Gold mine tailings material facilities are characterized by sparse vegetation and an abundance of dust. Mine tailings facilities are examples of extreme geotechnical and geochemical conditions which make it almost impossible for higher plants to establish and grow without rehabilitation intervention. In most cases higher plants such as grasses and trees are the focus areas for rehabilitation, but, having a look at something a little smaller such as biological crusts, it is seen that these micro-organisms play very important roles in any ecosystem. Various studies have shown that biological crusts, consisting of micro-organisms such as lichens, algae and cyanoprokaryotes enhance the soil quality by binding soil particles together, forming aggregates which counteract the erosive forces of wind and water. They play a part in nitrogen and carbon fixation, increase the soil surface temperature and increase the water retention of the soil. Thus, these organisms improve the overall health of the soil, which will in time encourage the successful establishment of higher plants. The aim of this study was to investigate the presence of cyanoprokaryotes and soil algae on mine tailings storage facilities that have been rehabilitated for different periods of time as well as to correlate the presence of these species with the physical and chemical characteristics of the mine tailings material. Chemical, physical and biological analyses of soil samples were done. Some of the ecologically important and dominant species were isolated and protocols were developed in order to identify the most successful manner in which to re-inoculate the organisms to a chosen substrate and how to measure biomass. Due to the immense cost of standard rehabilitation practices there is a need for a more cost effective, sustainable manner in which to protect the tailings material against the erosive forces of wind and water with as little input as possible. The influence of an organism cultured in normal Bold’s Basal medium (BBM) growth medium, BBM growth medium with half the phosphate concentration and BBM growth medium with half the nitrate concentration on the establishment of a biological soil crust (BSC) was tested. To test the influence of the inoculums already present in the tailings material and in the air, trials with mulch, water and nutrients without the addition of an organism was also investigated. This was done in the controlled environment of a glasshouse, as well as in field conditions. The biomass of the cyanoprokaryotes and algae, as well as the soil surface strength was also tested. The results show that the time of rehabilitation did not have an influence on the cyanoprokaryotes as well as algal species that occurred on the tailings material. Chlorella sp., Chlorococcum sp. and Klebsormidium sp. were present on all six sites, except on the fresh material and 15 year old material where no rehabilitation has been done. As for dominance; Chlamydomonas sp., Chlorococcum sp., Klebsormidium sp. and Phormidium sp. were dominant on all six sites except for the fresh material, where nothing grew. An array of methods exists for measuring algal biomass as a measure of growth. During the development of protocols for further use in investigating the growth of algae, the extraction solvent ethanol, for use in chlorophyll a extraction, was identified as the most sufficient. The re-inoculation of cyanoprokaryotes and soil algae onto a chosen substrate is most successful when pouring the organisms, cultured in growth medium and 0.1% agar, over the substrate. During the glasshouse trials the influence of the growth medium and growth medium with half the nitrate and half the phosphate concentrations showed that Chlamydomonas sp. produced the highest biomass when cultured in BBM. With Nostoc sp. the highest biomass occurred with culturing in BBM and BBM with half the phosphate concentration. Microcoleus vaginatus showed no significant difference when cultured in the three different growth mediums (BBM, BBM with half the nitrate concentration and BBM with half the phosphate concentration). Overall Nostoc sp. produced the highest biomass (34.33 μg/g), followed by Microcoleus vaginatus (17.05 μg/g) and Chlamydomonas sp. (6.12 μg/g). Soil surface strength, measured with a hand held penetrometer showed that Chlamydomonas sp. cultured in BBM growth medium produced the most stable crust (2.58 kg/cm2), although it had the lowest biomass measurements (6.12 μg/g). Nostoc sp. produced the highest biomass (34.44 μg/g), but had the lowest soil surface strength results (1.75 kg/cm2). Microcoleus vaginatus proved to be the species with high biomass production (17.05 μg/g), as well as high soil surface strength (2.08 kg/cm2). M. vaginatus is also a pioneer species and is therefore a good choice as primary inoculum on bare tailings material. It was decided to use Nostoc sp. in the field trials due to its high biomass and Microcoleus vaginatus due to the high soil surface strength produced. Despite the occurrence of a severe thunder storm on the afternoon of application and poor water management during the field trials the significance of water on the establishment of soil algae and cyanoprokaryotes on tailings material was determined. / MSc (Environmental Sciences), North-West University, Potchefstroom Campus, 2014

