The sedimentary structure of Upper Pleistocene-Holocene deposits in Venice and its effects on the stability of the historic centre

The results of a recent study (Geologia e Progettazione nel centro storico di Venezia", Secondo Convegno Nazionale: La riqualificazione delle cittA e dei territori, Venezia 7 dicembre 2007, Quad. IUAV 54, Il Poligrafo ed. 2008) focused on Venice's historic centre have led to the identification of a peculiar sedimentary structure (multistorey sandbody). This structure was created during the Upper Pleistocene by the multiple overlapping of sandbodies corresponding to stream channels, all of which crossed the area where the city now lies. The erosion and deposition events responsible for these stratigraphic conditions are the effects of both climatic variations during the Wurmian glaciation and changes in the fluvial regime. The sedimentary structure was completed during the Holocene by the deposition of sand in fluvial and tidal channels on top of the previous Pleistocene stream channel sediments. The deposits of this structure are heteropic with those of the rhythmic succession (cyclothemic organization) belonging to the alluvial plain of the Upper Pleistocene and the Holocene lagoon facies of tidal plain found in the surrounding area. The geological model of the city is able to explain the mechanical behaviour of the soil in the western and eastern areas of the historic centre where the losses in elevation are considered at present as a consequence of the urban development in the last centuries, since the vertical movements due to groundwater exploitation of the deep aquifers have more than likely ended. According to a methodology known as operational facies concept, which first determines the facies relationship, then discriminates the intervals with lithological variability and finally fits together the information coming from the terrain properties, it can be inferred that the losses in elevation are related to the evolution of the deformation processes acting nowadays in the urban settlement. The results of the aforementioned study reveal, in fact, that these processes are provoked by the residual component of a long-term secondary consolidation and by the geochemical subsidence which occurs between salt water intrusion and clay containing organic matter and peat.