and suffosion processes. In the plains and internal valleys of the regions of Veneto, Friuli and the Autonomous Province of Bolzano sinkhole phenomena are strictly controlled by the disintegration of evaporitic and carbonatic lithotypes that are found under a cover with a generally modest thickness. These are therefore referable to cover-collapse sinkholes. The phenomena that were found in Calabria, instead, are referable to small cavities that have filled up today. They are difficult to locate and almost all originated during earthquakes and are related to land liquefaction phenomena. The geological context appears substantially different in Sicily and Puglia, where sinkhole cases are influenced by the presence of evaporitic lands (chalk and salt) and by thinner clayey or sandy covers. These phenomena are concentrated in valleys between mountains, in alluvial valleys and in coastal plains. Subordinately, some phenomena were found on piedmont strips connecting with plain areas and in small hollows between hills. A common feature is the tectonic origin of the surveyed areas, which are structurally controlled by predominantly Apenninie and subordinately meridian faults. The distribution of more unique phenomena over vast areas has enabled the identification of sinkhole alignments and of risk areas on tectonic outlines of regional importance. They cover an area of several chilometres. These are: the Aterno Fault, the Pontine Fault, the Fiamignano-Micciani line and its extension up to the Fucino plain and the Ancona–Anzio line. This leads us to believe that these phenomena could be related to deep upflow mechanisms and active seismogenetic structures. In many cases, a close connection was found between an earthquake and the induction of the phenomena (136 cases). The soil responded to the event within 24 hours but many cases show that sinkholes can occur even ten days after the earthquake (up to over one month after the earthquake). In fewer cases, a connection was found through the alternation of dry and wet periods. Cover sediment thicknesses, mainly consisting of floods mixed with intervals of different granulometry, are generally as thick as one hundred metres and sometimes even thicker.
In most cases, a cavity drowning process takes place with formation (in many cases) of springs creating an upflow of mineralized liquids. The chemical composition of waters has mainly resulted to be of Calcium Bicarbonate type.