A coupled MFE poromechanical model of a large-scale load experiment at the coastland of Venice

A three-dimensional fully coupled mixed finite element (MFE) model based on Biot's consolidation equations is implemented to simulate the geomechanical response of a large-scale 5-year long loading/unloading test performed at the Venice coastland, Italy. The model uses linear piecewise polynomials and the lowest order Raviart-Thomas mixed space to represent the porous medium motion and the groundwater flow rate, respectively. The approach ensures an element-wise mass conservative formulation while preserving the stability of the numerical solution and providing at the same time an accurate calculation of the flow field. With the aim of characterizing the Late Pleistocene and Holocene deposits above, which the MoSE project, i.e. the mobile barriers to protect Venice from acqua alta, is under implementation, a 20-m radius, 6.7-m tall vertically walled cylinder was built from September 2002 to March 2003 and removed in June 2007. The maximum load exerted on the ground at the completion of the building activity was 0.105 MPa. The land displacements were accurately monitored at various depths and the center and outer boundary of the embankment by sliding deformeters, leveling, global positioning system, and persistent scatterer interferometry. Moreover, in situ tests and standard lab tests were performed to define the hydrological and geomechanical properties of the soil underlying the cylinder. The model addresses the actual lithostratigraphy of the subsurface down to 50-m depth below the embankment and prescribes the land surface loading versus time as an external source of strength. A hysteretic elastic constitutive law, with the Young modulus E in the loading phase between 2 to 36 Mpa according to lithology, a ratio s=15 of loading to unloading cycle E, and a small adjustment of the hydrological parameters allow to predict quite satisfactorily most of the observed pressure behavior, together with vertical and horizontal displacements.