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Earthen river embankments are typically in unsaturated conditions during their lifetime and the degree of saturation within their bodies may vary significantly throughout the year, due to seasonalfluctuations of the river stage, as well as infiltrations of meteoric precipitation and evapotranspiration phenomena. Given the significant effects of partial saturation on the hydro-mechanical behaviour of soils, realistic assumptions on the actual water content distribution inside the embankments are essential forproperly modelling their response to hydraulic loadings. In this framework, centrifuge modelling is a useful tool to get insights into the evolution of saturation conditions of a water retaining structure during flood events. It allows for the direct observation of the groundwater flow process, which is hardly detectable at the prototype scale, enabling, at the same time, the validation and calibration of predictive numerical tools.In this paper, the results of a centrifuge test carried out on small-scale physical model of a compacted silty clayey sand embankment subjected to a simulated high-water event, at the enhanced gravity of 50-g, are presented and discussed. The physical model was carefully instrumented with potentiometers, miniaturized pore pressure transducers and tensiometers. Pore pressures and suctions measured during the experiment showed that the stationary flow conditions were reached only after an unrealistic hydrometric peak persistence. It therefore emerges that, for the design and/or the assessment of the safety conditions of a river embankment similar to the one tested, the simplified hypothesis of a steady-state seepage, in equilibrium with the maximum river stage expected could result, in many cases, an excessively conservative assumption.

In the last few decades there has been a rapid increase in the number of piled foundations where the piles have been employed as settlement reducers; in some recent projects, the piles have been separated from the raft by a granular layer, which creates a more uniform pressure distribution on the raft bottom and reduces constraint reactions in the soil, foundation, and superstructure. A series of centrifuge model tests has been performed to investigate the load transfer mechanisms between a square rigid raft and a group of instrumented piles jacked in dry dense sand, in direct contact with the raft or separated f rom the raft by an interposed granular layer. The test results have shown that contact piles act as settlement reducers by diffusing the load applied to their heads to greater and deeper volumes of soil. The insertion of a deformable layer between a raft and pile heads does not ensure displacement compatibility, and the pressure diffused by the granular fill acts partly on the pile heads and partly produces shallow soil settlements, which mobilize negative skin friction on the upper part of the pile shaft. Noncontact piles act mainly as soil reinforcement. Key words: model tests, piled raft, granular layer, jacked piles in sand, negative friction. Au courant des dernieres annees, il y a eu une augmentation rapide du nombre de fondations sur pieux dans lesquelles les pieux sont utilises comme reducteur de tassement; de meeme que dans des projets recents, les pieux ont ete separes du radier par une couche de materiau granulaire pour creer une distribution plus uniforme des pressions sur le bas du radier et pour reduire les reactions de contrainte a l'interieur du sol, de la fondation et de la superstructure. Des essais a centrifuge ont ete effectuees afin d'eevaluer les mecanismes de transfert de charge entre un radier rigide carre et un group e de pieux instrumentees installees dans un sable dense sec, en contact direct avec le radier ou separes du radier par une couche interposee de materiau granulaire. Les resultats des essais ont montre que les pieux en contact direct avec le radier agisse nt en tant que reducteur de tassement en diffusant la charge appliquee sur leurs tetes a un plus grand volume de sol et plus en profondeur. L'insertion d'une couche deformable entre le radier et la tete des pieux ne garantit pas la compatibilite des deplacements, et la pression diffusee par le materiau granulaire agit en partie sur la teste des pieux et produit en partie des tassements peu profonds qui mobilisent le frottement superficiel neegatif dans la portion supeerieure du pieu. Les pieux sans contact servent principalement de renforcement du sol. Mots-cles: essais par modeles, radier sur pieux, couche granulaire, pieux enfoncees dans du sable, friction negative. [Traduit par la Redaction], Introduction When piles are used as settlement reducers, the objective of the design is to achieve an economic foundation by minimizing the number of piles while fully utilizing their bearing [...]