Masters of Science / Radon (222Rn) has been identified as an important factor that could result in a health hazard by studies all around the world. The health risks can be minimised by preventive measures where radon is highly concentrated as in some mines and homes. A study in the diffusion of the inert gas, will give us a better understanding of its possible pathways through soil into the air surrounding mine dumps where the radon releases can become hazardous. Measuring and modelling the radon concentrations in the mine dump soil, can help to deduce the radon flux to identify the problem areas for rehabilitation especially in the cases of gold and uranium mine tailings.

A presente pesquisa faz uma avaliação da influenciado regime de fluxo na estabilidade de uma barragem construída pela disposição hidráulica de rejeitos. Usualmente, essas estruturas são avaliadas com modelos de equilíbrio limite adotando-se, de modo simplificado, uma superfície freática em um regime hidrostático. No entanto, em casos em que a condutividade hidráulica da fundação ou de camadas na base do aterro é relevante em relação à dos rejeitos, o fluxo se torna verticalizado e o gradiente significativo, portanto, a premissa de regime hidrostático se torna não válida. Com o objetivo de avaliar a influência desses fatores, foram realizadas análises numéricas, simulando condições de fluxo e seus reflexos nos fatores de segurança. Ademais, foi desenvolvida uma extensa análise de dados e cálculos no estudo de caso de uma barragem, com expectativa de ampliar a vida útil de uma Planta de beneficiamento de minério de ferro. Ao se incorporar a real distribuição de pressões neutras, medidas na barragem, concluiu-se que o seu Fator de Segurança era cerca de 20% maior do que o obtido em cálculos simplificados, que desconsideram o regime de fluxo descendente, admitido, por hipótese, na barragem em estudo. Alguns comentários quanto à segurança à liquefação foram feitos, exceto a liquefação dinâmica, que foge do escopo desse trabalho. / The present research evaluates the influence of the foundations permeability in the scheme of flow and in the stability of a dam built through the hydraulic tailings disposal. Usually, these structures are evaluated with models of equilibrium limit, considering, in a simplified way, a phreatic surface in a hydrostatic scheme. However, in cases in which the foundations hydraulic conductivity is relevant when related to the tailings, the flow becomes verticalized and the gradient becomes significant. Therefore, the hydrostatic scheme premise becomes not valid. With the purpose of evaluating the influence of these factors, numerical analysis were made, simulating flow conditions and their influence in the factor of safety. Moreover, an extensive data and calculations analysis was developed in the case study of a dam, with the expectation of increasing the lifetime of an ore mining beneficiation plan. When incorporating the actual distribution of porepressure, measured at the dam, it was concluded that the safety factor of the dam was about 20% higher than that obtained for simplified calculations, that disregard the downward flow regime, admitted in the dam under study. Some comments about the safety of liquefaction were made, except the dynamic liquefaction, beyond the scope of this work.

A presente pesquisa faz uma avaliação da influenciado regime de fluxo na estabilidade de uma barragem construída pela disposição hidráulica de rejeitos. Usualmente, essas estruturas são avaliadas com modelos de equilíbrio limite adotando-se, de modo simplificado, uma superfície freática em um regime hidrostático. No entanto, em casos em que a condutividade hidráulica da fundação ou de camadas na base do aterro é relevante em relação à dos rejeitos, o fluxo se torna verticalizado e o gradiente significativo, portanto, a premissa de regime hidrostático se torna não válida. Com o objetivo de avaliar a influência desses fatores, foram realizadas análises numéricas, simulando condições de fluxo e seus reflexos nos fatores de segurança. Ademais, foi desenvolvida uma extensa análise de dados e cálculos no estudo de caso de uma barragem, com expectativa de ampliar a vida útil de uma Planta de beneficiamento de minério de ferro. Ao se incorporar a real distribuição de pressões neutras, medidas na barragem, concluiu-se que o seu Fator de Segurança era cerca de 20% maior do que o obtido em cálculos simplificados, que desconsideram o regime de fluxo descendente, admitido, por hipótese, na barragem em estudo. Alguns comentários quanto à segurança à liquefação foram feitos, exceto a liquefação dinâmica, que foge do escopo desse trabalho. / The present research evaluates the influence of the foundations permeability in the scheme of flow and in the stability of a dam built through the hydraulic tailings disposal. Usually, these structures are evaluated with models of equilibrium limit, considering, in a simplified way, a phreatic surface in a hydrostatic scheme. However, in cases in which the foundations hydraulic conductivity is relevant when related to the tailings, the flow becomes verticalized and the gradient becomes significant. Therefore, the hydrostatic scheme premise becomes not valid. With the purpose of evaluating the influence of these factors, numerical analysis were made, simulating flow conditions and their influence in the factor of safety. Moreover, an extensive data and calculations analysis was developed in the case study of a dam, with the expectation of increasing the lifetime of an ore mining beneficiation plan. When incorporating the actual distribution of porepressure, measured at the dam, it was concluded that the safety factor of the dam was about 20% higher than that obtained for simplified calculations, that disregard the downward flow regime, admitted in the dam under study. Some comments about the safety of liquefaction were made, except the dynamic liquefaction, beyond the scope of this work.