Tesis sobre el tema "Cemented paste backfills"

This thesis presents the results of an investigation of factors which influence the geotechnical properties of frozen mine backfill (FMB). FMB has extensive application potential for mining in permafrost areas. The uniaxial compressive strength (UCS) of hardened backfill is often used to evaluate mine backfill stability. However, the deformation behaviour and stiffness of the FMB are also key design properties of interest. In this thesis, uniaxial compressive tests were conducted on FTB and FCPB samples. Information about the geotechnical properties of FMB is obtained. The effects of FMB mix components and vertical compression pressure on the geotechnical properties of FMB are discussed and summarized. An optimum total water content of 25%-35% is found in which the strength and the modulus of elasticity of the FTB are 1.4-3.2 MPa and 35-58 MPa, respectively. It is observed that a small amount (3-6%) of cement can significantly change the geotechnical properties of FTB.

Mining has been one of the main industries in the course of the development of human civilization and economies of various nations. However, every industry has issues, and one of the problems the mining industry has faced is the management of waste, especially sulphide-bearing tailings, which are considered to be a global environmental problem. This issue puts pressure on the mining industry to seek alternative approaches for tailings management. Among the several different types of methods used, cemented paste backfilling is one of the technologies that offers good management practices for the disposal of tailings in underground mines worldwide. Cemented paste backfill (CPB) is a cementitious composite made from a mixture of mine tailings, water and binder. This technology offers several advantages, such as improving the production and safety conditions of underground mines. Among these advantages, CPB is a promising solution for the management of sulphidic tailings, which are considered to be reactive materials (i.e., not chemically stable in an atmospheric condition) and the main source of acid mine drainage, which constitutes a serious environmental challenge faced by mining companies worldwide. Such tailings, if they come into direct contact with atmospheric elements (mainly oxygen and water), face oxidation of their sulphidic minerals, thus causing the release of acidic drainage (i.e., acid mine drainage) and several types of heavy metals into surrounding water bodies and land. Therefore, the reactivity of sulphidic tailings with and without cement content can be considered as a key indicator of the environmental behavior and durability performance of CPB systems. For a better understanding of the reactivity, it is important to investigate the influencing factors. In this research, several influencing factors are experimentally studied by conducting oxygen consumption tests on different sulphidic CPB mixtures as well as their tailings under different operational and environmental conditions. These factors include time, curing temperature, initial sulphate content, curing stress, mechanical damage, binder type and content, and the addition of mineral admixtures. In addition, several microstructural techniques (e.g., x-ray diffraction and scanning electron microscopy) are applied in order to understand the changes in the CPB matrices and identify newly formed products. The results reveal that the reactivity of CPB is affected by several factors (e.g., curing time, initial sulphate content, ageing, curing and atmospheric temperature, binder type and content, vertical curing stress, filling strategy, hydration and drainage, etc.), either alone or in combination. These factors can affect reactivity either positively or negatively. It is observed that CPB reactivity decreases with increasing curing time, temperature (i.e., curing and atmospheric temperatures), curing stress, binder content, the addition of mineral admixtures, degree of saturation, and the binder hydration process, whereas reactivity increases with increases in sulphide minerals (e.g., pyrite), initial sulphate content, mechanical damage, and with decreased degrees of saturation and binder content. The effect of sulphate on the reactivity of CPB is based on the initial sulphate content as well as curing time and temperature. It is concluded that the reactivity of CPB systems is time- and temperature-dependent with respect to other factors. Also, binders play a significant role in lowering CPB reactivity due to their respective hydration processes.

Underground mining is a mineral acquisition technique that is critical to global economies, and human technological advancements. As shallow resource reserves are depleted, mine depths are increasing to accommodate global mineral demand. Increases in mine throughputs and excavation depths pose increased environmental concerns. Tailings surface disposal, and underground mine support are two considerable environmental and geotechnical factors of concern in current day mining. Underground waste disposal has been adopted by the mining industry in many forms. Cemented paste backfill (CPB) is a common best management practice developed to tackle these two specific resource industry related issues worldwide. CPB is a cement-stabilized material composed of tailings, water, and hydraulic binder. Tailings disposal areas on the earth’s surface are reduced by disposing of tailings in subsurface stopes that have been previously excavated. This increases underground safety by providing structural support to the mine. There are also economic benefits to this practice, as the additional support allows for adjacent pillars to be excavated. Although CPB greatly reduces tailings exposure to atmospheric elements, there are still underground environmental factors that must be considered with respect to environmental performance. CPBs are porous media, meaning they are susceptible to leaching of naturally occurring metals that are no longer in a stable condition as they were when incorporated in the parent rock. Arsenic and lead are metals of concern due to their association with many ore bodies. Leaching of these unstable metals may be influenced by the backfill curing temperature and the chosen hydraulic binder. Curing temperatures may be influenced by geographic location, local stope geology and depth, hydration and transport, among others. Hydraulic binders are chosen based on availability, cost, and desired mechanical properties of the paste. In this research, the effect of curing temperature and binder composition on the leachability of CPB are studied. ASTM C 1308 leaching protocol is used to determine the leachability of six CPBs. In addition, microstructural techniques (Powder X-Ray Diffraction, Mercury Intrusion Porosimetry, and Scanning Electron Microscopy) are used to relate the microstructural properties of the CPB to the leaching characteristics. Results reveal that CPBs cured with ordinary Portland cement (OPC) leach significantly less than CPBs cured with an OPC/Blast furnace slag (Slag) binder (50% blending ratio) as a result of CH consumption in slag hydration. Both CH and C-S-H are responsible for immobilizing arsenic in cement stabilized materials. OPC-CPBs contain greater relative quantities of CH, which aids in arsenic immobilization. Between the range of 2°C and 35°C OPC-CPB performed better at lower curing temperatures. Lower curing temperatures are favoured in OPC-CPB because the pore surface greater than the threshold pore diameter is reduced. Alternatively, OPC/Slag-CPB exhibited a decrease in cumulative mass leached at higher curing temperatures. The difference in cumulative mass leached by the OPC/Slag-CPBs is also related to the pore surface, and threshold pore diameter.

