1,721,085 research outputs found
Drying Behavior of Oil Sand Mature Fine Tailings Pre-dewatered with Superabsorbent Polymer
No full textOil sand processing to extract bitumen generates large volumes of slurry comprising water, silt, sand, clay, unrecovered bitumen, and residual chemical aides and solvents added during the extraction process. The by-product stream of the bitumen extraction is pumped into constructed tailings ponds. Managing these tailings is one of the most difficult environmental challenges for the oil sand industry. This study aims to develop a novel technique to assist in the assessment of the technologies for managing mature fine tailings (MFT) in oil sands. Innovative application of a superabsorbent polymer in the oil sands industry may provide a new method for tailings management. However, thus far, no study has addressed this research gap. In fact, fundamental knowledge of the behavior of MFTs pre-dewatered with the superabsorbent polymer could provide an important way to positively affect the speed of reclamation. To this end, comprehensive instrumentation, geo-environmental, and geotechnical analyses are carried out to obtain essential knowledge on the behavior of MFTs pre-dewatered with the polymer. The results of this study reveal that the mechanical, hydraulic, and thermal properties of the MFTs are related. Evaporation and drying shrinkage can affect the hydro-mechanical properties of the tailings and have a significant influence on the developed shear strength of the MFTs. In addition, the process-affected water includes a high concentration of the dissolved ions and organic chemicals that stem from ore extraction chemicals and tailings treatments, or that may be released from oil sands ores. Through the application of a superabsorbent polymer in the dewatering of oil sand MFTs, the chemical components are entrapped in the polymer chains, thus limiting the mobility of the major ions and trace metals in water bodies beneath the oil sand tailings pond. Results show that the application of the superabsorbent polymer considerably reduces the rate of drainage from the oil sand MFTs into water bodies, which can help mitigate the risk of seepage.
The author believes that the superabsorbent polymer dewatering technique can be considered as an environmentally friendly promising approach for management of oil sands MFTs. This new technique can accelerate the pace of reclamation and thus minimize the footprint of the oil industry in Canada
Geotechnical Behaviour of Frozen Mine Backfills
No full textThis 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
Experimental Characterization of the Thermal, Hydraulic and Mechanical (THM) Properties of Compost Based Landfill Covers
No full textLandfills are considered to be one of the major sources of anthropogenic methane (CH4) emissions in the environment. A landfill biocover system optimizes environmental conditions for biotic CH4 consumption that controls the fugitive and residual emissions from landfills. A compost material has more oxidation potential in comparison to any other material due to its high porosity, organic content, free flux for gases and water holding capacity. Thermal, hydraulic, bio – chemical and mechanical (THMCB) properties are important factors that can significantly affect the performance of biocover material with regards to CH4 oxidation potential as well as structural stability. Technical data on the thermal, hydraulic and mechanical (THM) properties of compost based biocover materials are quite limited. Hence, a detailed experimental program has been carried out at the University of Ottawa to study the THM properties and behaviour of compost biocover material by conducting experimental tests on small compost samples as well as by performing column experiments.
The test results indicate that lower water content (dry of optimum for compaction curve) shows more free air space (FAS) in comparison to higher water content. The compost has almost the same shear strength for various initial water contents and dry unit weights; however, it settles and swells more at higher water content than lower water content per mechanical test results. The thermal and hydraulic properties of compost are a function of the compaction degree in addition to various other parameters. It is also found that the THM properties of compost are strongly coupled and the degree of saturation greatly affects the FAS
Geotechnical and Geo-Environmental Behaviour of Landfill Biocover under Freeze-Thaw Condition
No full textLandfill biocovers have been proven as a green and efficient technology to mitigate landfill methane emissions. Thermal, hydraulic, mechanical and bio-chemical (THMBC) factors regulate biocover behaviour. The aim of the current study is to evaluate the geotechnical and geo-environmental response or performance of compost based biocovers under freeze-thaw conditions. A comprehensive experimental program, including tests on samples as well as biocover column experiments, has been conducted.