Gold mine tailings material facilities are characterized by sparse vegetation and an abundance of dust. Mine tailings facilities are examples of extreme geotechnical and geochemical conditions which make it almost impossible for higher plants to establish and grow without rehabilitation intervention. In most cases higher plants such as grasses and trees are the focus areas for rehabilitation, but, having a look at something a little smaller such as biological crusts, it is seen that these micro-organisms play very important roles in any ecosystem. Various studies have shown that biological crusts, consisting of micro-organisms such as lichens, algae and cyanoprokaryotes enhance the soil quality by binding soil particles together, forming aggregates which counteract the erosive forces of wind and water. They play a part in nitrogen and carbon fixation, increase the soil surface temperature and increase the water retention of the soil. Thus, these organisms improve the overall health of the soil, which will in time encourage the successful establishment of higher plants. The aim of this study was to investigate the presence of cyanoprokaryotes and soil algae on mine tailings storage facilities that have been rehabilitated for different periods of time as well as to correlate the presence of these species with the physical and chemical characteristics of the mine tailings material. Chemical, physical and biological analyses of soil samples were done. Some of the ecologically important and dominant species were isolated and protocols were developed in order to identify the most successful manner in which to re-inoculate the organisms to a chosen substrate and how to measure biomass. Due to the immense cost of standard rehabilitation practices there is a need for a more cost effective, sustainable manner in which to protect the tailings material against the erosive forces of wind and water with as little input as possible. The influence of an organism cultured in normal Bold’s Basal medium (BBM) growth medium, BBM growth medium with half the phosphate concentration and BBM growth medium with half the nitrate concentration on the establishment of a biological soil crust (BSC) was tested. To test the influence of the inoculums already present in the tailings material and in the air, trials with mulch, water and nutrients without the addition of an organism was also investigated. This was done in the controlled environment of a glasshouse, as well as in field conditions. The biomass of the cyanoprokaryotes and algae, as well as the soil surface strength was also tested. The results show that the time of rehabilitation did not have an influence on the cyanoprokaryotes as well as algal species that occurred on the tailings material. Chlorella sp., Chlorococcum sp. and Klebsormidium sp. were present on all six sites, except on the fresh material and 15 year old material where no rehabilitation has been done. As for dominance; Chlamydomonas sp., Chlorococcum sp., Klebsormidium sp. and Phormidium sp. were dominant on all six sites except for the fresh material, where nothing grew. An array of methods exists for measuring algal biomass as a measure of growth. During the development of protocols for further use in investigating the growth of algae, the extraction solvent ethanol, for use in chlorophyll a extraction, was identified as the most sufficient. The re-inoculation of cyanoprokaryotes and soil algae onto a chosen substrate is most successful when pouring the organisms, cultured in growth medium and 0.1% agar, over the substrate. During the glasshouse trials the influence of the growth medium and growth medium with half the nitrate and half the phosphate concentrations showed that Chlamydomonas sp. produced the highest biomass when cultured in BBM. With Nostoc sp. the highest biomass occurred with culturing in BBM and BBM with half the phosphate concentration. Microcoleus vaginatus showed no significant difference when cultured in the three different growth mediums (BBM, BBM with half the nitrate concentration and BBM with half the phosphate concentration). Overall Nostoc sp. produced the highest biomass (34.33 μg/g), followed by Microcoleus vaginatus (17.05 μg/g) and Chlamydomonas sp. (6.12 μg/g). Soil surface strength, measured with a hand held penetrometer showed that Chlamydomonas sp. cultured in BBM growth medium produced the most stable crust (2.58 kg/cm2), although it had the lowest biomass measurements (6.12 μg/g). Nostoc sp. produced the highest biomass (34.44 μg/g), but had the lowest soil surface strength results (1.75 kg/cm2). Microcoleus vaginatus proved to be the species with high biomass production (17.05 μg/g), as well as high soil surface strength (2.08 kg/cm2). M. vaginatus is also a pioneer species and is therefore a good choice as primary inoculum on bare tailings material. It was decided to use Nostoc sp. in the field trials due to its high biomass and Microcoleus vaginatus due to the high soil surface strength produced. Despite the occurrence of a severe thunder storm on the afternoon of application and poor water management during the field trials the significance of water on the establishment of soil algae and cyanoprokaryotes on tailings material was determined. / MSc (Environmental Sciences), North-West University, Potchefstroom Campus, 2014