One of the most novel technologies developed in the past few decades is to convert mine wastes into cemented construction materials, otherwise known as cemented tailings backfill (CTB). CTB is an engineered mixture of tailings (waste aggregates), water and hydraulic binders. It is extensively used worldwide to stabilize underground cavities created by mining operations and maximize the recovery of ore from pillars. Moreover, the application of CTB is also an environmentally friendly means of disposing potential acid generating tailings underground. During and after its placement into underground mine excavations or stopes, complex multiphysics processes (including thermal, T, hydraulic, H, mechanical, M, and chemical, C, processes) take place in the CTB mass and thus control its behavior and performance. With the interaction of the multiphysics processes, the field variables (temperature, pore water pressure, stress and strain) and geotechnical properties of CTB undergo substantial changes. Therefore, the prediction of the field performance of CTB structures during their life time, which has great practical importance, must incorporate these THMC processes. Moreover, the self-weight effect, water drainage through barricades, thermal expansion and chemical shrinkage can contribute to the volumetric deformation of CTB. Consequently, CTB exhibits unique consolidation behavior compared to conventional geomaterials (e.g., soil). Furthermore, the consolidation processes can result in relative displacement between the rock mass and CTB. The resultant rock mass/CTB interface resistance can reduce the effects of the overburden pressure or the vertical stress (i.e., arching effect). Hence, a full understanding, through multiphysics modeling and simulation of CTB behaviors, is crucial to reliably assess and predict the performance of CTB structures. Yet, there are currently no models or tools to predict the fully coupled multiphysics behavior of CTB. In this Ph.D. study, a series of mathematical models which include an evolutive elastoplastic model, a fully coupled THMC model, a multiphysics model of consolidation behavior and a multiphysics model of the interaction between the rock mass/CTB interface are developed and validated. There is excellent agreement between the modeled results and experimental and/or in-situ monitored data, which proves the accuracy and predictive ability of the developed models. Furthermore, the validated multiphysics models are applied to a series of engineering issues, which are relevant for the field design of CTB structures, to investigate the self-desiccation process, consolidation behavior of CTB structures as well as to assess the pressure on barricades and the strength development in CTB structures. The obtained results show that CTB has different behaviors and performances under different backfilling conditions and design strategies, and the developed multiphysics models can accurately model CTB field behavior. Therefore, the research conducted in this Ph.D. study provides useful tools and technical information for the optimal design of CTB structures.

Difficult ground conditions negatively affect both mine production and the safety of underground workers. Underhand cut-and-fill mining is a potential solution to these issues. Discussions with mine sites revealed the use of sill beams in underhand cut-and-fill mining is not optimized. Optimization in ground support, development of in-situ strength test, and revisions of design standards are desired. Ultimately, the operations require the minimum cemented paste backfill (CPB) strength for a stable span. Operational concerns were addressed by a multi-prong approach investigating stability of CPB sill beams using observational, experimental and analytical techniques. A case study approach summarizing the design of five mines utilizing underhand cut-and-fill with CPB is presented for different ground conditions. A historical study of span widths and beam strengths for the Stillwater mine is presented. Laboratory testing determined the stress-strain behaviour of CPB. CPB follows a hyperbolic elastic loading path to peak stress followed by a strain-softening associated with the decay of the cohesion values. Additional testing found that cohesive and tensile strength values were on average 35 and 20 percent respectively of the unconfined compressive strength. This finding impacts sill beam design strengths as previous assumptions were conservative. Test database analyses from three mine sites found that sample size and location preparation has no effect on the strength of the sample. In-situ testing methods common in other industries were not practical with CPB; rather the in-situ strength can be represented by a site specific moisture content index relationship. Review of current design methodology noted closure stresses were absent from the majority; a method was developed to assess closure for sill beam stability. The potential for critical failures were determined through a Monte Carlo probabilistic model. Methods reducing the risk of failure based on the simulation are investigated. Analysis found ground support does not improve the structural stability of the sill beam. Ground support keeps the beam intact: beam equations govern stability. The stability of sill beam in a seismic environment was analyzed based on the strain-energy density of the beam. The research concludes with a design guideline for CPB sill beams.

Underground mining produces a huge amount of voids and an even larger quantity of mine waste. Overlooking these voids could lead to the possibility of ground subsidence, as well as safety issues during mining operation; while ignoring the waste, could cause environmental pollution and significant suffering. One solution to remedy both (the voids and the waste) is cemented paste backfill (CPB), which is gaining increased recognition in both the mining industry and academic research. Transforming tailings into cemented paste, and transporting this back to underground stopes, not only negates these safety issues to a large degree, but also makes it possible to put waste to good use.However, most studies involving CPB have been conducted at temperatures above 0°C; knowledge of CPB in sub-zero environments is still lacking. For this reason, this thesis investigates the mechanical behaviour of CPB in a the latter type of environment.Uniaxial compressive strength tests were carried out on a series of frozen CPB (FCPB) samples to evaluate the mechanical behaviour (e.g. compressive strengths, geotechnical features, and the stress-strain relationships) of FCPB. It has been discovered in this thesis that FCPB exhibits remarkable strength compared to CPB and, has a great resemblance to frozen soil. Factors which may affect the behaviour of FCPB were thoroughly examined. Binder contents and types were found to be irrelevant; water content, in contrast, plays a dominant role, with an optimum value of around 26% by weight. Sulphate was confirmed to have an adverse effect on the strength of FCPB due to the increasing unfrozen water content and the formation of legible ice lenses. Hydraulic conductivity tests, scanning electron microscope observations, thermal gravimetric analyses, and mercury intrusion porosimetry were also performed as subsidiary experiments to understand the geotechnical features of FCPB. This information will be of significant value for numerous practical applications.

