Mining & Process Engineering

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    DEM simulations of critical state behaviour of granular materials under various drained triaxial stress path tests
    (2022) Kodicherla, Shivaprashanth K; Gong, Guobin; Wilkinson, Stephen
    The present study investigates the critical state behaviour of granular assemblies composed of clumped particles under four different drained axisymmetric triaxial stress paths, using the discrete element method (DEM). A series of numerical samples were prepared at initial states with different density indexes (𝐼𝐷) and different initial confining pressures (𝑝𝑜 ′ ). These samples were sheared to large strains, at which constant stresses and volumes were maintained to reach the critical state. The evolution of stress ratio under the same loading mode (for the same intermediate principal stress ratio, b) is shown to yield an almost identical behaviour independent of stress paths, whereas the stress-strain response depends on the stress paths. Four different axisymmetric stress paths all share the same unique friction angle at critical state, indicating the Mohr-Coulomb failure criterion is the appropriate critical state strength criterion, which is at least true for the axisymmetric stress conditions. A unique coordination number (CN) is achieved at the critical state for a given 𝑝𝑜 ′ , which is independent of the stress path. The critical state CN is found to increase with the increase in 𝑝𝑜 ′ , which could be attributed to the decrease in the critical state void ratio (ec) as mean effective stress (𝑝 ′ ) increases. Interestingly, a unique linear functional relationship is found between the critical state values of CN and ec, and a unique polynomial functional relationship is found between the critical state values of CN and 𝑝 ′ . These functional relationships indicate no dependency on the stress paths or loading modes, thus characterizing unique features at critical states at both macroscopic and microscopic levels for a given type of granular material.
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    Geochemistry and potential environmental impact of the mine tailingsat Rosh Pinah, southern Namibia
    (Elsevier, 2015-02-26) Nejeschlebová, L; Sracek, O; Mihaljevic, M; Ettler, V; Kríbek, B; Knésl, I; Vanek, A; Penízek, V; Dolnícek, Z; Mapanie, B
    Mine tailings at Rosh Pinah located in semiarid southern Namibia were investigated by the combinationof mineralogical methods and leaching using water and simulated gastric solution. They are well-neutral-ized with leachate pH > 7 and neutralization potential ratios (NPR) up to 4. Neutralization is mainly dueto abundant Mn-rich dolomite in the matrix. Concentrations of released contaminants in water leachatefollow the order Zn > Pb > Cu > As. Relatively high leached concentrations of Zn and partly also of Pb arecaused by their link to relatively soluble carbonates and Mn-oxyhydroxides. In contrast, As is almostimmobile by binding into Fe-oxyhydroxides, which are resistant to dissolution. Barium is released bythe dissolution of Ba-carbonate (norsethite) and precipitates in sulfate-rich pore water as barite.Dissolved concentrations in neutral mine drainage water collected in the southern pond are low, butwhen total concentrations including colloidal fraction are taken into account, more than 70% of Zn isin colloidal form. Groundwater upgradient of the mine tailings is of poor quality and there seems tobe no negative impact on groundwater downgradient from mine tailings.Contaminant concentrations in simulated gastric leachates are in the order Ba > Pb > Zn > Cu > As witha maximum gastric bioaccessibility of 86.6% for Ba and a minimum of 3.3% for As. These results demon-strate that total contaminant content and toxicity in the solid phase are poor predictors of risk, and there-fore mineralogical and bioavailability/bioaccessibility studies are necessary for evaluation ofcontaminant environmental impact.
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