Large scale hydraulic conductivity of the soil deposits of the Venezia Lagoon from numerical back analysis

In the framework of the Mo.S.E. (Electro-mechanical Experimental Module) project for the safeguard of the city of Venezia, an 8.7 m deep basin is being used in the provisional stage for gate caissons precasting. The basin perimeter embankment is delimited by a cofferdam realised by sheet piling on the sea side, and by a CSM (Cutter Soil Mixing) diaphragm wall on the land side. Dewatering operations lasted about 5 months to reach the steady state regime, and they affected the silty foundation deposits which were partially desaturated. The results of FEM numerical analyses of the dewatering operations are presented, focusing on the hydraulic regime and groundwater control. A non-linear analysis was performed, accounting for desaturation of the upper deposit layers, promoted by the dewatering operations. Parametric analyses were run to analyse the effects of hydraulic conductivities ratios and of their absolute values on the dewatering operations. The comparison between recorded data and numerical results allowed for the determination of the large scale hydraulic conductivities of the Venetian deposits, and showed the role played by anisotropy and possible non homogeneous permeability of the barriers on the performance of the cofferdam.