Etude du comportement de couvertures a effets de barriere capillaire placees sur des haldes a steriles en climat semi-aride (French text)

In the mining industry, covers with capillary barrier effects (CCBE) may be used to isolate potentially toxic tailings, and reactive rock wastes. They can protect the environment from the effects of acid mine drainage (AMD) which is formed in presence of sulphide minerals, water and oxygen, causing the contamination of mining effluents. Capillary effects created in CCBEs appear when an unsaturated water flow occurs in materials for which contrasts in the saturated hydraulic conductivities exist (i.e. fine grained material over a coarser grained material). These effects limit water infiltration or gas flux, thus allowing control of AMD formation.; This research project focuses on the study of the short term behaviour of covers with capillary barrier effects (CCBE) placed on waste rock piles under semi-arid (and arid) climate. More than one third of the continents' surface is affected by these climates, characterized by low and irregular precipitations, excessive temperatures and occasional violent rainfalls. The objective of this work is to better understand the influence of precipitation, hydraulic properties and thickness of the water retention layer on the efficiency of CCBE; especially the «store, divert, and release» (SDR) type which particularly limits water infiltration. The numerical models are based on a typical waste dump, and calculations have been performed with the SEEP/W software from GEO-SLOPE. The variation of the DDL position («Down Dip Limit») has been observed; this is the position beyond which a multi-layer capillary cover efficiency is decreasing. The study shows the existence of a treshold precipitation rate value, specific to a given problem (configuration and hydraulic properties of the CCBE); capillary effects are reduced above this value. The greater the precipitation intensity, the faster capillary effects tend to disappear. This phenomenon is also linked to the saturated hydraulic conductivity and the thickness of the CCBE.