Study Of Hydro Mechanical Instabilities Caused By Fluidization In Immersed Granular Areas

The overall objective of the study is to better understand fluid-grain coupling and its role in the initiation and the development of hydro mechanical instabilities of vertical fluidization type in a granular area subjected to an internal liquid flow.The specific objectives were to:assess the risk of rupture of structures in earth caused by fluidization phenomenon in order to predict an initiation to the discontinuity of the dam foundation and predict the evacuation alert;simulate the environmental impact of dam failure in order to determine the evolution of the flow,the water volume and height and to predict the evacuation alert;determine the influence of the granular stack’s initial density on the transient homogeneous fluidization in order to preserve and protect civil engineering structures(dam,protective dikes)by reproducing the condition in which they collapse or partially break in the presence of water infiltration by fluidization;determine an alternative experimental approach in order to remove the porous structure’s optical access restriction by using the hydro-gels beads,composed of about 99%of water and having substantially the same refraction indexes,as solid phase;and develop a numerical coupling approach for the study of destabilization by localized fluidization of an immersed dense granular material and the transient expansion and propagation phases of homogeneous fluidization within an immersed granular medium in order to realistically report and confirm the predominant physical mechanisms involved in the experimental studies.Hydro mechanical instability is a degradation process at the heart of the problem of internal erosion of earthen hydraulic structures.Understanding its mechanisms requires a rigorous description of the coupling and interaction between fluid and soil particles.For this purpose,to do finer analyzes,a series of experiments were carried out using an internal visualization technique of a granular stack combining the adjustment of the refractive index of the two phases(liquid and solid).A first experimental approach has consisted to use hydrogel beads,composed of about 99%water and having almost the same refractive index as water,and LED lighting with the ultimate aim of visualizing the evolution of the internal structure of the granular medium without resorting to either a two-dimensional geometry or a costly laser lighting system and nor to a mixture of several liquids.Moreover,siliceous sand with a density of 2500 kg/m3 and an average diameter of 1.6×10-3 m has constituted a granular sample for the study of the influence of initial density of granular stack on a transient regime of homogeneous fluidization.Other series of experiments are in progress.Over and above that,in order to overcome the difficulties of experimentally measuring the complex collective movements resulting from interactions between grains,numerical modeling has been performed.A model of hydro mechanical coupling of two particle methods(EDEM and FLUENT),FLUENT treating the fluid part and the EDEM relating to the solid part,makes it possible to calculate the hydrodynamic interactions between the fluid and the particles and the inter-grain contact interactions.In most cases,fluidization has been caused by a liquid flow at the base of the granular stack through a small section relative to the dimensions of the system.However,in the limiting case of homogeneous fluidization,the injection diameter reached the total width of the domain.The steady state reached by a locally fluidized grain layer has been observed and analyzed through the spatio-temporal development resulting from an image processing performed with the free software ImageJ developed by the National Institute of Health.The findings on environmental dynamics revealed that the granular stack’s initial density influences the transient homogeneous fluidization.Depending on the material initial volume fraction,there was a difference in grain dynamics.For an initially loose stack,homogeneous turbulent fluidization was observed,whereas,for an initially dense stack,there was a mass takeoff of the stack.The propagation of wave porosity instability,from the bottom to the top of the stack with fast kinetics that decompacted the medium,followed this mass takeoff.Similarly,the findings on the investigation on the piping phenomenon predicted an initiation to the discontinuity of the dam foundation,where the hydraulic gradients are the strongest.Further,the hydraulic piping is difficult to detect and evolves very quickly,which leaves little time to act against it.When it happens,it is often too late.Further,a real economic alternative experimental approach for the experimental study of fluid-grain coupling during destabilization by localized fluidization of granular material has been created.The optical access restriction is removed.To visualize the evolutions of the internal structure of the granular medium,the hydro-gels beads,composed of about 99%of water and having substantially the same refraction indexes,are used as solid phase.A LED lighting system is used in place of a laser lighting system.The findings on the risk assessment following a rupture scenario of a dam(case of Mali Selingue dam)revealed that the magnitude of the damage caused is strongly related to the distance separating each zone to the dam.The extent of the damage generated in the closest parts to the dam is approximately five to six times greater than that of the others zones.In just a few hours after the break,the water level in the closest zones can reach more than 1 m then the low altitude areas of this zone should be urgently evacuated.Moreover,a three-dimensional model coupling of the two particulate methods:the discrete element method(EDEM)to describe the mechanical behavior of the particle assembly and the FLUENT method to render an account of the hydrodynamics of the interstitial fluid,has been developed for carrying out a numerical study of the destabilization mechanism.