Mining is an important industry that plays a significant role in the development of human civilization and economies. However, the underground mining process produces a large volume of mine wastes (e.g., tailings) as well as creates large voids that require filling, typically with an engineering backfill material. Filling the voids with mine waste materials provides an environmental-friendly way of disposing mining waste. It is also an effective way of increasing ore recovery and improving the safety of miners. One of the best techniques of mine backfill is called cemented paste backfill (CPB), which is typically a mixture of tailings, binder and water. The most common binder used in the preparation of CPB is Portland cement (PC). PC is not only a costly binder, but its production is highly energy-intensive and also generates a large amount of CO2. The cement consumption can represent up to 75% of the cost of CPB. These above-mentioned factors have compelled mining companies to seek for cement alternatives that enhance the engineering properties of the CPB, decrease the cement content and reduce the carbon footprint of the mining industry. Sodium silicate is the most recent chemical additive that is proposed to reduce the binder content in CPB. Sodium silicate is an alkaline solution that is used to activate a pozzolanic material, such as cement, slag and Fly ash. However, the effect of sodium silicate on the strength and key environmental properties (permeability or saturated hydraulic conductivity, reactivity) of CPB is not well understood. The objective of this thesis is to investigate the possibility of using sodium silicate as an activator in cemented paste backfill and obtain an improvement in the aforementioned engineering properties of CPB. In order to determine the effect of the sodium silicate on backfill properties, some CPB testing methods were developed to fulfill the objectives of this research. Thus, the evolution of hydraulic, mechanical and microstructural properties of CPB samples containing sodium silicate (SS-CPB) have been tested or monitored at different curing ages (1, 3, 7, 28 and 90 days) and different CPB mixtures as well. The results of these studies show that activating CPB with sodium silicate develop CPB strength faster than CPB samples without sodium silicate. In addition, hydraulic conductivity and reactivity results show a positive change in samples containing sodium silicate compared to free sodium silicate CPB samples. Indeed, this activation leads to decreasing permeability and reactivity due to the formation of cement hydration products and acceleration of the binder hydration process. Moreover, binder type and content in the presence of sodium silicate as an alkali activator in the CPB play a significant role in lowering hydraulic conductivity and reactivity of CPB.

The main result of underground mining extraction is creating of large underground voids (mine stopes). These empty openings are typically backfilled with an engineering cementitious material called cemented paste backfill (CPB). The main purpose of CPB application in underground mining is to provide stability and ensure the safety of underground openings, maximize ore recovery, and also provide an environmental-friendly means of underground disposal of potential acid generating tailings. CPB is a mixture of mine tailings, cement binder and water. CPB has a complex geotechnical behaviour when poured into mine voids. This is because of the different thermal (T), hydraulic (H), mechanical (M) and chemical coupled processes and interactions that take place in CPB soon after placement. In addition to these THMC behaviours, various external factors, such as stope geometry, drainage condition and arching effects add more complexity to its behaviour. In order to acquire a full understanding of CPB behaviour, there is a need to consider all of these THMC factors and processes together. So far, there has not been any study that addresses this research need. Indeed, fundamental knowledge of the THMC behaviour of CPB provides a key means for designing safe and cost-effective backfill structures, as well as optimizing mining cycles and productivity of mines. Innovative experimental tools and CPB testing methods have been developed and adopted in this research to fulfill the objectives of this research. In the first phase of the study, experiments with high columns are developed to study the THMC behaviour of CPB from early to advanced ages with respect to height of the column and curing time. The column experiments simulate the mine stope and filling sequence and provide an opportunity to study external factors, such as evaporation, on the THMC behaviour of CPB. However, an important factor is the overburden pressure from the stress due to self-weight that cannot be simulated through column experiments. Therefore, in the second phase of this study, a novel THMC curing under stress apparatus is developed to study the THMC behaviour of CPB under various pressures due to the self-weight of the CPB, drainage conditions, and filling rate and sequence. Comprehensive instrumentation and geotechnical testing are carried out to obtain fundamental knowledge on the THMC behaviour of CPB in different curing conditions from early to advanced ages. The results of these studies show that the THMC properties of CPB are coupled. Important parameters, such as curing stress, self-desiccation due to cement hydration, temperature, pore water chemistry, and mineralogical and chemical properties of the tailings, have significant influence on the shear strength and compressive strength development of CPB. Factors such as evaporation and drying iii shrinkage can also affect the hydro-mechanical properties of CPB. The curing conditions (such as curing stress, drainage and filling rate) also has significant impact on CPB behaviour and performance. The THMC interactions and the degree of influence of each factor should be included in designing backfill structures and planning mining cycles. This innovative curing under stress technique can be replaced the conventional curing of CPB (curing under zero stress and no THMC loadings), in order to optimize CPB mechanical strength assessment, increase mine safety and enhance the productivity.