The results demonstrate that the thermal properties (thermal conductivity and thermal diffusivity) of the biocovers change due to the FTCs. Moreover, the outcomes of the column experiments demonstrate that biocover performance remains at an acceptable level even after experiencing two FTCs despite that most of the THMBC parameters in the biocover have changed due to the impacts of the FTCs and methane injection. The findings presented in this thesis will contribute to a better understanding and design of compost biocovers in cold regions
Thermally and Chemically Induced Changes in Interface Shear Behavior of Landfill Liners
No full textComposite liners are used in landfills to isolate solid waste from the local environment. The combination of a high-density polyethylene (HDPE) geomembrane and compacted clay liner (CCL) is commonly used worldwide. In the Ontario region, bentonite sand mixtures (BSMs) and the local clay i.e. Leda clay, can be considered as appropriate CCL materials. However, the interface failure between smooth HDPE and CCL is a critical issue for landfill safety. The shear stress behavior and strength parameters at the interface between the HDPE and CCL can be affected by many factors, such as temperature and chemicals. The temperature difference between winter and summer in the Ontario region is approximately 50°C, which causes a freeze-thaw (F-T) phenomenon in local landfills. Leachate and heat are generated during the solid waste stabilization process. Landfill leachate usually contains a high concentration of cations, which can carry heat, thus affecting the landfill liner properties. As a result, the interface shear stress behavior and strength parameters are affected by the aforementioned conditions.
In this thesis, a series of experiments were conducted on the shear stress behavior at the interface of Leda clay / HDPE and bentonite sand mixture (BSM) / HDPE. In order to understand the influence of the F-T phenomenon, the samples were tested by varying the number of F-T cycles. Meanwhile, in order to understand the combined influence of cations and heat, the samples were saturated with different solutions, i.e. distilled water, potassium chloride and calcium chloride solutions. Then they were cured in an oven with different temperatures and room temperature, respectively. All of the laboratorial shear tests have been performed by using a direct shear machine. Results show that the BSM /HDPE and Leda clay/ HDPE interfaces are both influenced by the F-T cycles. The BSM/HDPE interface shear of the samples between 0 and 5 F-T cycles has more obvious differences, while the friction angle of compacted Leda clay/HDPE exhibits distinct reduction in the first 3 cycles, after which, the difference becomes hard to differentiate. The results also indicate that both high temperature and high concentration of cations from leachate can slight reduce the interface shear stress of BSM/HDPE. However, the combined influence of thermal-chemical conditions is not much more obvious compared to the effects of a single thermal or chemical condition. The BSM materials, which were saturated with different solutions, are also tested by using X-ray diffraction to examine the mineral changes in the BSM. The calcium and potassium cations convert sodium-bentonite into calcium-rich bentonite and illite/semectie mixtures, respectively. Nevertheless, the changess of clay part caused by the combined effect of heat and leachate have limited influence on the BSM/HDPE interface shear behavior
SAP Based Rapid Dewatering of Oil Sands Mature Fine Tailings
No full textMature fine tailings (MFT), as a mixture of residual bitumen, sand, silt, fine clay particles and water, are a byproduct of oil sands extraction. The large volume, and poor consolidation and water release ability of MFT have been causing significant economic and environmental concerns. Therefore, several studies have been implemented on finding innovative dewatering/disposal techniques. As a result, different methods have been introduced and tested at a laboratory or a field scale, yet very few of these are commercially used in the oil sands industries. Despite the extensive research, an optimal solution has not been found due to the lack of technical or economic feasibility.
In the present study, a novel approach that consists of the rapid dewatering of MFT by using a super absorbent polymer (SAP) to produce dense MFT is proposed. A comprehensive laboratory investigation on the geotechnical characteristics and behavior before and after treatment of MFT is conducted. The effects of SAP based dewatering and freeze/thaw cycles on the undrained shear strength of dewatered MFT by using a vane shear apparatus are studied. Furthermore, the ability of recycled SAP to dewater and densify MFT is assessed. Finally, this study provides the results of consolidation and hydraulic conductivity testing to evaluate the void ratio versus effective stress and hydraulic conductivity of MFT. The effects on the behavior and characteristics of MFT after amendment with usage of recycled SAP are also investigated.
