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The heterogeneity of waste rock piles is due to the wide and variable grain size distribution of waste rock and construction methods leading to complex internal structures. The general objective of this work was to better understand the effect of such heterogeneity on the coupled transfer processes acting within waste rock piles producing Acid Mine Drainage (AMD). For this purpose, parametric numerical simulations were conducted with the TOUGH AMD numerical simulator, considering 1) three random spatial distributions of the same material properties to assess the resulting behavior, 2) four ranges of material properties with the same spatial distribution to evaluate the effect of the degree of heterogeneity, and 3) the effect of compacted layers due to circulation of heavy equipment during construction. Results show that fine-grained (denser with lower permeability) material present near the boundary of a pile can limit air entry. Coarse materials promote preferential flow of gas and water vapor. Fine-grained materials beneath the pile surface favor the internal condensation of water vapor and thus minimize water loss. The initiation of secondary gas convection cells requires a minimal degree of heterogeneity, which is closely related to the range of permeability between the coarse and the finer material ratio (k(coarse)/k(fine)). The presence of coarse grained material in the pile does not necessarily lead to more convection and higher AMD production. The magnitude of convection rather depends on the amount of fine-grained material and its distribution in the pile. Results also show that low-permeability compacted layers strongly limit convection. Results thus support waste rock pile construction methods integrating fine-grained materials or compacted layers to minimize AMD production.

A fatal accident at a mine waste rock dump was thought to be due to the downward low of O 2deicient air originating from the dump. A numerical model tested the plausibility of various physical mechanisms hypothesized to control gas low in the dump. The main initial hypotheses were that gas low leading to the incident was due to the sole or combined effect of changes in barometric pressure, temperature, or till cover water saturation.

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The occurrence of thermally driven convective air flow within waste rock or natural soil profiles has been well established; however, the potential impact that convective air flow may have on water storage within reclamation soil covers has not been previously explored. We conducted a numerical modeling study to evaluate the effect that convective air flow may have on stored water within a soil reclamation cover placed over a coke stockpile at an oil sands mine in Alberta, Canada. Coke is a carbon, sand‐like byproduct of heavy oil processing. Two‐dimensional simulations of thermally driven convective air flow were conducted for two different field sites based on available field data. The elevated temperature within the coke stockpile resulted in the development of strong convective air flow cells that drew in drier atmospheric air over the lower slope positions while releasing it across the upper slope and plateau areas of the cover. The magnitude of the gas flux and the intensity of the convection within the cell were a function of the air permeability of the coke and cover material, the depth of the coke, and the slope of the stockpile. It was estimated that convective air movement through the cover could produce as much as 1 to 2 mm/d of enhanced drying of the cover in lower slope positions. Field observations of water content distributions within the cover provided corroboration that the cover has undergone enhanced drying at lower slope positions.