This study is focused on developing a method to predict the dynamic behavior of mine tailings dams under earthquake loading. Tailings dams are a by-product of coal mining and processing activities. Mine tailings impoundments are prone to instability and failure under seismic loading as a result of the mechanical behavior of the tailings. Due to the existence of potential seismic sources in close proximity to the coal mining regions in the United States, it is necessary to assess the post-earthquake stability of these tailings dams. To develop the aforementioned methodology, 34 cyclic triaxial tests along with vane shear tests were performed on undisturbed mine tailings specimens from two impoundments in Kentucky. Therefore, the liquefaction resistance and the residual shear strength of the specimens were measured. The laboratory cyclic strength curves for the coal mine specimens were produced, and the relationship between plasticity, density, cyclic stress ratio, and number of cycles to liquefaction were identified. The samples from the Big Branch impoundment were generally loose samples, while the Abner Fork specimens were dense samples, older and slightly cemented. The data suggest that the number of loading cycles required to initiate liquefaction in mine tailings, NL, decreases with increasing CSR and with decreasing density. This trend is similar to what is typically observed in soil. For a number of selected specimens, using the results of a series of small-strain cyclic triaxial tests, the shear modulus reduction curves and damping ratio plots were created. The data obtained from laboratory experiments were correlated to the previously recorded geotechnical field data from the two impoundments. The field parameters including the SPT blow counts (N1)60, corrected CPT cone tip resistance (qt), and shear wave velocity (vs), were correlated to the laboratory measured cyclic resistance ratio (CRR). The results indicate that in general, the higher the (N1)60 and the tip resistance (qt), the higher the CSR was. Ultimately, practitioners will be able to use these correlations along with common state-of-practice geotechnical field methods to predict cyclic resistance in fine tailings to assess the liquefaction potential and post-earthquake stability of the impoundment structures.

Platinum mining activities contribute substantially to South Africa's economy since it exceeded gold as economical contributor in 2001. Mining activities contribute to large amounts of waste production in the form of tailings and rock waste, deposited in the surrounding environment of the mine premises. Mining companies are held responsible for damages caused to the surrounding environment. These companies are required to introduce the cost of ecological rehabilitation in their operation costs as well as compile an environmental management plan. Numerous attempts to rehabilitate mine waste have proven unsuccessful. New and improved rehabilitation techniques are required to facilitate in the rehabilitation of these mine spoils. Woodchip-vermicompost produced from platinum mining wastes (woodchips and sewage sludge) was used as an alternative amendment to inorganic fertilisers during the rehabilitation of platinum mine tailings. The effectiveness of the woodchip-vermicompost as an alternative amendment during the platinum mine tailings rehabilitation were monitored using different ecosystem components. A natural veldt in the vicinity of the mine area was randomly selected to serve as a reference site. These ecosystem components selected have previously been shown to be effective as indicators of ecosystem quality. The components selected for this study includes the use of microbial enzymatic activity, microbial community structure, nematode trophic structures, and other mesofaunal groups such as micro-arthropods. The physical and chemical properties of the platinum mine tailings and reference area as well as the vegetation cover of the platinum mine tailings were determined. Statistical and multivariate analyses were use to determine the correlation between the dependent microbial components and dominate independent chemical properties. Nematode trophic structure, Maturity Index, and Plant-Parasitic nematode Index were used to compare the two rehabilitation techniques in terms of nematodes as indicators. Microarthropods family structures were used to compare the two amendments in terms of diversity and abundance. Enzymatic activity was positively affected by the addition of woodchip-vermicompost, than in the sites treated with inorganic fertilisers. The microbial community structure showed no statistically significant (p < 0.05) differences between the two amendments. A higher abundance of nematodes especially plant-parasitic nematodes and bacterivorous nematodes were observed in the woodchip-vermicompost sites than in the inorganic fertilised sites. According to the Maturity Index, both amendments became more enriched during the study period, while the Plant-Parasitic nematode Index showed that the carrying capacity for plantparasitic nematodes on the woodchip-vermicompost sites increased while it decreased in the inorganic fertilised sites, which can be related to the decrease in vegetation cover on the inorganic fertilised sites. Both coloniser (Prostigmata) and persister (Cryptostigmata and Mesostigmata) groups of the micro-arthropods, as well as a higher diversity of micro-arthropods, were present on the