The results indicate that SAP has the ability to significantly dewater, densify and increase the undrained shear strength of MFT. Furthermore, when subjected to freeze/thaw cycles, the MFT dewatered with SAP shows an additional increase in strength and solid content. It is also found to be possible to regenerate the polymer (still within sachets) through light thermal drying, and the regenerated SAP can still significantly dewater and thus increase the shear strength and solid content of the MFT. In addition, the obtained high solid content affects and improves the compressibility of the material, thus resulting in low initial void ratios. On the other hand, low hydraulic permeability that is derived from low initial void ratios and consolidation is improved by the freeze/thaw process due to the interconnected voids created during the freezing process
Coupled Hydro-Mechanical Modelling of Gas Migration in Saturated Bentonite
No full textBentonite is regarded as an ideal geomaterial for the engineering barrier system of a deep geological repository (DGR) where nuclear wastes are disposed, as it has several desirable properties for sealing the nuclear wastes, including low permeability, low diffusion coefficient, high adsorption capacity and proper swelling ability. Nevertheless, gas migration in saturated bentonite may undermine the sealing ability of the geomaterial. Previous experimental studies showed that the gas migration process is accompanied by complex hydromechanical (HM) behaviors, such as gas breakthrough phenomenon, development of preferential pathways, build-up of water pressure and total stress, nearly saturated state after gas injection test, localized consolidation, water exchange between clay matrix and developed fractures and self-sealing process. These experimentally observed behaviors should be properly modelled for conducting a reliable performance assessment for the geomaterial over the lifespan of DGR. In this thesis, two different coupled HM frameworks, i.e., one based on double porosity (DP) concept, referred to as coupled HM-DP framework, and the other on phase field (PF) method, referred to as coupled HM-PF framework, are proposed to simulate the gas migration process in saturated bentonite.
For the coupled HM-DP framework, the saturated bentonite is assumed as a superposition of a MAcro-Continuum (MAC) and a MIcro-Continuum (MIC). Two-phase flow is only allowed in the MAC, whereas the MIC is impermeable to both water and gas. Nevertheless, the water can transfer between the MIC and the MAC under the water pressure gap. The first coupled HM model in this framework is based on a double effective stress concept. Mechanical behaviors of the MAC and the MIC are respectively governed by Bishop-type effective stress and Terzaghi’s effective stress. The model can well simulate the evolutions of both gas pressure and gas outflow rate, the water exchange between clay matrix and developed pathways, the high degree of saturation and the consolidation of clay matrix. To account for the development of preferential pathways, the damaging effect has been introduced in the framework. In this improved model, Bishop-type effective stress for the MAC is replaced by the independent stress state variables, i.e., net normal stress and suction, since using the net normal stress is beneficial to simulating tensile failure under high gas pressure. Numerical results showed that the damage-enhanced model can well describe the effect of the development of preferential pathways on the build-up of water pressure and total stress. In addition, the proposed hysteretic models for intrinsic and relative permeabilities make the coupled HM framework more flexible to reproduce the experimental results.
To explicitly simulate the development of preferential pathways, a coupled HM-PF framework is developed by using Coussy’s thermodynamic theory and the microforce balance law. The coupled HM-PF framework is implemented in the standard Finite Element Method (FEM). To avoid the pore pressure oscillation and enhance the computational efficiency, a stabilized mixed finite element, in which linear shape functions are selected for interpolating all primary variables, is adopted to discretize the whole domain. In the developed framework, swelling pressure (initial stress) is accounted for by introducing a modified strain tensor that is the sum of the strain tensor due to deformation and the strain tensor calculated from the initial stress. The numerical results showed that the developed coupled HM-PF framework can capture some important behaviors, such as the discrete pathways, localized gas flow, built-up of water pressure and total stress under constant volume condition and nearly saturated state in clay matrix. A spatially autocorrelated random field is introduced into the framework to describe the heterogeneous distribution of HM properties in bentonite. The heterogeneity is beneficial to simulating the fracture branching and the complex fracture trajectory. Numerical results showed that some factors, such as Gaussian random field, coefficient of variation, boundary condition and injection rate, have significant influences on the fracture trajectory.