Cemented paste backfill (CPB) is a novel technology developed in the past few decades to better manage mining wastes (such as tailings) in environmentally friendly way. It has received prominent interest in the mining industry around the world. In this technology, up to 60% of the total amount of tailings is reused and converted into cemented construction material that can be used for secondary support in underground mine openings (stopes) and to maximize the recovery of ore from pillars. CPB is an engineered mixture of tailings, water, and hydraulic binder (such as cement), that is mixed in the paste plant and delivered into the mine stopes either by gravity or pumping. During and after placing it into the mine stopes, the performance of CPB mainly depends on the role of the hydraulic binder, which increases the mechanical strength of the mixture through the process of cement hydration. Similar to other fine-grained soils undergoing cementations, CPB’s behavior is affected by several conditions or factors, such as cement hydration progress (curing time), chemistry of pore water, mixing and curing temperature, and filling strategy. Also, it has been found that fresh CPB placed in the mine stopes can be susceptible to many geotechnical issues, such as liquefaction under ground shaking conditions. Liquefaction-induced failure of CPB structure may cause injuries and fatalities, as well as significant environmental and economic damages. Many researches studied the effect of the aforementioned conditions on the static mechanical behavior of CPB. Other researches have evaluated the liquefaction behavior of natural soils and tailings (without cement) during cyclic loadings using shaking table test technique. Only few studies investigated the CPB liquefaction during dynamic loading events using the triaxial tests. Yet, there are currently no studies that addressed the liquefaction behavior of CPB under the previous conditions by using the shaking table technique. In this Ph.D. study, a series of shaking table tests were conducted on fresh CPB samples (75 cm × 75 cm ×70 cm), which were mixed and poured into a flexible laminar shear box (that was designed and build for the purpose of this research). Some of these shaking table tests were performed at different maturity ages of 2.5 hrs, 4.0 hrs, and 10.0 hrs, to investigate the effect of cement hydration progress on the liquefaction potential of CPB. Another set of tests were conducted to assess the effect of the chemistry (sulphate content) of the pore-water on the cyclic response of fresh CPB by exposing cyclic loads on couple of CPB models that contain different concertation of sulphate ions of 0.0 ppm and 5000 ppm. Moreover, as part of this study, series of shaking table test was conducted on CPB samples that were prepared and cured at different temperatures of 20oC and 35oC, to evaluate the effect of temperature of the cyclic behavior of CPB. Furthermore, the effect of filling strategy on the cyclic behavior of fresh CPB was assessed by conducting set of shaking tables tests on CPB models that were prepared at different filling strategies of continuous filling, and sequential or discontinuous (layered) filling. The results obtained show that CPB has different cyclic behavior and performance under these different conditions. It is observed that the progress of cement hydration (longer curing time) enhances the liquefaction resistance of CPB, while the presence of sulphate ions diminishes it. It is also found that CPB mixed and cured in low temperature is more prone to liquefaction than those prepared at higher temperatures. Moreover, the obtained results show that adopting the discontinuous (layered) filling strategy will improve the liquefaction resistance of CPB. The finding presented in this thesis will contribute to efficient, cost effective and safer design of CPB structures in the mine areas, and will help in minimizing the risks of liquefaction-induced failure of CPB structures.

Cemented Paste Backfill (CPB) is extensively used in underground mine operations. Several studies have been conducted to investigate the mechanical properties of CPB. However, little attention has been devoted to the thermal conductivity of CPB. The knowledge of this thermal property is vital for the design of cost-effective and durable CPB materials. This paper presents the results of a comprehensive laboratory study on the thermal conductivity of CPB. Influencing factors on the thermal conductivity of CPB were quantitatively investigated. The measurements of thermal conductivity were performed by using the KD2 Thermal Properties Analyzer. Valuable results with regards to the effects of CPB's mix components, curing time and temperature, water saturation degree on the thermal conductivity of CPB were gained. It is felt that the present study would contribute to the better optimization of CPB mixtures and the design of more cost-effective and durable CPB underground structures. Keywords. Cemented Paste Backfill; Thermal Conductivity; Hydration; Temperature; Sulphate; Tailings.

Over the past decades, cemented paste backfill (CPB) has become a common, environmentally friendly method of managing mine wastes (such as tailings). This technology allows up to 60% of the total amount of tailings to be reused and filled in the mine stopes after converting them into cemented material. Beside reducing the environmental risks associated with the traditional disposal of these materials, turning them into cemented material and placing them in the underground mine stopes can also provide secondary support for these stopes in addition to minimizing the risk of ground subsidence in the mine area. CPB is an engineered mixture of tailings, water, and hydraulic binder (such as cement, blast furnace slag, and fly ash) that is mixed in the paste plant and delivered into the mine stopes through a gravity or pumping based transportation system. During the transportation of CPB through the delivery system pipelines, the flowability of CPB depends on the rheology of the transported CPB, which is affected by different factors, such as the transportation time, temperature variation, binder type, and chemical composition of these mixtures. In addition, the performance of CPB, after placing the CPB mixture into the mine stopes, is mainly dependent on the role of the hydraulic binder, as it increases the mechanical strength of the mixture through the process of cement hydration. The mechanical strength is also influenced by different factors, such as time progress, temperature variation, and presence of chemical additives. It has previously been found that fresh CPB transported and/or placed in the mine stopes can be susceptible to temperature variation of different sources, such as the climatic effects, heat generated from the surrounding rocks, and heat generated during the process of cement hydration. Unsuitable flowability of CPB through the delivery system might lead to significant financial losses due to clogging of pipelines with unexpected hardening of CPB during transportation, which will cause delay in work and possible damages to the pipelines. Also, failure of CPB structure in the mine stopes due to inappropriate mechanical strength may cause casualties to the mine workers as well as significant environmental and economic damages. Many researchers studied the rheological properties and/or strength development of CPB under the individual effect of any of the aforementioned factors. Additionally, many researchers have evaluated the coupled effect of some of these factors on the rheology and mechanical strength of CPB material. Hitherto, there are currently no studies that addressed the combined effect of all these conditions on the rheological properties and strength development of CPB. At the first stage of this M.A.Sc. study, a series of experimental tests was conducted on fresh CPB in order to determine the combined effect of time, temperature, binder content, and chemical additives on the rheological properties of CPB. These experiments include rheological properties test (yield stress and viscosity), microstructural analysis (thermal analysis and XRD), chemical analysis (pH and Zeta potential), and monitoring tests (electrical conductivity), which were conducted on 125 CPB samples that were mixed and prepared at different temperatures (2oC, 20oC, 35oC) and cured for different curing time (0 hrs., 0.25 hrs., 1 hr., 2hrs, and 4 hrs.). These samples were prepared with different blends of hydraulic binders (PCI, PCI/Slag, and PCI/FA) and contained different dosages of sodium silicate (0%, 0.1%, 0.3%, and 0.5%). The results obtained show that rheology of CPB increases with the progress of curing time. It also increases with the increase in the initial (mixing and curing) temperature and content of sodium silicate. It was also found that the partial usage of slag and FA reduces the rheological properties. However, CPBs containing PCI/FA as binder have lower rheological properties, and thus better flowability, than those that contain PCI/Slag as binder. At the second stage of this M.A.Sc. study, in order to understand the combined effect of time, temperature and sodium silicate content on the strength development of slag-CPB, unconfined compression (UCS) test, microstructural analysis (thermal analysis and MIP), and monitoring tests (electrical conductivity, suction, and volumetric water content) were conducted on 72 CPB samples that were prepared with PCI-Slag as a binder, cured for different times (1 day, 3 days, 7 days, and 28 days) under different curing temperatures of (2oC, 20oC, 35oC), and contained different dosages of sodium silicate (0%, 0.3% and 0.5%). The results obtained at this stage showed that the strength development of slag-CPB increases with the progress of curing time and temperature. It also increases with the increase in the sodium silicate content. Also, the combined effect of high temperature, high dosage of sodium silicate and longer curing time showed significant enhancement in the mechanical strength of slag-CPB. The findings of this M.A.Sc. research will contribute to cost effective, efficient, and safer design of CPB structures in the mine areas. It will also help in minimizing financial loss associated with unsuitable flowability of CPB transported in the CPB delivery system besides reducing the risks of human loss, and the environmental and economic damages associated with the failure of CPB structures.