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2016. / Gold mining of the Witwatersrand Basin reefs has been responsible for the rise of Johannesburg as an economic centre of South Africa. While mining provided a base for business and infrastructure development for the region, it has also generated social and environmental problems for the country. Tailings storage facilities (TSFs), a common sighting around Johannesburg and across the entire basin, have been built to contain the processed waste following extraction of gold from the pyrite containing quartzite ore. When the fine grained waste is exposed to atmospheric conditions, oxidation of remnant sulphides occurs resulting in acidic, metal rich and sulphate rich plumes that enter the environment through surface and groundwater systems. This thesis sought to better understand the release, transport, dispersal and fate of metals emanating from TSFs and their remnant footprints on the Witwatersrand. These metals included aluminium, copper, chromium, iron, manganese, nickel and uranium and are known to be toxic to humans depending on their concentration and speciation. Traditionally, analytical methods have been employed in studies focussing on the characterisation of some of these processes in the region. While these studies have generally conducted quantitative assessment of the extent of pollution, little comprehensive interrogation and fingerprinting of the processes that are influential in determining the potential risk posed by metals has been done. This has largely been due to the shortcomings of analytical methods to determine these. To this end, this research has employed geochemical modelling to complement the traditional analytical methods. The approach to study the release of metals from TSFs involved assessment of the partitioning of metals within tailings and their potential release using batch and sequential extraction methods. Processes of metal release within the tailings were simulated through geochemical modelling (using the PHREEQC and Geochemist’s Workbench codes). The simulations were based on the percolation of rainwater through these layers and the changes in its chemistry along the path. The potential seepage of this plume along the path was then correlated to observed efflorescent mineral crusts that are temporary sinks for metals and are a common feature in the vicinity of the tailings and water bodies such as ponds and streams. The potential impact of the mineral crusts on the water chemistry of receiving water systems following their dissolution was assessed using forward geochemical modelling. The transport of the metals in groundwater was also studied. This involved simulations of the transition in chemistry of a plume from a TSF along an aquifer of known composition. This was based on a 1-D reactive transport model constructed using information from sequential extraction work on the aquifer rock (to identify the key minerals to consider) and site data (mainly flow rates) from previous studies. The processes occurring in the removal of metals from acid mine drainage (AMD) through a permanent sink in the form of a pump-and-treat plant in the Central Goldfield of the basin were simulated using PHREEQC. The findings from the research showed that two different plumes were produced from an abandoned TSF as a result of rainwater percolation, notably a plume produced from the dissolution of secondary salts formed in the oxidised layer and a sulphuric acid rich plume in the unoxidised layer. These differences were apparent in the geochemical composition of the mineral crusts collected on the walls of tailings dumps and from a pond into which the plumes were draining. On dissolution, mineral crusts were found to produce acidic solutions with crusts containing predominantly Fe producing pH values below 3. The simulated dissolution of various types of mineral crusts gave insight into the impact of minerals present in the smallest amount. This showed that the bulk mineralogy as determined by analytical techniques such as PXRD and remote sensing could not be used with confidence to deduce the impact of the mineral crusts on receiving water bodies. The characteristics of surface plumes released from tailings TSF were compared to other water systems in the area around Soweto, with complementary interpretation conducted using chemometric methods. From principal component analysis (PCA), surface water systems were found to form distinct groups largely influenced by mineral solubility, alkalinity and dissolved oxygen content. The 1-D reactive transport simulations involved