At the end of the thesis, the obtained numerical results are synthesized and analyzed. Based on the analysis, the pros and cons of the developed numerical models are discussed. Corresponding to the limitations, some recommendations are proposed for future studies
Multiphysics Modeling and Simulation of the Behavior of Cemented Tailings Backfill
No full textOne 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
Coupled Thermo-Hydro-Mechanical-Chemical (THMC) Processes in Cemented Tailings Backfill Structures and Implications for their Engineering Design
No full textThe 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
Testing and Multiphysics Modelling of the Shear Behaviour of Rock-Cemented Paste Backfill Interface
No full textCemented paste backfill (CPB) is an innovative technology developed in the mining industry during the last few decades. It has been adopted worldwide by many underground mines for its tremendous advantages: (1) mining space is stabilized by pumping cemented paste backfill into the underground cavities created by mining activity, which is critical to the safety of mine workers; (2) the consumption of tailings (which is stored at the ground surface and is a major source of acid mine drainage (AMD)) is beneficial for environmental protection and community safety; (3) due to the supporting effect of the CPB structure on underground cavities, the recovery ratio is significantly increased; and (4) CPB structures can also carry heavy equipment when mining the adjacent orebody, facilitating mining operations.
How to design a safe and cost-effective CPB structure is a key task or challenge for mining engineers and researchers. Mechanical stability is one of the most important design criteria. This stability is mainly a function of the uniaxial compressive strength (UCS) of CPB body and the shear strength/behaviour of the CPB–rock interface. Given the lower friction angle and adhesion of the CPB–rock interface (in comparison with the friction angle and cohesion of CPB body), a thorough understanding of the shear strength/behaviour of the interface is critical for a cost-effective geotechnical design of underground CPB structures. However, only limited studies have been conducted to date on the shear performance of the CPB–rock interface, and no studies have taken into consideration the effects of different factors (e.g., temperature, sulphate ions, self-weight or surface morphology) on the shear behaviour of the CPB–rock interface. Moreover, no multiphysics interface model is currently available that incorporates the aforementioned factors to describe and predict the CPB–rock interface shear behaviour. This research gap was therefore addressed in this PhD study.
In this PhD research, a series of laboratory tests were conducted assessing the effects of sulphate content, temperature, curing stress, drainage condition and interface roughness on the shear strength/behaviour of the interface between CPB and rock. The results obtained so far indicated that sulphate and temperature can either positively or negatively affect the shear strength of the CPB–rock interface, depending on the initial sulphate contents and curing time. In terms of the effect of temperature, the shear strength and shear strength properties generally increased with temperature. However, high temperature (≥ 35°C) resulted in an adverse effect on the shear strength because of the crossover effect. In addition, higher curing stress benefitted to the shear strength acquisition of the interface and, due to the increased effective stress and matrix suction, the drained condition increased shear strength as well. As for the effect of surface morphology, the shear strength of the CPB–rock interface rose with surface roughness. Furthermore, chemo-elastic as well as coupled thermo-chemo-mechanical cohesive zone models (CZMs), which take the sulphate attack and temperature-induced acceleration in the cement hydration into consideration, are also developed to simulate the shear strength and behaviour of the CPB–rock interface. The proposed models can well capture the shear behaviour of the interface under different loading conditions. Besides, they also numerically attest to the importance of the shear resistance of the CPB-rock interface in controlling stress distribution in CPB structures.
The results obtained from experimental tests, numerical modelling and simulations concerning the shear behaviour of the CPB–rock interface under different multiphysics conditions provided useful information for understanding and more effectively assessing the shear strength and behaviour of the interface between a CPB structure and rock mass, which is critical for the design of safer and more cost-effective CPB structures
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