Neste trabalho são abordados dois aspectos relacionados com modelos físicos fraturados: o primeiro, referido à resistência, é abordado através da avaliação de ensaios triaxiais em modelos fraturados pelo critério empírico de resistência de Hoek-Brown, e por análise numérica através do Código Universal de Elementos Distintos (UDEC). O segundo, referido a modos de ruptura, é abordado através da simulação em termos de deformabilidade e resistência de modelos fraturados e a simulação de taludes de diferentes alturas com o intuito de estudar a influência do tamanho do bloco no modo de ruptura. Ambos aspectos estão baseados nos resultados experimentais de Brown (1970) e de Singh (1997). A influêcia do tamanho do bloco no modo de ruptura foi estudada utilizando RMR89\", levando em consideração a escala do maciço (altura do talude). Através de análises numéricas preliminares e de um processo de retroanálises, foi simulado o comportamento mecânico dos modelos fraturados referidos. A avaliação da resistência aplicando o critério de resistência empírica de Hoek-Brown mostrou resultados coerentes quando comparados com os resultados experimentais de Brown (1970). Através de RMR89\" foi possível observar a influência do tamanho do bloco nos modos de ruptura e na estabilidade dos taludes de diferentes alturas. / This works treats about two aspects related to jointed physical models: one related to strength, by back-analyzing data using the Hoek-Brown criterion by means of numerical analyses with the Universal Distinct Element Code (UDEC). The record refers to failure modes, is analyzed by means of numerical simulations taking into account deformability and strength of jointed physical models, as well as the simulation of slopes in jointed rock masses. Slopes of different heigths and different block sizes were analyzed to investigate scale effects. The analyses were made taking experimental results obtained by Brown (1970) and Singh (1997). The influence of the block size was studied using RMR89\", taking the rock mass scale into consideration. Strength evaluation adopting Hoek-Brown empirical strength criterion proved consistent with Brown\'s model test results. By means of RMR89\" the influence of the block size could be analyzed on failure models and in the stability of slopes with different heights.

Underground excavations break the balance of the initial stress field and cause stress redistributions in the surrounding rock masses. Problems normally arise as the stress exceeds the rock mass strength. In addition, the rock mass contains preexisting defects, such as the fissures, fractures, joints, faults, shear zones, dikes, etc., which could significantly weaken the rock mass strength and make the rock mass behavior complicated. The stability of underground excavations is of great importance to an operating mine project since it ensures the safety of the working environment and the successful ore exploration. Due to the complex geological conditions and engineering disturbances, the assessment of rock mass stability for a practical engineering problem is extremely challenging and difficult, which needs to be solved by the modern numerical methods. In this dissertation, the rock mass stability around tunnels in an underground mine in the USA was investigated by performing three-dimensional modeling using the 3DEC 3-Dimensional Distinct Element Code. Comprehensive stress analyses were respectively carried out on a preliminary model and a more advanced model. In the preliminary study, the built model contains the inclined lithologies, a non-persistent fault, and a convoluted tunnel system. The geomechanical property values used for the rock masses and discontinuities in the numerical model were estimated using the available geotechnical information and the experience of the research group. The Mohr-Coulomb and strain softening constitutive relations were prescribed for the rock masses; the coulomb slip joint model was assigned for the discontinuities. The influence of the boundary conditions, block constitutive models, horizontal in situ stress and rock support system on the tunnel stability was investigated. The rock mass behavior was quantified using the results of stress, displacement, and yielded zones around the tunnels. It showed that the roller boundary conditions resulted in slightly different but comparable results with the combined boundary conditions (roller and stress combined) where K0 equals to 0.4 or 0.5. Whereas the in-situ stress field for a complex geological system can only be obtained by applying proper boundary stresses and then by performing stress analysis. The softening behavior of the rock masses caused more deformations and yielded zones around the tunnels; the rock masses around the tunnels were observed to reach the residual strength values, which can be treated as failed areas. In addition, the M-C and s-s rock masses reacted differently as the K0 value changed. At K0=1.0, the tunnels seemed to be the most stable; K0=1.5, however, provided the worst scenario with roof and floor problems. With respect to the effectiveness of the support system, a large amount of the bonds of the supports was failing, thus, the deformations and yielded zones around the tunnels were slightly improved. Finally, comparisons between the numerical modeling results and the field measurements implied the applicability of strain softening behavior and a K0 value between 0.5 and 1.0 for the mine. Based on the specific geological, geotechnical, and construction information, a numerical model incorporating accurate features was developed. It includes a non-planar, weak interlayer, the persistent and non-persistent faults, and the open and backfilled excavations. The mechanical property values used for the rock masses and faults were estimated based on the laboratory test results of the intact rock and smooth joints. The strain softening behavior was specified for the rock masses belonging