The effects of sulphate and curing temperature play an important role on the strength development and durability of Cemented Paste Backfill (CPBs). Depending on the application of the CPB, different strength values, measured as unconfined compressive strength (UCS), are targeted. There is a lack of proven theory to predict the UCS for a specific CPB mix due to the complexity of the interactions between the variables that affect the CPB strength. This thesis presents an approach to use the artificial neural network (ANN) methodology in order to develop two models that can predict the effects of sulphate and curing temperature on the UCS of CPBs. The ANN models here developed illustrate an outstanding accuracy in the UCS prediction for the simulation of sulphate and its coupled effect with curing temperature. The ANN models provide a better understanding of the effects of sulphate and/or temperature on the strength of CPBs.

Les métaux précieux (tels que l'or et l'argent), et les métaux de base (tels que le cuivre et d'autres) sont extraits du sous-sol par excavation, en créant des vides de différentes tailles appelés (chantiers d’abattage) qui sont reliés entre eux par des galeries (de circulation et de soutirage). Dans le cas de l’exploitation par chambres-remblayées, ces vides ou chambres sont généralement remplis avec du remblai en pâte cimenté (RPC) qui est un mélange fait des rejets de concentrateur (appelés résidus), d’un agent liant (ex. ciment) et de l’eau de malaxage. Vu que le RPC est déposé à l’état liquide (mélange solide-liquide) dans les chantiers d’abattage, il est nécessaire d’utiliser un ouvrage de retenue afin de le contenir pendant le remblayage. Cet ouvrage de retenue est appelé barricade et peut être construit en bois, en béton, en briques, en béton projeté ou à partir des roches stériles disponibles sous terre et qui sont issues du développement des galeries. Les barricades construites à partir des roches stériles sont les plus courantes au Québec et au Canada car elles sont économiques, disponibles (sous terre) et favorisent le drainage de l'eau lors du remblayage; ce qui favorise la consolidation gravitaire du RPC, et donc, la réduction de la pression interstitielle. À ce jour, peu d'informations existes sur les caractéristiques réelles in situ de ces barricades (telles que leur granulométrie, leur résistance à la rupture, le mécanisme probable de leur rupture et les dimensions standards utilisées) afin d’appuyer leur conception de manière à assurer la sécurité des travailleurs et des équipements miniers; ce qui contribuerait à la diminution du cycle de minage, et par conséquent, à l'augmentation de la productivité minière. Les travaux de cette thèse se sont appuyés sur les modélisations physiques et numériques afin de mieux comprendre le comportement géomécanique complexe des barricades de roches stériles. Un modèle physique à l'échelle réduite d’un chantier d’abattage a été développé et construit à partir de plaques en plexiglass translucides, afin de simuler le remblayage dans les mines souterraines. Une méthodologie spécifique a été développée pour l’exécution des essais : instrumentation du modèle réduit à l’aide de capteurs de pression (totale et interstitielle), calibrage des capteurs, remplissage du modèle réduit avec du RPC, suivi en continu des essais avec des caméras haute définition. Les essais réalisés ont permis de mettre en évidence le principal mécanisme probable de rupture des barricades de roches stériles, ainsi que l’estimation de la pression maximale au moment de leur rupture. L'effet de la distribution de la taille des particules de roches stériles sur la stabilité et l’intégrité des barricades de roches stériles à la suite de la poussée exercée par le RPC a également été analysé. Une partie des essais réalisés sur le modèle réduit a été modélisée à l’aide du code de calculs numériques Geostudio 2018 (GeoSlope Intl.) par calibrage avec les résultats expérimentaux. Les résultats des simulations réalisées reproduisaient correctement le comportement général observé lors des essais sur le modèle réduit, avec une différence significative au niveau des valeurs des pressions. Des solutions analytiques simplifiées basées sur l’équilibre limite ont également été proposées sur la base des observations expérimentales pour l’analyse de stabilité (par rapport au glissement et au frottement) des barricades de roches stériles. Des recommandations ont été proposées afin de pousser cette étude plus loin en incluant l’effet de différents facteurs (ex. la position de la barricade dans la galerie de soutirage, la viscosité et le seuil d’écoulement du remblai ou son pourcentage de solides, les paramètres de cisaillement des barricades de roches stériles, l’effet d'arche, etc.)
Precious metals (such as gold and silver), and base metals (such as copper and others) are mined from the underground by excavation, creating voids of various sizes called (stope) which are interconnected by galleries or drifts (for circulation and draw point). In the case of cut-and-fill mining, these voids are usually filled with cemented paste backfill (CPB) which is a mixture made of concentrator mill tailings, of a binding agent (e.g., cement) and mixing water. Since the CPB is placed in the liquid state (solid-liquid suspension) in the underground stopes, it is necessary to use a retaining structure to contain it during backfilling. This retaining structure is called a barricade and can be constructed from wood, concrete, bricks, shotcrete or from waste rock available underground and which come from the drift’s development. Barricades built from waste rock are the most common in Quebec and Canada because they are economical, readily available (underground) and promote water drainage during backfilling, which promotes self-weight consolidation of the CPB, and therefore, reduction of pore water pressure. To date, little information exists on the real in situ characteristics of these barricades (such as their grain size distributions, their failure strength, the probable mechanism of their rupture and the standard dimensions used) to support their design in a meaningful way to ensure the safety of workers and mining equipment, which would contribute to the reduction of the mining cycle, and consequently, to the increase of mining productivity. The work of this thesis project was based on physical and numerical modeling to better understand the complex geomechanical behavior of waste rock barricades. A reduced-scale physical model of a mine stope was developed and constructed from translucent plexiglass plates to simulate backfilling in underground mines. A specific methodology was developed for the execution of the tests: instrumentation of the reduced-scale model using pressure sensors (total and pore water), calibration of the sensors, filling of the reduced-scale model with CPB, continuous monitoring of the tests using high-definition cameras. The tests carried out have made it possible to highlight the main probable mechanism of rupture of the waste rock barricades, as well as the estimation of the maximum pressure at the time of their rupture. The effect of waste rock particle size distribution on the stability and integrity of waste rock barricades due to the CPB pressure was also analyzed. Part of the tests carried out on the reduced-scale model were modeled using the Geostudio 2018 numerical code (GeoSlope Intl.) through calibration with the experimental results. The results of the simulations performed reproduced well the general behavior observed during the tests on the reduced-scale model, but with a significant difference in the pressure values. Simplified analytical solutions based on limit equilibrium have also been proposed based on experimental observations for the stability analysis (with respect to sliding and friction) of waste rock barricades. Some recommendations were proposed to take this study further by including the effect of various factors (e.g., the position of the barricade in the drift or draw point, the viscosity, and the shear yield stress of the backfill or its solids mass concentration, the shear parameters of the waste rock barricades, the arching effect, etc.)