acidic, metal and sulphate rich water ingressing the aquifer (below the TSF). Several scenarios were modelled including simulations with different dolomite contents; allowing for surface complexation and the presence of cation exchange surfaces. At a point 500 m from the water ingress in the dolomite rich aquifer, Fe and Mn were largely precipitated out (as confirmed by sequential extraction results on the aquifer rock) while the sulphate concentration was reduced by almost half. On the other hand, Ca concentrations were conservative largely because of continuous dissolution of dolomite and precipitation of gypsum along the flow path. The simulations of the high density sludge treatment plant involved forward modelling of the treatment process with the sludge responsible for the removal of trace metals from the incoming acid mine drainage. The model can be of use for cost and process optimisation at the facility. This research has had notable outputs in the form of publications; models on metal release, transport and attenuation; and models on pump-and-treat processes. These will form an important repository of information and for benchmarking any further studies related to AMD. / MT2016

Today mines produce metals which are used for everyday products by people worldwide. When metals are produced, waste products known as tailings are generated. These tailings are commonly stored in impoundments, surrounded by embankment dams. The demands from the society are constantly increasing regarding the quality and safety of dams. One step in development towards a better control regarding safety and quality of tailing dams, could be to compact the beach. Today there is a lack of methods to easy check the dry densities over large areas for a compacted material. Since these dams can be large structures with embankment lengths of several kilometers, it is necessary to be able to check the density with a fast method. In this thesis it was investigated if the dry density, and correspondingly the compaction, can be checked with a Dynamic Cone Penetrometer (DCP). In the thesis the dry density of tailings was compared with penetration rate of the DCP. A laboratory setup was made with a test box filled with tailings provided by Boliden AB from the Aitik mine. Six different box tests were performed, each test with different compaction. From the tests a trend was observed, for which the tailings increased in density as the DCP indexes showed an increased resistance. A relationship between DCP index and dry densities was found. Though a relationship was found, it is important to emphasize that the tests were carried out in an environment that was easy to control. One test with high water content showed that water was influencing the DCP index results. Therefore it was concluded that if the DCP will be used in field, the water content also has to be checked.

[pt] A pesquisa apresenta uma análise do comportamento dinâmico da barragem de rejeitos Alpamarca localizada em região sismicamente ativa no centro do Peru. Estudos de ameaça sísmica constituem um aspecto fundamental no projeto de barragens de rejeito tendo em vista os grandes prejuízos ambientais, econômicos e sociais que a ruptura da estrutura, por carregamento dinâmico, possa produzir. Nesta pesquisa foram feitos estudos probabilísticos de ameaça sísmica utilizando o software de código aberto Openquake, que inclui leis de atenuação específicas para as condições geológicas do Peru. A simulação do comportamento dinâmico da barragem demandou a geração de terremotos de projeto com base no espectro de acelerações uniformemente provável determinado na investigação de ameaça sísmica e em função da classificação da barragem em relação a níveis de risco. No trabalho também são discutidas técnicas de tratamento dos acelerogramas, com o objetivo de diminuir o esforço computacional dos cálculos no domínio do tempo, porém sem perder precisão requerida para os resultados estimados. / [en] The research presents an analysis of the dynamic behavior of the Alpamarca tailings dam located in Junín, central Peru, because of the catastrophic effects of earthquakes that frequently occur on the western coast of South America. Dam from the 4670 m elevation. Up to the elevation 4703 m. Under the sea level, with the purpose of supplying the demand for tailings generation in the mining area. The tailings dams are important structures due to the storage of contaminant residues that would shave the environment in the event of a collapse or rupture, which is indispensable to carry out studies to characterize their seismic behavior. In this research, probabilistic seismic threat studies were performed using the OpenQuake software, working with the regional seismicity and soil attenuation law of Peru. Dynamic behavior analysis was performed using the FLAC 2D software, based on the finite difference method. The simulation of the seismic behavior of the dam is made by discussing several important aspects that should be considered for a correct analysis such as the selection of the project earthquake, the filtering of high frequencies to minimize the number of elements of the mesh, the introduction of Quiet conditions, the choice of constituent models including the incorporation of hysteretic damping, among other points.