woodchip-vermicompost sites whereas the inorganic fertilised sites showed only the presence of colonisers, with a decrease in diversity and abundance of micro-arthropods over the study. The colonisation of micro-arthropods may have been affected by the addition of woodchip-vermicompost and vegetation cover, which contribute to the establishment of suitable microhabitats for these soil biota. By intercorrelating the results, it may be concluded that the addition of woodchip-vermicompost may be an essential part of the rehabilitation process, by contributing to soil organic material to the ecosystem system, which may improve the recolonisation of soil biota and ecosystem processes. However further studies need to be conducted in order to determine the long-term sustainability of the woodchip-vermicompost in providing organic material and sustaining the ecosystem processes. The study also showed the necessity to integrate various ecosystem components when evaluating ecosystem development due to the unique role each component plays and the impact it may have on other components. / Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2006.

Resíduos de mineração, dispostos em barragens de rejeito, são materiais de baixas densidades e resistências, em geral – muitas vezes, suscetíveis à liquefação. Desta forma, técnicas de melhoramento de solos, tais como o Deep Soil Mixing, surgem como possibilidade à estabilização destes materiais. Neste sentido, baseado nesta técnica, este trabalho visa avaliar o comportamento mecânico de resíduos de mineração de ouro estabilizados com cimento, comparativamente aos rejeitos no estado natural, analisando fatores de influência (quantidade de agente cimentante, umidade inicial das amostras e líquido empregado na moldagem). Para este propósito, ensaios de resistência à compressão simples, de compressão diametral, de ondas ultrassônicas e triaxiais não-drenados (CIU) foram empregados. Os resultados mostram que os resíduos de mineração, nas condições representativas de campo, apresentam baixas resistências e suscetibilidade à liquefação a baixas tensões confinantes. Por sua vez, os ensaios com resíduo estabilizado com cimento revelaram que: o aumento nas taxas de cimentação, a diminuição dos teores de umidade iniciais das amostras e uso de licor proveniente das barragens de rejeito nas moldagens, repercutiram, em geral, em aumentos nas resistências (qu e qt) e na rigidez inicial (G0) das misturas As análises de variância comprovaram que todos os fatores controláveis estudados foram significativos para as respostas avaliadas e demonstraram que a influência da dosagem de cimento foi muito mais pronunciada, em relação às outras variáveis. O comportamento tensão-deformação das misturas, sob condições não-drenadas, foi típico de materiais cimentados submetidos à baixas tensões confinantes – com geração de poro-pressões negativas devidas à tendência à dilatação. Ainda, ficou demonstrado que o índice porosidade/teor volumétrico de cimento (η/Civ) é adequado para a previsão do comportamento mecânico das misturas resíduo-cimento, considerando os parâmetros estudados – ampliando a possibilidade de aplicação deste índice. Foi observado, que existe uma proporcionalidade direta entre as resistências à tração e à compressão das misturas, que independe da relação η/Civ, sendo esta traduzida através de um valor escalar único (ξ = 0,17). / Mine tailings are materials of low in situ densities and strengths, in general – often susceptible to liquefaction. In view of this, soil improvement methods such as Deep Soil Mixing can be alternatives for the stabilization of these materials. In this sense, based on DSM techniques, this research aims to analyze the mechanical behaviour of cemented gold mine tailings, comparing to its natural condition, evaluating factors that influence the stabilization (cement content, initial water content and type of water for preparing specimens). For this purpose, unconfined compressive tests, splitting tensile tests, ultrasonic pulse velocity tests and undrained triaxial (CIU) tests were used. The results showed that remoulded mine tailings samples presented low strength and liquefaction susceptibility under low confining pressures. In its turn, the tests with cemented gold tailings revealed that: increasing the cement content, decreasing initial water content and using water from the tailings pond (liquor) in moulding provided strength (qu and qt) and initial stiffness (G0) gains on the mixtures of gold mine tailings-Portland cement. In this regard, the variance analysis statistically demonstrated that all the factors chosen on the experiment were significant to the parameters assessed It is also showed that the cement content influence was much more pronounced than other factors. Furthermore, the stress-strain behaviour of the blends is typical of cemented materials under low confining pressures and undrained conditions – with negative pore-pressures build-up. In addition, it is demonstrated that the porosity/cement index controls the mechanical performance of gold tailings-Portland cement blends, considering the whole range of densities and cement contents studied – what broadens the applicability of such index. Finally, it was found a single relationship between tensile (qt) and compressive strength (qu) equal to 0.17, being independent of the porosity/cement ratio.

Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2006.

Platinum mining activities contribute substantially to South Africa's economy since it exceeded gold as economical contributor in 2001. Mining activities contribute to large amounts of waste production in the form of tailings and rock waste, deposited in the surrounding environment of the mine premises. Mining companies are held responsible for damages caused to the surrounding environment. These companies are required to introduce the cost of ecological rehabilitation in their operation costs as well as compile an environmental management plan. Numerous attempts to rehabilitate mine waste have proven unsuccessful. New and improved rehabilitation techniques are required to facilitate in the rehabilitation of these mine spoils. Woodchip-vermicompost produced from platinum mining wastes (woodchips and sewage sludge) was used as an alternative amendment to inorganic fertilisers during the rehabilitation of platinum mine tailings. The effectiveness of the woodchip-vermicompost as an alternative amendment during the platinum mine tailings rehabilitation were monitored using different ecosystem components. A natural veldt in the vicinity of the mine area was randomly selected to serve as a reference site. These ecosystem components selected have previously been shown to be effective as indicators of ecosystem quality. The components selected for this study includes the use of microbial enzymatic activity, microbial community structure, nematode trophic structures, and other mesofaunal groups such as micro-arthropods. The physical and chemical properties of the platinum mine tailings and reference area as well as the vegetation cover of the platinum mine tailings were determined. Statistical and multivariate analyses were use to determine the correlation between the dependent microbial components and dominate independent chemical properties. Nematode trophic structure, Maturity Index, and Plant-Parasitic nematode Index were used to compare the two rehabilitation techniques in terms of nematodes as indicators. Microarthropods family structures were used to compare the two amendments in terms of diversity and abundance. Enzymatic activity was positively affected by the addition of woodchip-vermicompost, than in the sites treated with inorganic fertilisers. The microbial community structure showed no statistically significant (p < 0.05) differences between the two amendments. A higher abundance of nematodes especially plant-parasitic nematodes and bacterivorous nematodes were observed in the woodchip-vermicompost sites than in the inorganic fertilised sites. According to the Maturity Index, both amendments became more enriched during the study period, while the Plant-Parasitic nematode Index showed that the carrying capacity for plantparasitic nematodes on the woodchip-vermicompost sites increased while it decreased in the inorganic fertilised sites, which can be related to the decrease in vegetation cover on the inorganic fertilised sites. Both coloniser (Prostigmata) and persister (Cryptostigmata and Mesostigmata) groups of the micro-arthropods, as well as a higher diversity of micro-arthropods, were present on the woodchip-vermicompost sites whereas the inorganic fertilised sites showed only the presence of colonisers, with a decrease in diversity and abundance of micro-arthropods over the study. The colonisation of micro-arthropods may have been affected by the addition of woodchip-vermicompost and vegetation cover, which contribute to the establishment of suitable microhabitats for these soil biota. By intercorrelating the results, it may be concluded that the addition of woodchip-vermicompost may be an essential part of the rehabilitation process, by contributing to soil organic material to the ecosystem system, which may improve the recolonisation of soil biota and ecosystem processes. However further studies need to be conducted in order to determine the long-term sustainability of the woodchip-vermicompost in providing organic material and sustaining the ecosystem processes. The study also showed the necessity to integrate various ecosystem components when evaluating ecosystem development due to the unique role each component plays and the impact it may have on other components. / Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2006.