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Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

L’objectif de la thèse est de comprendre le comportement géochimique de l’arsenic dans les remblais miniers en pâte cimentés. Cette technique est employée depuis plusieurs années pour remblayer les galeries de mine exploitées à l’aide des rejets de concentrateurs. Pour cette étude, deux types de remblais sont élaborés en laboratoire : des échantillons de remblai minier en pâte synthétiques, fabriqués à partir de silice et enrichis artificiellement en arsenic, et des remblais miniers en pâte élaborés à partir de rejets miniers arséniés. Dans les deux cas, différents liants cimentaires sont testés. Plusieurs types d’investigations sont menés sur les échantillons de remblai en pâte cimentés. Une caractérisation minéralogique est effectuée à l’aide de divers outils. En parallèle, les échantillons de remblais sont soumis à différents tests de lixiviation complémentaires, afin d’évaluer la mobilité/rétention de l’arsenic dans ces matrices. Enfin, une modélisation géochimique est mise en œuvre à partir des résultats issus des deux études précédentes, afin d’affiner les connaissances sur la nature et la stabilité des composés arséniés dans les remblais étudiés. Les résultats obtenus indiquent que l’arsenic est mieux stabilisé dans les matrices à base de ciment Portland et de laitier de haut-fourneau que dans les remblais à base de cendres volantes. Le comportement à la lixiviation variable d’une matrice à l’autre s’explique par des spéciations et des mécanismes de piégeage de l’arsenic variés. L’arsenic peut précipiter sous forme de minéraux arséniés, principalement sous forme d’arséniates de calcium, mais aussi de divers autres composés secondaires variables d’une matrice à l’autre. Un piégeage physique des grains de rejets miniers arséniés par les hydrates cimentaires peut aussi avoir lieu, par formation d’un revêtement limitant l’oxydation des sulfures porteurs d’arsenic. Tous ces mécanismes interviennent dans la stabilisation/solidification de l’arsenic
The objective of this study is to understand the geochemical behaviour of arsenic in cemented paste backfills. This technique consists in transporting the tailings in the mine openings. Two types of backfills are prepared in the laboratory for this study. First, synthetic cemented paste backfills artificially spiked with arsenic are synthesized, using silica in replacement of the tailings. In parallel, other cemented paste backfill specimens are prepared with arsenic-rich tailings. In the two cases, various types of hydraulic binders are tested. Several types of investigations are conducted on the cemented paste backfill specimens. A mineralogical characterization is carried out with the help of specific tools. At the same time, cemented paste backfill samples are submitted to several complementary leaching tests, to assess the mobility/immobilization potential of arsenic in these matrices. Finally, geochemical modeling is implemented, based on the results of the two previous studies, in order to refine the understanding of the nature and stability of the arsenic compounds. The results show that arsenic is better immobilized in Portland cement and slag-based matrices, rather than in fly ash-based matrices. The variable leaching behaviour from a given matrix to another is due to different arsenic trapping mechanisms. Arsenic can precipitate and form several arsenic minerals, mainly calcium arsenates, but also various other secondary compounds, which are different from a matrix to another. Physical entrapment of the tailings grains by the cementitious minerals can also occur, by formation of a coating around the grains, limiting the oxidation and dissolution of arsenic-bearing sulfides (passivation). These mechanisms are involved in the stabilization/solidification of arsenic by cemented paste backfills