Assessment of post-liquefaction residual strength is needed for the development of empirically-based, predictive correlations for earthquake engineering design. Previous practice commonly assigned negligible strengths to liquefied materials for engineering analysis, producing overly-conservative designs. Increasingly available case history data, and improved analytical tools have allowed for more accurate and less overly-conservative estimation of soil residual strength, improving empirical predictive models. This study provides a new case history to the limited suite of (approximately 30) liquefaction failure case histories available for post-liquefaction in-situ strength predictive correlations. This case history documents the Las Palmas gold mine tailings dam failure, resulting from seismic-induced liquefaction during the moment magnitude 8.8 February 27, 2010 Maule, Chile earthquake; the sixth largest since 1900. Forensic analysis provides reasonably well-constrained values of 1) back-calculated representative post-liquefaction residual strength, 2) representative penetration resistance, and 3) representative vertical effective stress along the suspected liquefied failure surface. This study employs the incremental momentum method to incorporate momentum effects of a moving soil mass. The incremental momentum method requires a series of cross sections animating the geometry of failure progression from initiation to termination, converging on the observed final geometry. Using interpreted soil strength characteristics, an iterative procedure approximates the back-calculated value of post-liquefaction residual strength. Findings of this case history plot well with existing empirical deterministic regression charts and are in general agreement with previous, related efforts. Results yield representative, well-constrained values of: 1) post-liquefaction residual strength ≈ 173 psf, 2) penetration resistance of N1,60,CS ≈ 5 and N1,60 ≈ 2.5, and 3) vertical effective stress ≈ 4,300 lb/ft2, or ≈ 2.0 atm.

In mining industry, waste storage is a very prominent issue / in this respect, safety of storage structures is one of the leading problems in the industry. Most of the tailings dams require remedial measures, throughout their lifespan to increase their reliability. The objective of the study is to investigate stability problems of formerly constructed but newly raised Eti Copper Mine tailings dam and alternative dam geometries for future raises. Plenty of methods were developed to analyze the reliability of structures / limit equilibrium methods, finite element methods and finite difference methods are among them. In this case, stability of the dam was analyzed with finite element method under static loading conditions. In order to determine input parameters properly, disturbed samples obtained at the field investigations were used. For this purpose, several laboratory experiments were conducted to determine natural moisture content, grain size distribution, specific gravity, Atterberg limits, maximum dry density and shear strength parameters of tailings and embankment material.

[pt] O Peru está localizado em uma área de alta atividade sísmica conhecida como cinturão de fogo. Os terremotos registrados com diferentes mecanismos de falha resultam da interação entre as placas tectônicas de Nazca e Sul-americana. É nesta região onde acontecem atividades de mineração no país, juntamente com a construção de grandes estruturas civis, como barragens de rejeitos, projetadas para armazenar grande quantidade de material na menor área possível. Neste trabalho, foi avaliado o comportamento sísmico de uma barragem de rejeitos localizada em Arequipa, Peru, construída com o método a montante. Em evento sísmico ocorrido em 2013, a barragem sofreu trincas no seu corpo, recalques na crista e pequenos vulcões de areia, indicando a redução dos parâmetros de resistência no rejeito devido a possível fenômeno de liquefação. Nesta pesquisa, a estimativa da ameaça sísmica foi feita por métodos probabilísticos, com o terremoto artificial de projeto obtido por método de ajuste espectral. Foram feitas análises pseudo-estáticas de estabilidade dos taludes, bem como estimados deslocamentos permanentes e o potencial de liquefação dinâmica por métodos simplificados. A análise global da barragem de rejeitos foi executada pelo método dos elementos finitos, considerando o modelo constituivo UBC3D-PLM para simular o comportamento mecânico dos rejeitos sob carregamento cíclico. / [en] Peru is located in an area of high seismic activity known as the fire belt. Earthquakes recorded with different failure mechanisms result from the interaction between the Nazca and South-American tectonic plates. It is in this region where mining activities take place in the country, along with the construction of large civil structures, such as tailings dams, designed to store large amounts of material in the smallest possible area. In this work, the seismic behavior of a tailings dam located in Arequipa, Peru, built with the upstream method, was evaluated. In a seismic event that occurred in 2013, the dam suffered cracks in its body, settlement in the crest and small sand boils, indicating the reduction of the tailings resistance parameters due to possible dynamic liquefaction. In this research, the seismic hazard assessment was made by probabilistic methods, with the artificial design earthquake obtained by the spectral adjustment method. Pseudo-static analyzes of slope stability were performed, as well as the estimated permanent displacements and the potential of dynamic liquefaction were obtainded by simplified methods. The global seismic analysis of the tailings dam was carried out through the finite element method, considering the UBC3D-PLM constitutive model to simulate the mechanical behavior of tailings under cyclic loading.