to the average quality, and the rock masses that reached residual strengths were assumed to be failing. The linear relations between the fault stiffnesses and normal stress were described using the continuously yielding joint model. To simulate the mine construction process in the field, the sequential excavation, backfilling, and supporting procedures were numerically implemented; additionally, a novel routine was applied to account for the delayed installation of the supports. Results showed that the tunnels close to the fault and the backfilled area were less stable. Most of the displacements around the tunnels occurred within a distance of zero to 2 or 3 m from the tunnel surface. The varying K0 value caused great changes in the rock mass behavior and the shear behavior of the major fault; significant instability of the tunnels was triggered by the high horizontal in situ stress. Parametric studies on the rock mass condition, rock mass residual strengths, and fault property values showed that the tunnel stability was more sensitive to the former two factors than the last one. A systematic investigation was conducted to evaluate the current rock supports installed at the mine where the increasing stress relaxation was incorporated. The deformations and of the failure zone thicknesses around the tunnels were reduced up to 8% and 20% after applying the supports instantaneously, and the reductions were improved by the delayed installation of supports. Additionally, the safety of supports was evaluated by the bond shear and bolt tensile failures, which was also improved with incorporation of delayed supporting. It was found that the current rock supports are insufficient in length, bond and tensile strengths. Therefore, a stronger support system was suggested. The stronger supports worked better in stabilizing the tunnels. Based on the deformations and failures of the rock masses, the length of the bolts on walls was suggested to be 4-5 m. At the end, the horizontal convergence strain predicted by the numerical simulations were calculated at two locations where the tape extensometers were installed. Good agreements with the field measurements were obtained for the cases that have the average rock mass properties and K0 values in the range 0.5-1.25.

The behavior of the ballast particles during their interaction with tamping tines in tamping operation is studied by developing a simulation model using the Discrete Element Model (DEM), with the aim of optimizing the railroad tamping operation. A comprehensive literature review is presented showcasing the applicability of DEM techniques in modeling ballast behavior and its feasibility in studying the fundamental mechanisms that influence the outcome of railroad tamping process is analyzed. The analysis shows that DEM is an excellent tool to study tamping operation as its important and unprecedented insights into the process, help not only to optimize the current tamping practices but also in the development of novel methods for achieving sustainable improvements in the track stability after tamping. The simulation model is developed using a commercially available DEM software called PFC3D (Particle Flow Code 3D). A detailed explanation is provided about how to set up the DEM model of railway ballast considering important parameters like selection and calibration of particle shapes, ballast mechanical properties, contact model, and parameters governing the contact force models. Tamping operation is incorporated into the simulation model using a half-track layout with a highly modular code that enables a high degree of adjustability to allow control of all process parameters for achieving optimized output. A parametric study is performed to find the best values of tine motion parameters to optimize the linear tamping efficiency and a performance comparison has been made between linear and elliptical tamping. It is found that squeeze and release velocity of the tines should be lesser for better compaction of the particles and linear tamping is better compared to elliptical tamping. / Master of Science / Railway track stability is the resistance of the tracks to deformation and is affected by the rail traffic, ballast fouling (contamination of ballast) and the changing environmental conditions. The track stability depends on the normal and frictional support provided by the ballast to the sleepers. Non-uniform ballast consolidation below the railway sleeper results in erratic wheel-rail contact forces, low traffic speeds, poor ride quality, and derailments. Thus, tamping is a railway track maintenance method done periodically on the railway tracks to ensure track stability. Tamping process involves compacting the railroad ballast underneath the sleeper. The sleeper is lifted by a desired height and then vibrating tamping tools called tines are inserted into the ballast below the sleeper to fill the void created by lifting of the sleeper and the sleeper is dropped back on to the ballast. So, it is important to understand the ballast mechanics, dynamics and ballast’s behavioral response to the tamping operation. Since, large scale experiments such as this are difficult, this operation has been simulated in a commercially available software called PFC3D using a Discrete Element Model (DEM) to represent the railway ballast. It is shown through a simulation that though spherical particles provide better computational efficiency, they cannot capture the exact ballast behavior like clumps (a collection of spherical pebbles). So using clumps to represent ballast, efforts are made to optimize the linear tamping efficiency. This is done by changing the values of parameters like tine amplitude, tine frequency, insertion velocity and squeeze velocity and finding their optimum values. Linear tamping results are compared with elliptical tamping. Thus, an optimum tamping cycle would help save money spent on the track maintenance activities.