Gelfill (GF) is made of tailings, water, binder and chemical additives (Fillset, sodium silicate gel). The components of GF are combined and mixed on the surface and transported (by gravity and/or pumping) to the underground mine workings, where the GF can be used for both underground mine support and tailings storage. Thermal (T), hydraulic (H), and mechanical (M) properties are important performance criteria of GF. The understanding of these engineering properties and their evolution with time are still limited due to the fact that GF is a new cemented backfill material. In this thesis, the evolution of the thermal, hydraulic, mechanical, and microstructural properties of small GF samples are determined. Various binder contents of Portland cement type I (PCI) are used. The GF is cured for 3, 7, 28, 90, and 120 days. It is found that the thermal, hydraulic and mechanical properties are time-dependent or affected by the degree of binder hydration index. Furthermore, a relationship is found between the compressive strength and the saturated hydraulic conductivity of the GF samples. The unsaturated hydraulic properties of GF samples have also been investigated. The outcomes show that unsaturated hydraulic conductivity is influenced by the degree of binder hydration index and binder content, especially at low suction ranges. Simple functions are proposed to predict the evolution of air-entry values (AEVs), residual water content, and fitting parameters from the van Genuchten model with the degree of hydration index (α). Furthermore, two columns are built to simulate the coupled thermo-hydro-mechanical (THM) behaviour of GF under drained and undrained conditions. The obtained results from the GF columns are compared with the small samples. It is observed that the mechanical properties, hydraulic properties (suction and water content), and temperature development are strongly coupled. The magnitude of these THM coupling factors is affected by the size of the GF. The findings also show that the mechanical, hydraulic and thermal properties of the GF columns are different from samples cured in plastic moulds.

Modern mines require systems that quickly deliver backfill to support the rock mass surrounding underground openings. Cemented Paste Backfill (CPB) is one such backfilling method, but concerns have been raised about CPB’s liquefaction susceptibility especially when the material has just been placed, and if it is exposed to earthquakes or large mining induced seismic events. Conventional geotechnical earthquake engineering for surface structures is now relatively advanced and well accepted, and so the objective of this thesis is to consider how that framework might be extended to assess the liquefaction potential of CPB. Seismic records were analyzed for earthquakes and for large mining induced events. Important seismological trends were consistent for rockbursts and earthquakes when the signals were recorded at distances as proximate as one kilometre, suggesting that the conventional earthquake engineering approach might plausibly be adapted for such design situations. For production blasts and for more proximate locations to rockbursts, much higher frequencies dominate and therefore new design methods may be required. Monotonic triaxial tests conducted on normally consolidated uncemented mine tailings demonstrated that the material is initially contractive up to a phase transition point, beyond which dilation occurs. Most importantly the material never exhibits unstable strain softening behaviour in compression, and only temporary or limited liquefaction in extension. The addition of 3% binder results in initial sample void ratios that are even higher than their uncemented counterparts, and yet the material friction is slightly enhanced when tested at 4 hours cure. These results suggest that the flow liquefaction phenomenon commonly associate with undrained loose sand fills will not occur with paste backfill. Cyclic triaxial test results analyzed in terms of number of cycles to failure for a given cyclic stress ratio exhibited a trend consistent with previous tests on similar materials. However, the addition of 3% binder and testing at 4 hours cure resulted in an order of magnitude larger number of cycles to failure – a surprising and dramatic increase, suggesting good resistance of the material to cyclic mobility. Future research is recommended to build on these results and develop more robust methods for liquefaction assessment of CPB.

Measurement of the strength development of Cemented Paste Backfill in laboratory cast cylinders does not replicate the in situ strengths of CPB in mine stopes. The mass of CPB in a filled stope is large and temperature rises due to the heat of hydration of the cementing materials, thus accelerating the gain in strength, relative to laboratory specimens stored at ambient temperature. The purpose of this study was to determine the impact on strength development when CPB test cylinders were subjected to a temperature profile mimicking that in a large mass, such as a mine stope. Also, maturity (the integral of time and temperature during hydration of the CPB) was compared to actual strengths, and the maturity – strength concept used in concrete technology was applied. It was found that the strength- maturity relationship was applicable to CPB once the base line or datum temperature was adjusted.

Cemented Paste Backfill (CPB) is a relatively new backfilling technology for which a better understanding of binder hydration is required. This research uses electromagnetic (EM) wave-based techniques to non-destructively study a CPB consisting of tailings, sand, process water and binder (90% blast-furnace slag; 10% Portland cement). EM experiments were performed using a broadband network analyzer (20 MHz to 1.3 GHz) in the lab and capacitance probes (70 MHz) in the lab and field. Results showed that the EM properties are sensitive to curing time, operating frequency and specimen composition including binder content. The volumetric water content interpreted from dielectric permittivity varied little with curing. Temporal variations in electrical conductivity reflected the different stages of hydration. Laboratory results aided interpretation of field data and showed that a reduction in binder content from 4.5% to 2.2% delays setting of CPB from 0.5 days to over 2 days, which has important implications for mine design.

Underground mining can be summarized as the removal of economically viable volumes of rock which creates underground voids. In order to optimize ore extraction, a material is used to backfill these openings prior to creating any adjacent openings. The use of cemented paste backfill (CPB), a mixture of mine tails, water, and cement binder, has gained prominence as it not only provides a material that has engineered strength and can be deployed rapidly, but also decreases the surface storage volume of the mine tails. There is limited knowledge about the behavior of the stresses within the CPB during the filling of an underground opening, particularly during the early curing ages of the hydrating CPB which is critical to the design of fill barricades. This thesis presents a design procedure which can be used to determine the in situ stresses within the CPB. Three methodologies were used in the development of this design procedure. The first was to develop a laboratory testing method that determined the time-dependent consolidation characteristics and strength parameters of the hydrating cemented paste material. The second was to collect several field-data sets. The third methodology was to numerically model the CPB using Itasca’s FLAC3D, which incorporated the underground void’s geometry, backfilling strategy, and time-dependent backfill parameters in order to determine the in situ stresses of the CBP. This simulation allowed for the prediction of both total and effective stress throughout the stope. The model and the laboratory results were used to model the stresses in several test stopes so that a comprehensive comparison could be made between the model and field instrumentation results. Four case studies were examined using a total of six different field instrumentation datasets. The results from these case studies showed that the modeling approach, given some model calibration, is capable of quantitatively representing the important geomechanical aspects of paste filling and curing.

