Mixed finite element analysis of a trial embankment at the coastland of Venice, Italy

In the safeguard works currently under way at the inlets of the Venice Lagoon, an instrumented trial embankment was built with the aim of investigating the lagoon subsoil response to loading/unloading and assessing the relevant geomechanical parameters. A 20 m radius, 6.7 m high vertically-walled sand cylinder was constructed on the Venice littoral from September 2002 to March 2003, and removed in June 2007. The movements of the ground surface were accurately monitored at the center and side of the embankment, as well as at a reference benchmark 100 m apart by leveling, GPS, and persistent scatterer interferometry. Moreover, cone and standard penetration tests, geotechnical borings, and standard lab tests were performed to characterize the soil below the embankment. This large amount of information is used to set-up a fully coupled three-dimensional (3D) mixed finite element (MFE) model based on the Biot consolidation equations. Linear piecewise polynomials and the lowest order Raviart-Thomas mixed space are selected to approximate the medium displacement and the fluid flow rate, respectively. The approach ensures an element-wise mass conservative formulation while preserving the practical advantage of low-order interpolation elements. This helps to stabilize the numerical solution and obtain a more accurate calculation of the flow field. A finite difference scheme is used for the integration in time. The model is implemented over the actual lithostratigraphy of the subsurface down to 60 m depth by prescribing the surface loading vs time as a forcing factor. A small adjustment of the geomechanical and hydrological parameters allows for a satisfactory reproduction of most of the observed displacements both in the vertical and horizontal directions despite the simple linear elastic constitutive law used in the modelling approach.