Doctor of Philosophy / Department of Industrial & Manufacturing Systems Engineering / Shuting Lei / This study experimentally and numerically investigates laser assisted milling (LAMill) of silicon nitride ceramics. Experiments are conducted to study the machinability of Si3N4 under LAMill. The effects of temperature on cutting forces, tool wear, surface integrity, edge chipping and material removal mechanisms are investigated. It is shown that when temperature increases, cutting force and tool wear are significantly decreased, surface integrity is improved, chip size is increased and material removal demonstrates more plastic characteristics. The mechanisms of edge chipping at elevated temperature are investigated theoretically and experimentally. When temperature is above the softening point and below the brittle/ductile transition temperature, the mechanism is mainly through softening. When temperature is above the brittle/ductile transition temperature, toughening mechanism contributes significantly to the reduced edge chipping. The coupled effect of softening and toughening mechanisms shows that temperature range between 1200 to 1400°C has the most significant effect to reduce edge chipping. Distinct element method (DEM) is applied to simulate the micro-mechanical behavior of Si3N4. First, quantitative relationships between particle level parameters and macro-properties of the bonded particle specimens are obtained, which builds a foundation for simulation of Si3N4. Then, extensive DEM simulations are conducted to model the material removal of machining Si3N4. The simulation results demonstrate that DEM can reproduce the conceptual material removal model summarized from experimental observations, including the initiation and propagation of cracks, chip formation process and material removal mechanisms. It is shown that material removal is mainly realized by propagation of lateral cracks in machining of silicon nitride. At the elevated temperature under laser assisted machining, lateral cracks are easier to propagate to form larger machined chips, there are fewer and smaller median cracks therefore less surface/subsurface damage, and crushing-type material removal is reduced. The material removal at elevated temperature demonstrates more plastic characteristics. The numerical results agree very well with experimental observations. It shows that DEM is a promising method to model the micro-mechanical process of machining Si3N4.

In this thesis, fractured rock masses are treated asequivalent continua for large-scale analyses of rockengineering projects. Systematic developments are made for thedetermination of equivalent mechanical and hydraulic propertiesof fractured rock masses using a hybrid discrete fracturenetwork - distinct element method (DFN-DEM) approach. Thedetermined equivalent properties are then used for a far-fieldfinite element analysis of the thermo-mechanical impacts on thestress, deformation and permeability of fractured rockssurrounding a hypothetical geological repository of nuclearwaste. The geological data were extracted from the results ofan extensive site investigation programme at Sellafield, UK,conducted by Nirex UK Ltd. The scale dependencies of the hydraulic and mechanicalproperties were investigated by using multiple realizations ofthe fracture system geometry with increasing model sizes untilproperly defined hydraulic and mechanical representativeelementary volumes (REVs) were reached. The validity of thesecond order permeability tensor and the fourth-ordermechanical compliance tensor were tested for continuum analysesat larger scales. The REV was determined to be around 5 m formechanical and hydraulic data in this study. Analysis of the stress-dependent mechanical and hydraulicproperties shows that the effect of rock stresses is crucial.The elastic moduli increase significantly with the increase ofstress and an empirical equation of stress-dependent elasticmodulus is suggested based on results of numerical experiments.Calculations of the Poisson's ratios suggest greater valuesthan are normally assumed in practice. Depending on the stateof stress, permeability decreases or increases with increasingcompressive stress. Stress-induced flow channeling effect iscaptured by numerical modeling for the first time and detailedmechanisms of shear dilation of fractures are provided. Basedon the numerical experiments, a set of empirical equations wassuggested for the stress-dependent permeability, consideringboth normal deformation and shear dilation of fractures. Thermo-mechanical impact on the performance of ahypothetical repository at a far-field scale (5 km by 1 km) wasinvestigated with the stress-dependent equivalent propertiesdetermined at the REV scale. This analysis shows thatmechanical responses vary significantly depending on how themechanical properties were determined. The change ofpermeability due to the thermal loading is, however, notsignificant in this particular case. The thesis provides a framework for systematic analysis oflarge-scale engineering applications in fractured rock masses,such as geological repositories of nuclear wastes. Keyword:Fractured rock masses, Equivalent Continuum,Discrete Fracture Network (DFN), Distinct Element Method (DEM),Finite Element Method (FEM), Nuclear Waste Disposal, CoupledThermo-Hydro-Mechanical Processes