Resíduos de mineração, dispostos em barragens de rejeito, são materiais de baixas densidades e resistências, em geral – muitas vezes, suscetíveis à liquefação. Desta forma, técnicas de melhoramento de solos, tais como o Deep Soil Mixing, surgem como possibilidade à estabilização destes materiais. Neste sentido, baseado nesta técnica, este trabalho visa avaliar o comportamento mecânico de resíduos de mineração de ouro estabilizados com cimento, comparativamente aos rejeitos no estado natural, analisando fatores de influência (quantidade de agente cimentante, umidade inicial das amostras e líquido empregado na moldagem). Para este propósito, ensaios de resistência à compressão simples, de compressão diametral, de ondas ultrassônicas e triaxiais não-drenados (CIU) foram empregados. Os resultados mostram que os resíduos de mineração, nas condições representativas de campo, apresentam baixas resistências e suscetibilidade à liquefação a baixas tensões confinantes. Por sua vez, os ensaios com resíduo estabilizado com cimento revelaram que: o aumento nas taxas de cimentação, a diminuição dos teores de umidade iniciais das amostras e uso de licor proveniente das barragens de rejeito nas moldagens, repercutiram, em geral, em aumentos nas resistências (qu e qt) e na rigidez inicial (G0) das misturas As análises de variância comprovaram que todos os fatores controláveis estudados foram significativos para as respostas avaliadas e demonstraram que a influência da dosagem de cimento foi muito mais pronunciada, em relação às outras variáveis. O comportamento tensão-deformação das misturas, sob condições não-drenadas, foi típico de materiais cimentados submetidos à baixas tensões confinantes – com geração de poro-pressões negativas devidas à tendência à dilatação. Ainda, ficou demonstrado que o índice porosidade/teor volumétrico de cimento (η/Civ) é adequado para a previsão do comportamento mecânico das misturas resíduo-cimento, considerando os parâmetros estudados – ampliando a possibilidade de aplicação deste índice. Foi observado, que existe uma proporcionalidade direta entre as resistências à tração e à compressão das misturas, que independe da relação η/Civ, sendo esta traduzida através de um valor escalar único (ξ = 0,17). / Mine tailings are materials of low in situ densities and strengths, in general – often susceptible to liquefaction. In view of this, soil improvement methods such as Deep Soil Mixing can be alternatives for the stabilization of these materials. In this sense, based on DSM techniques, this research aims to analyze the mechanical behaviour of cemented gold mine tailings, comparing to its natural condition, evaluating factors that influence the stabilization (cement content, initial water content and type of water for preparing specimens). For this purpose, unconfined compressive tests, splitting tensile tests, ultrasonic pulse velocity tests and undrained triaxial (CIU) tests were used. The results showed that remoulded mine tailings samples presented low strength and liquefaction susceptibility under low confining pressures. In its turn, the tests with cemented gold tailings revealed that: increasing the cement content, decreasing initial water content and using water from the tailings pond (liquor) in moulding provided strength (qu and qt) and initial stiffness (G0) gains on the mixtures of gold mine tailings-Portland cement. In this regard, the variance analysis statistically demonstrated that all the factors chosen on the experiment were significant to the parameters assessed It is also showed that the cement content influence was much more pronounced than other factors. Furthermore, the stress-strain behaviour of the blends is typical of cemented materials under low confining pressures and undrained conditions – with negative pore-pressures build-up. In addition, it is demonstrated that the porosity/cement index controls the mechanical performance of gold tailings-Portland cement blends, considering the whole range of densities and cement contents studied – what broadens the applicability of such index. Finally, it was found a single relationship between tensile (qt) and compressive strength (qu) equal to 0.17, being independent of the porosity/cement ratio.

