A case study on soil settlements induced by preloading and vertical drains

The method of preloading associated with prefabricated vertical drains was used to accelerate the consolidation process and anticipate the long-term settlements of the foundation soil of two cylindrical oil tanks founded on an alluvial deposit mainly consisting of silty clays. In-situ investigations, including boreholes and cone penetration tests (CPTs), and laboratory tests were carried out to define the geotechnical profile of the construction site and the soil mechanical properties. Dissipation tests were also carried out during the CPTs and allowed evaluating the horizontal consolidation and permeability coefficients through several procedures. An extensive field monitoring of the site was carried out during the embankment construction, the preloading period and, after the embankment removal, during the hydraulic leakage test of the tanks. Differential settlements and angular distortions of the tank foundation evaluated from the measured settlement profiles were compared with expected profile shapes for tanks overlying homogeneous compressible soil layers and with available empirical relationships. A general fair agreement was observed even if the heterogeneity of the alluvial soil deposit affects the tank response. Observed absolute and differential settlements and distortions are consistent with the allowable limits provided by the literature and with the design prescriptions, thus confirming the effectiveness of the preloading and drainage technique adopted in the project and envisaging a satisfactory performance of the tank under service conditions.