Liquefaction is a major challenge in geotechnical engineering in which soil strength and stiffness are compromised due to earthquake activity. Understanding and predicting the behaviour and liquefaction susceptibility of soils under cyclic loading is a critical issue in civil engineering, mining and protective engineering. Numerous earthquake-induced ground failure events (e.g., substantial ground deformation, reduced bearing capacity) or liquefaction in natural fine-grained soils or manmade fine-grained soils (i.e., fine tailings) produced by mining activities have been observed and reported in the literature. Tailings are manmade soils that remain following the extraction of metals and minerals from mined ore in a mine processing plant. Traditionally, such tailings are stored in surface tailings impoundments at the mine’s surface. However, geotechnical and environmental risks and consequences related to conventional tailings impoundments have attracted the attention of the engineering community to develop novel methods of tailings disposal and management to minimize geotechnical and environmental risks. Thus, engineers have introduced and implemented innovative tailings technologies—thickened tailings and paste tailings—as cost-effective means for tailings management in mining operations. As both thickened tailings and paste tailings have lower water content and higher solid content than tailings in conventional impoundments, these tailings may be more resistant to liquefaction. However, it should be noted that the seismic or cyclic behaviour of these thickened and paste tailings, with and without heavy rainfall effects, are not fully understood. There is little technical information or data about the behaviour and liquefaction of thickened and paste tailings under seismic or cyclic loading conditions. The objective of the present PhD research is to investigate the response of layered thickened and paste tailings deposits, with and without heavy rainfall effects, to cyclic loads by conducting shaking table tests. To simulate the field deposition of thickened and paste tailings, tailings were deposited in three thin layers in a flexible laminar shear box (FLSB) attached to the shaking table equipment. A sinusoidal seismic loading at a frequency of 1 Hz and peak horizontal acceleration of 0.13g was applied at the bottom of the layered tailings deposits. Acceleration, displacement and pore water pressure responses to the cyclic loading were monitored at the middle depth of each layer of the tailings deposits. Regarding the acceleration response of these thickened and paste tailings deposits (without the effect of heavy rainfall), there was no difference between the middle of the bottom and middle layers or at the base of the shaking table. However, the acceleration at the middle of the top layer differed from the acceleration at the base of the shaking table. Throughout shaking, the layered tailings deposits (with and without the effect of heavy rainfall) exhibited contraction and dilation responses. The excess pore water pressure ratios of the layered thickened tailings deposit that was not exposed to heavy rainfall prior to shaking were found to exceed 1.0 during shaking. However, for the layered paste tailings deposit that was not exposed to the effect of heavy rainfall prior to shaking, the excess pore water pressure ratios were found to be lower than 0.85 during shaking. This reveals that without the effect of heavy rainfall, the layered thickened tailings deposit was susceptible to liquefaction, whereas the layered paste tailings deposit was resistant to liquefaction during shaking. The excess pore water ratios of the layered thickened and the paste tailings deposits that were exposed to heavy rainfall prior to shaking were found to be lower than 0.8 during shaking. This reveals that with the effect of heavy rainfall, the layered thickened and paste tailings deposits were resistant to liquefaction during shaking. The results and findings of this PhD research thus provide valuable information for the implementation of tailings in earthquake-prone areas.