Resíduos de mineração, dispostos em barragens de rejeito, são materiais de baixas densidades e resistências, em geral – muitas vezes, suscetíveis à liquefação. Desta forma, técnicas de melhoramento de solos, tais como o Deep Soil Mixing, surgem como possibilidade à estabilização destes materiais. Neste sentido, baseado nesta técnica, este trabalho visa avaliar o comportamento mecânico de resíduos de mineração de ouro estabilizados com cimento, comparativamente aos rejeitos no estado natural, analisando fatores de influência (quantidade de agente cimentante, umidade inicial das amostras e líquido empregado na moldagem). Para este propósito, ensaios de resistência à compressão simples, de compressão diametral, de ondas ultrassônicas e triaxiais não-drenados (CIU) foram empregados. Os resultados mostram que os resíduos de mineração, nas condições representativas de campo, apresentam baixas resistências e suscetibilidade à liquefação a baixas tensões confinantes. Por sua vez, os ensaios com resíduo estabilizado com cimento revelaram que: o aumento nas taxas de cimentação, a diminuição dos teores de umidade iniciais das amostras e uso de licor proveniente das barragens de rejeito nas moldagens, repercutiram, em geral, em aumentos nas resistências (qu e qt) e na rigidez inicial (G0) das misturas As análises de variância comprovaram que todos os fatores controláveis estudados foram significativos para as respostas avaliadas e demonstraram que a influência da dosagem de cimento foi muito mais pronunciada, em relação às outras variáveis. O comportamento tensão-deformação das misturas, sob condições não-drenadas, foi típico de materiais cimentados submetidos à baixas tensões confinantes – com geração de poro-pressões negativas devidas à tendência à dilatação. Ainda, ficou demonstrado que o índice porosidade/teor volumétrico de cimento (η/Civ) é adequado para a previsão do comportamento mecânico das misturas resíduo-cimento, considerando os parâmetros estudados – ampliando a possibilidade de aplicação deste índice. Foi observado, que existe uma proporcionalidade direta entre as resistências à tração e à compressão das misturas, que independe da relação η/Civ, sendo esta traduzida através de um valor escalar único (ξ = 0,17). / Mine tailings are materials of low in situ densities and strengths, in general – often susceptible to liquefaction. In view of this, soil improvement methods such as Deep Soil Mixing can be alternatives for the stabilization of these materials. In this sense, based on DSM techniques, this research aims to analyze the mechanical behaviour of cemented gold mine tailings, comparing to its natural condition, evaluating factors that influence the stabilization (cement content, initial water content and type of water for preparing specimens). For this purpose, unconfined compressive tests, splitting tensile tests, ultrasonic pulse velocity tests and undrained triaxial (CIU) tests were used. The results showed that remoulded mine tailings samples presented low strength and liquefaction susceptibility under low confining pressures. In its turn, the tests with cemented gold tailings revealed that: increasing the cement content, decreasing initial water content and using water from the tailings pond (liquor) in moulding provided strength (qu and qt) and initial stiffness (G0) gains on the mixtures of gold mine tailings-Portland cement. In this regard, the variance analysis statistically demonstrated that all the factors chosen on the experiment were significant to the parameters assessed It is also showed that the cement content influence was much more pronounced than other factors. Furthermore, the stress-strain behaviour of the blends is typical of cemented materials under low confining pressures and undrained conditions – with negative pore-pressures build-up. In addition, it is demonstrated that the porosity/cement index controls the mechanical performance of gold tailings-Portland cement blends, considering the whole range of densities and cement contents studied – what broadens the applicability of such index. Finally, it was found a single relationship between tensile (qt) and compressive strength (qu) equal to 0.17, being independent of the porosity/cement ratio.

