## 1 Introduction  
Many industrial applications rely on fluidized particle bed processes, in most cases induced by a uniform fluid injection [1; 2]. The configuration, corresponding to the particular situation where an ascending liquid flow creates a partial fluidization of a granular material, has motivated a large number of studies [3; 4; 5; 6; 7; 8; 9]. The specific situation of the transient expansion phase of the homogeneous fluidization, which corresponds to the case where the injection size reaches the total width of the domain, has attracted all our attention. In this more general topic of hydromechanical instabilities within a drained granular material, we studied, experimentally and numerically, the antagonistic roles of the fluidization and the dilatancy of an initially dense or loose material, controlled by the geometrical confinement imposed by the walls.  
Localized fluidization by a gas is also present in some direct industrial applications via spouted bed processes and has resulted in a large number of studies [10; 11; 12; 13; 14; 15]. Furthermore, for hydraulic structures in earth, the hydraulic flow of infiltration into the foundations generates downstream of the structure an overpressure below the surface layer of the soil, which can cause an overall lifting of the structure granular and possibly hydraulic fracturing (internal erosion) [16; 17; 18]. Moreover, this underground internal flow, under the effect of a sufficiently high-pressure gradient, can also locally fluidize the soil and form vertical chimneys, possibly leading to the surface (phenomenon of sand boil). On the other hand, water leaks in underground pipes, for the distribution and drain of wastewater, can erode granular soil particles. Underground cavities can develop and affect the surrounding infrastructures. A better understanding of fluid-grain coupling and its role in the development of hydromechanical instabilities can help to answer the concrete problem of safety of hydraulic structures in which localized fluidization at the foot appears as a mechanism initiating regressive erosion. This soil erosion problem can be studied in the laboratory through a model based on a granular stack subjected to an upward liquid flow.  
The present study focuses on the dynamics of destabilization by homogeneous fluidization of a submerged granular stack in dense regime. The collapse or partial breakage of a civil engineering structure in the presence of water infiltration by fluidization is modeled. More specifically, we were studying the transient expansion and propagation phases of homogeneous fluidization within an immersed dense granular medium. This study of the fluid-grain interaction is motivated by the fact that the resulting phenomena are sometimes at the origin of the appearance of hydromechanical instabilities. A three-dimensional model, based on the coupling of two particulate methods, EDEM and the CFD-FLUENT code, has been developed to realistically report and confirm the predominant physical mechanisms involved in the experimental studies. EDEM, used for describing the solid particle dynamics, is a commercial software of discrete element method (DEM) expanded for taking into account the bulk material simulation. The continuous model, based on the Eulerian multiphase models code in CFD-FLUENT, is used for solving the fluid dynamics equation in confined geometry, more precisely in the pore space between grains.