<p>In this thesis, fractured rock masses are treated asequivalent continua for large-scale analyses of rockengineering projects. Systematic developments are made for thedetermination of equivalent mechanical and hydraulic propertiesof fractured rock masses using a hybrid discrete fracturenetwork - distinct element method (DFN-DEM) approach. Thedetermined equivalent properties are then used for a far-fieldfinite element analysis of the thermo-mechanical impacts on thestress, deformation and permeability of fractured rockssurrounding a hypothetical geological repository of nuclearwaste. The geological data were extracted from the results ofan extensive site investigation programme at Sellafield, UK,conducted by Nirex UK Ltd.</p><p>The scale dependencies of the hydraulic and mechanicalproperties were investigated by using multiple realizations ofthe fracture system geometry with increasing model sizes untilproperly defined hydraulic and mechanical representativeelementary volumes (REVs) were reached. The validity of thesecond order permeability tensor and the fourth-ordermechanical compliance tensor were tested for continuum analysesat larger scales. The REV was determined to be around 5 m formechanical and hydraulic data in this study.</p><p>Analysis of the stress-dependent mechanical and hydraulicproperties shows that the effect of rock stresses is crucial.The elastic moduli increase significantly with the increase ofstress and an empirical equation of stress-dependent elasticmodulus is suggested based on results of numerical experiments.Calculations of the Poisson's ratios suggest greater valuesthan are normally assumed in practice. Depending on the stateof stress, permeability decreases or increases with increasingcompressive stress. Stress-induced flow channeling effect iscaptured by numerical modeling for the first time and detailedmechanisms of shear dilation of fractures are provided. Basedon the numerical experiments, a set of empirical equations wassuggested for the stress-dependent permeability, consideringboth normal deformation and shear dilation of fractures.</p><p>Thermo-mechanical impact on the performance of ahypothetical repository at a far-field scale (5 km by 1 km) wasinvestigated with the stress-dependent equivalent propertiesdetermined at the REV scale. This analysis shows thatmechanical responses vary significantly depending on how themechanical properties were determined. The change ofpermeability due to the thermal loading is, however, notsignificant in this particular case.</p><p>The thesis provides a framework for systematic analysis oflarge-scale engineering applications in fractured rock masses,such as geological repositories of nuclear wastes.</p><p><b>Keyword:</b>Fractured rock masses, Equivalent Continuum,Discrete Fracture Network (DFN), Distinct Element Method (DEM),Finite Element Method (FEM), Nuclear Waste Disposal, CoupledThermo-Hydro-Mechanical Processes</p>

The goal of this research is to investigate the fundamentals of fracturing processes in heterogeneous materials such as concrete using geophysical methods and dynamic micromechanical models. This work describes how different aspects of fracture formation in concrete can be investigated using a combination of Acoustic Emission (AE) techniques, ultrasonic wave velocity imaging, and high resolution Computed Tomography (CT). Fracture formation and evolution were studied during shear failure of large reinforced concrete beams and compressive failure of concrete samples. AE analysis includes studying complex spatial and temporal fracture development that precedes shear failure. Predominant microcrack mechanisms were analyzed at different stages of fracture formation. CT images were used to investigate the influence of concrete microstructure on fracture topography. Combined AE and CT damage evaluation techniques revealed different aspects of fracture development, thus expanding our understanding of AE events and their mechanisms. These images show how aggregate particles influence fracture nucleation and development. An emphasis has been placed on the role of coarse aggregates during the interlocking of fracture surfaces at transferring shear stresses. Ultrasonic wave velocity and AE techniques have been applied to uniaxial compression tests of concrete with various aggregate sizes and strengths similar to that of the concrete beams. AE parameters, p-wave velocities, and stress-strain data have been analyzed concurrently to image damage evolution under compression. Influence of material composition on microcracking and material state changes during loading has been investigated in detail. The results of compressive tests were used as building blocks for developing realistic micromechanical numerical models of concrete. The models were designed using a distinct element code, where material is modelled through the combination of bonded particles. A number of procedures were developed to transfer the exact microstructure of material incorporating its visual representation into the model. The models’ behaviour has been verified against experimental data. It was shown that these models exhibit realistic micromechanical behaviour. The results of the experimental investigation of concrete fracturing were expanded by modelling more cases with aggregate size and strength variations. It was shown that geophysical imaging techniques, along with advanced micromechanical numerical modelling, can help us understand damage formation and evolution.

