Modélisation numérique des phénomènes de ségrégation et de stratification dans les haldes à stériles

Astract Mining activities typically need excavation works to access the ore. Those operations produce large quantities of waste rock which does not have any major economic value. Such material is generally stored at the surface with repeated granular dumps building waste rock piles. Waste rock dumping is an operation known to cause the development of segregation phenomena among the material which favors the concentration of coarser fraction particles, or blocks, at slopes toe. Eventually, waste rock pilesexpose particular granular patterns, segregated or stratified, attributed to waste rock physical properties disparities between each particle. The development of segregation and stratification phenomena in waste rock piles represents indeed a risk, since grains heterogeneous distributions within the dumped structure might alter its geotechnical and hydrogeological properties, depending on piles areas, therefore leading to mechanical and hydrogeological sources of instabilities. This work aims to develop numerical models able to reproduce segregation and stratification mechanisms by simulating granular heaps disposal. The study has begun by reproducing experimental trials able to demonstrate grading phenomena development through granular material pouring in quasi-2D cells. Qualitative comparison of obtained particles heaps configurations with similar former published works has demonstrated tests consistency. Results has been quantified by measuring repose angles and stratification patterns wavelengths. Developing numerical models to further investigate granular material disposal has been decided. Numerical models have been created thanks to the open-source software LMGC90, developed at the university of Montpellier in France. LMGC90 is an alternative for simple or complex dynamic systems modelling which has proven itself capable of solving problems involving granular mixtures similar to those considered within the scope of this study. Simulations have been performed by means of a discrete-element method (DEM) known as contact dynamics (CD). Results interpretation and formatting requested the use of various programming languages such as Python and Fortran. The numerical study considered 2D and 3D modelling geometries.LMGC90 allowed to create granular mixtures whose physical properties have been determined to generate samples with grain size and shape dispersities. The main numerical model simulates a dumping scenario and varying pouring parameters: flow regimes, injection height, cell's dimensions (disposal environment). Waste rock piles thus reproduced expose patterns which have been investigated qualitatively and quantitatively through multiple indices. Grains aspect distribution in terms of sizes or shapes have been drawn within each deposit. Granular flows kinematics during piles construction were also examined. Granular mixtures properties and dumping parameters influence on the development of grading phenomena have been discussed according to simulations results. Complementary works have also widened the scope of the study on segregation phenomena in granular media thanks to the development of a second numerical model. This new set of simulations models an elementary granular volume under simples hearing using the same samples previously built. Shearing parameters have been defined with the computation of an associated inertial number for each simulation. The evolution of segregation with various inertial number values and pressures applied during shearing was studied. Segregation and stratification phenomena numerical DEM study notes three dimensional (3D) models to be relevant designs to reproduce granular mixtures grading mechanisms in discharge operations. Completed simulations present segregation patterns. Such result is observed with consistency despite deposition parameters variations in each scenario. The close control on pouring settings seems necessary in order to reproduce mechanisms leading to stratified patterns in rock piles. Shape dispersity in bidisperse samples appears to be a catalyst for stratification. Shearing simulations show segregation kinematics evolution in bidisperse samples as a function of the imposed inertial number. Shearing and segregation localizations are shown for specific pressure conditions. Study's results plan to serve as a basis for the resolution of challenges addressing grading phenomena prediction in waste rock disposal operations. Various model examples and interpretation methods presented constitute a range of solutions able to characterize grains ordering patterns in waste rock piles and their intensity. Future works may consider to create reliable numerical warning tools for the prediction, and description, of segregation and stratification phenomena in waste rock dumps.