Résumé : Ce projet de recherche s’inscrit dans le cadre d’une requête de l’entreprise partenaire, Golder Associés Ltée. Cette dernière s’interrogeait sur la consolidation d’un résidu minier épaissi, une approche de plus en plus utilisée dans le domaine de la gestion des aires de déposition des résidus miniers. Ce type de matériau est épaissi jusqu’à un point où une fois déposé, il ne connaît pas ou peu de ségrégation, rejette moins d’eau et possède généralement un angle de repos plus grand que les résidus conventionnels. L’intérêt de cette approche dans le cadre de cette étude consiste en la réduction des empreintes des aires de déposition par la combinaison des résidus épaissis et des rehaussements par l’amont. La consolidation de ce type de résidus a été peu étudiée depuis leur développement. En rassemblant les observations rapportées dans les écrits scientifiques, il est possible de ressortir que le mécanisme physique générant la dissipation des pressions interstitielles en excès et le tassement au moment de la déposition est essentiellement la consolidation sous le poids propre. Un essai en colonne de tassement instrumentée de transmetteurs de pression a permis l’étude de la consolidation sous le poids propre en termes de dissipation des pressions d’eau avec une grande précision. Un essai en consolidomètre a permis de définir la compressibilité du matériau durant cette étape de consolidation sous le poids propre avec une précision acceptable. Les résultats expérimentaux ont été reproduits à l’aide du modèle CS2 suite à quelques ajustements des relations constitutives. CS2 considère la consolidation sous le poids propre et les grandes déformations d’une déposition instantanée. Le rapprochement entre le modèle CS2 et les résultats expérimentaux est sans équivoque et permet de prendre connaissance de certains aspects de la consolidation sous le poids propre qui étaient peu détaillés jusqu'à maintenant dans les écrits scientifiques. L’importance de la consolidation sous le poids propre et son intégration dans le plan de déposition proposé par l’entreprise partenaire a été évaluée à l’aide du logiciel SIGMA/W. Les capacités de ce logiciel ont tout d’abord été étudiées en 1D. Ces vérifications visaient la considération de la consolidation sous le poids propre tout en examinant la formulation de SIGMA/W pour un remplissage progressif. Ces simulations 1D ont pu être comparées aux résultats du modèle CS4, un modèle calqué sur CS2 qui permet un remplissage progressif. Finalement, une campagne de simulation à grande échelle en 2D visant la simulation d’une aire de déposition de résidus épaissis a permis d’examiner les éléments importants dans la considération de ce mécanisme à l’aide du logiciel commercial SIGMA/W. Il en ressort que le logiciel SIGMA/W n’est pas le logiciel idéal pour simuler la déposition progressive de résidus épaissis en considérant la consolidation sous le poids propre et ce pour maintes raisons. Les résultats de cette étude viennent situer l’importance de la consolidation sous le poids propre dans les résidus épaissis et fournissent une base solide pour la compréhension et la modélisation numérique de ce mécanisme en 1D ou en 2D. // Abstract : This research project was done through an academic partnership between the Université de Sherbrooke and Golder Associés Ltée., a mining consultant from Montréal. The latter had interrogations at regarding consolidation mechanisms controlling thickened tailings deposition, a relatively new approach in tailings management. This type of material are thickened to a point from which they don’t show any segregation, they expulse less water and can be stacked with a greater beach angle than conventional tailings. The combination of this method with the upstream raising method results in the possible reduction of the footprint of the tailings disposal facility. Thickened tailings have not been studied extensively. However, by gathering available information from the existing technical literature, it is apparent that the sole mechanism creating both a dissipation of excess pore water pressure and a settlement following deposition is self-weight consolidation. A settling column monitored with pressure transmitters was used to define accurately the self-weight consolidation process in terms of dissipation of excess pore water pressure. A consolidometer setup was used to define the compressibility of this material during the self-weight consolidation. Experimental results were reproduced with the numerical model CS2 which considers the self-weight consolidation and large strains. Following a few adjustments of the constitutive relationships, the numerical reproduction is clear; it represents closely the experimental results. This made it possible to document certain elements that were not well defined in the technical literature about the self-weight consolidation. The use of the finite element model SIGMA/W and its capability to consider self-weight consolidation within a tailings deposition scheme were evaluated. Firstly, the model capabilities were verified through 1D simulations that helped to document the method to adopt to model the self-weight consolidation. The formulation of SIGMA/W related to the sequential tailings placement was also studied during this phase. A comparison with the model CS4 was also performed. Secondly, the tailings disposal facility was modeled at large scale by including the sequential tailings placement and the consideration of the self-weight consolidation for each deposition. This process allowed evaluating the capabilities of SIGMA/W in large scale analysis. It appears that this numerical tool presents some numerical weaknesses especially with regards to the sequential tailings placement. Moreover, the results of this study place the self-weight consolidation of hard rock thickened mine tailings as an important mechanism that needs to be considered in a deposition scheme as it controls the short term displacements of the impoundment. Finally, they document the self-weight consolidation mechanism and provide reliable information for modelling this process in one or two-dimensional numerical analysis.