Etudes numeriques des effets de la configuration des couvertures a effets de barrieres capillaires inclinees sur la diffusion de l'oxygene

Acid mine drainage (AMD) generation resulting from the oxidation of sulphide tailings constitutes one of the main environmental problems facing the mining companies in the world. Different techniques including covers with capillary barrier effects (CCBEs) were developed to limit the environmental impacts of AMD during the last years. The goal of this study is to show the influence of various parameters affecting the diffusion of oxygen through CCBE.;The effect of the inclined CCBE geometry (slope and length of slope) was studied by numerical simulations on models having inclinations of 10, 18, and 30 degrees, and lengths of 15m, 30m, and 50m. The results obtained in drainage condition corresponding to 46 days of drought (as applied by in situ tests on the LTA site) showed that the more the inclination and the length of the slope increased, the more the CCBE was desaturated, particularly in the top zone of the slopes. This desaturation induced an increase in cumulative oxygen fluxes. While previous studies showed that the length of the slope had little effect on the hydraulic behaviour of the inclined CCBE, the results obtained here, based on the X oxygen fluxes, indicate that the length of the slope has a significant effect (even if it is weak compared to that of the slope inclination).;To attenuate the desaturation in top of slope, a suction break made of bentonitic geocomposite was built in the middle of the slope in the water retention layer. The suction break created a zone of water accumulation which brought back suctions around 0 and thus improved saturation. The studies performed showed a reduction in cumulative oxygen fluxes of a maximum of 15% at the end of a 46 days drainage period for the slopes with high inclination angles. Moreover, the localised effect of suction break was highlighted as in previous studies. The position and the influence of two breaks were also considered. When breaks were installed apart from their mutual zones of influence, it was noted that their effects on cumulative oxygen fluxes were cumulated.;The effect of reactivity of the tailings under the inclined CCBE was studied by considering tailings with 7 values of apparent reactivity K r* between 0/year and 10000/year. The results of the oxygen cumulative flux show that the reactivity of tailings plays a key role in the diffusion of oxygen through the CCBE. As the value of the apparent reactivity of tailings increases, the cumulative oxygen fluxes at the base of the CEBC are increased, whatever the drainage duration considered. However, there was very little variation in fluxes when the reactivity increased beyond a value of apparent reactivity Kr *around 2000/year.;The hydraulic properties of the material used for the water retention layer is also a determining factor in the role of barrier to the diffusion of oxygen from a CCBE. Three types of material having air entry values (AEV) of 1.2 m, 6 m, and 3 m and having different saturated hydraulic conductivities were studied. Numerical simulations indicated that the desaturation of the water retention layer and the cumulative oxygen flux decreased when the AEV increased. The oxygen flux calculated on all models studied involving the material with a 6m AEV are almost zero.;The numerical models also helped to highlight the effect of the thickness of the water retention layer on the cumulative oxygen flux at the base of the CCBE. Thicknesses of 40 cm, 80 cm, and 160 cm were considered, and the results showed that an increase in the thickness induced a considerable decrease in the cumulative oxygen fluxes reaching the tailings. Furthermore, the use of a reactive CCBE led to a further reduction in the fluxes, because part of the oxygen was consumed by the cover.;All these studies were used to determine the efficiency of various inclined CCBE studied. When the cover efficiency is obtained by comparing the cumulative oxygen fluxes on the surface of uncovered tailings and at the. CEBC-tailings interface, the efficiency is very high and exceeds 90% for Kr* = 200/year and a water retention layer made by the MNR tailings. The geometry greatly influences the effectiveness of the cover. Indeed, the more the inclination and/or the longer the slope, the lower the effectiveness of the CCBE. On the other hand, when the cover efficiency is obtained by comparing the cumulative oxygen flux from previously mentioned models to those of a model in which the water retention layer is maintained at a constant degree of saturation of 85%, the CCBE studied are less effective after some drainage time, which decreases when the angle of the slope increases. (Abstract shortened by UMI.)...