Rapid delivery of backfill to support underground openings attracted many mines to adopt paste backfilling methods. As a precaution to prevent liquefaction and to improve the mechanical performance of backfills, a small portion of a binder is added to the paste to form the cemented paste backfill (CPB). Recently, adding sand to mine tailings (MT) in CPB mixes has attracted attention since it enhances the flow and mechanical characteristics of the pastefill. This thesis investigates the effects of adding sand to CPB on the undrained mechanical behavior of the mixture (CPBS) under monotonic and cyclic loads. Liquefaction investigations took place at the earliest practically possible age. Beyond this age, the present research focused on characterizing the evolution of stiffness and obtaining the values of the stiffness parameters that could be useful for designing and modeling backfilling systems. The liquefaction investigation involved monotonic compression and extension triaxial tests. Neither flow nor temporary liquefaction was observed for all cemented and uncemented specimens under monotonic compression, while temporary liquefaction was observed for all specimens under monotonic extension. The addition of binder and sand to MT was found to slightly strengthen the pastefill in compression while weakening it in extension. Under cyclic loading, the addition of sand negatively impacted the cyclic resistance. However, binder was found to be more effective in the presence of sand. All specimens exhibited a cyclic mobility type of response. The evolution of effective stiffness parameters for two CPB-sand mixtures was monitored in a non-destructive triaxial test for five days. Self-desiccation was found to not be influential on the development of early age stiffness. Moreover, a framework is suggested to predict the undrained stiffness at degrees of saturation representative of the field. The credibility of the proposed test in providing stiffness parameters at representative strain levels of the field was verified.

A two-fold study was carried out to 1) characterize the evolution of the reactivity of pyrite in the early cycles of kinetic test leaching, using cyclic voltamperometry, and 2) document the weathering characteristics of various paste backfill mixtures that contain pyritic tailings, when exposed to leaching environments similar to those encountered in mine settings. Pyrite leaching experiments were carried out on 6 different pyrite samples from existing mines. Cyclic voltamperometry was performed on carbon paste electrodes (CPE) containing fine grained pyrite samples on the unleached samples and after leaching periods of 4, 10 and 20 weeks. Pyrite reactivity profiles, supported by scanning electron microscope (SEM) observations and leachate chemistry data showed that minor phases of sphalerite and galena present in the pyrite samples were the most important parameters affecting pyrite reactivity in the initial leaching cycles. Sphalerite and galena were found to effectively retard the oxidation of pyrite in the early leaching cycles. As sphalerite and galena were leached out, an increase in the reactivity of pyrite was observed, followed by a gradual loss of reactivity from precipitate coatings. At a fundamental level, mineral surface characterization by cyclic voltamperometry was found useful in the interpretation of kinetic test data for the prediction of acid rock drainage (ARD) generation. For the backfill weathering study, paste backfill samples of 4 different mines were leached in deionized water (pH 5.5) in flooded and alternating air-flooded environments and in a simulated ARD solution (Fe=500 mg/1, SO4=1.5 g/1 and pH 2.5) for 20 weeks. SEM, solid phase chemistry, paste pH, acid-base accounting measurements and leachate chemistry were also used to document the weathering characteristics of cemented paste backfill (CPB). This study revealed that hydrated portland cement minerals are pH sensitive and highly soluble. Short-term exposures of portland-CPB to circum-neutral water or to ARD solution promoted the dissolution of the binder material, increasing the porosity of the backfill and further infiltration of aqueous solution. Long-term exposure or flooding of CPB was found to promote the precipitation of secondary, expansive minerals such as gypsum in addition to solubilizing primary cement minerals. Detailed chemical analyses and acid-base accounting indicated that the neutralizing potential added to the material by the cement phase is short-lived and the small volumes added are insufficient to neutralize the acid generating potential of the mixture. All ARD solution-leached CPB samples formed an increasingly thick crust of precipitates that, with time, reduced the ability of the CPB to neutralize the ARD solution. [Scientific formulae used in this abstract could not be reproduced.]

Current design of cemented paste backfill (CPB) barricades tends to be of unknown conservativeness due to limited understanding of their behaviour. Previous work done to characterize barricade response has not accounted for the effects of the surrounding rock stiffness, which can have significant impact on the development of axial forces which enhance capacity via compressive membrane action. Parametric analyses were performed with the finite element analysis program Augustus-2 to determine the effects of various material and geometric properties on barricade capacity. Equations based on Timoshenko and Boussinesq solutions were developed to model rock stiffness effects based on boundary material properties. An iterative simulation process was used to account for secondary moment effects as a proof of concept. It was found that, for a range of typical rock types, barricade capacity varied significantly. The commonly made design assumption of a fully rigid boundary resulted in unconservative overpredictions of strength.

Large voids created by underground mining are backfilled to provide regional ground support. This thesis examines using conventional oedometer techniques and electromagnetic (EM) techniques to characterize consolidation and binder hydration in mine backfill so that EM monitoring can be used in the field to provide real-time feedback to operators to optimize the backfilling process. New techniques are given for interpreting the full range of deformation (initial compression, primary and secondary consolidation). Deformation due to initial compression is non-trivial and may have to be accounted for in numerical back-analyses of field case studies. EM parameters are sensitive to binder content, progress of hydration and loss of water caused by consolidation and binder hydration. The integrated interpretation of consolidation and EM behaviours has significant potential impact on real-time monitoring of mine backfill operations, and recommendations are made to advance the technique for this purpose.