Lo stato idrologico del sottosuolo nelle aree a ridosso dei cantieri

In the Mo.S.E. project for the protection of Venice against exceptional high tides, the development of Treporti (Lido’s inlet) and Ca’ Roman (Chioggia’s inlet) harbours in the Venice Lagoon required the realization of two provisional sea basins (cofferdams) which will be drained to provide access to the building sites, positioned below the sea level. The dewatering systems installed on the perimeter of the basins, which have been active since 2005 in Punta Sabbioni and since 2007 in Ca’ Roman, permitted to dry up the cofferdams, where the concrete lodging caissons of mobile barriers are to be built. Two groundwater monitoring systems have been emplaced to measure hydraulic heads in the first and second aquifer levels, in order to monitor the effects of the dewatering systems in the proximity of the constructions sites. The monitoring network is composed of more than twenty piezometers intercepting both aquifer layers in Punta Sabbioni (Lido’s inlet) and two in Ca’ Roman (Chioggia’s inlet). The monitoring systems emplaced in both sites permitted to discriminate between the effects of natural driving forces (rainfall, atmospheric pressure, tidal oscillations) from the anthropogenic ones (construction and dewatering of the cofferdam). The effects of construction works in the first phreatic aquifer have been highlighted by shallow piezometers. The measurements indicate a partial isolation of this groundwater body from the sea with a consequent increase in the hydraulic head and a decrease in the groundwater salinity. The deep piezometers next to the cofferdam showed instead a decrease of hydraulic head due to dewatering operations. The lecture will describe the natural and anthropogenic system driving forces, the characteristics of the piezometric monitoring network and of the installed instrumentation, and will analyse in the details the groundwater level monitoring operations relating them with the pre-established high and low level thresholds.