The goal of this research is to investigate the fundamentals of fracturing processes in heterogeneous materials such as concrete using geophysical methods and dynamic micromechanical models. This work describes how different aspects of fracture formation in concrete can be investigated using a combination of Acoustic Emission (AE) techniques, ultrasonic wave velocity imaging, and high resolution Computed Tomography (CT). Fracture formation and evolution were studied during shear failure of large reinforced concrete beams and compressive failure of concrete samples. AE analysis includes studying complex spatial and temporal fracture development that precedes shear failure. Predominant microcrack mechanisms were analyzed at different stages of fracture formation. CT images were used to investigate the influence of concrete microstructure on fracture topography. Combined AE and CT damage evaluation techniques revealed different aspects of fracture development, thus expanding our understanding of AE events and their mechanisms. These images show how aggregate particles influence fracture nucleation and development. An emphasis has been placed on the role of coarse aggregates during the interlocking of fracture surfaces at transferring shear stresses. Ultrasonic wave velocity and AE techniques have been applied to uniaxial compression tests of concrete with various aggregate sizes and strengths similar to that of the concrete beams. AE parameters, p-wave velocities, and stress-strain data have been analyzed concurrently to image damage evolution under compression. Influence of material composition on microcracking and material state changes during loading has been investigated in detail. The results of compressive tests were used as building blocks for developing realistic micromechanical numerical models of concrete. The models were designed using a distinct element code, where material is modelled through the combination of bonded particles. A number of procedures were developed to transfer the exact microstructure of material incorporating its visual representation into the model. The models’ behaviour has been verified against experimental data. It was shown that these models exhibit realistic micromechanical behaviour. The results of the experimental investigation of concrete fracturing were expanded by modelling more cases with aggregate size and strength variations. It was shown that geophysical imaging techniques, along with advanced micromechanical numerical modelling, can help us understand damage formation and evolution.

[pt] O presente trabalho apresenta a aplicação das metodologias de análise de estabilidade para taludes rochosos de grande altura comumente aplicados dentro do mundo da mineração. Mostra os principais fatores que influem dentro de uma análise de estabilidade como são a determinação das propriedades geomecânicas do maciço rochoso (rocha intacta e descontinuidades) e a caracterização geológica estrutural da região estudada. Esses parâmetros geralmente são obtidos a partir de ensaios de laboratório e de campo, além da informação que fornecem os mapeamentos geológicos. Outra forma de determinar estas propriedades de resistência é através de retro-análises em regiões onde tenha ocorrido algum tipo de colapso ou pela recopilação de dados que foram utilizados em análises anteriores e possam ser extrapolados. Também é apresentado um passo a passo das distintas etapas de um estudo de estabilidade e a obtenção de dados que finalizam na aplicação de softwares especializados na área de geotécnia e geomecânica, os quais permitirão determinar e pré-visualizar os possíveis problemas de instabilidade dentro dos taludes de um pit mineiro. Serão apresentados dois tipos de análises de estabilidade, o primeiro baseado na teoria do método de equilíbrio limite, o qual procura a possível superfície crítica de deslizamento dentro de um talude e que, condiciona a estabilidade da parede aos valores do fator de segurança. A segundo análise está relacionada com um modelo numérico, o qual aplica o método de elementos distintos ou de blocos, que permite a aplicação de um modelo constitutivo que descreva o comportamento dos materiais e das descontinuidades dentro do maciço rochoso, fornecendo como resultados a pré-visualização de variação de deslocamentos e vetores de velocidade para cada etapa de escavação. / [en] This thesis presents the application of procedures for the stability analysis of high slopes commonly applied in the mining world. It presents the main parameters that influence the stability analysis: determination of the rock mass geomechanical properties (intact rock and discontinuities) and the geological structures characterization of the studied region. These parameters generally are the result of laboratory and field tests, in addition, the information about the geological mapping. Another procedure for determining the properties strength may be through a back analysis of a collapsed zone or extrapolation of a data compilation from previous analyses. Apart from strength parameter determination and concept applying of rock mechanics, this research describes step by step the different stages of a stability analysis and its obtained data, this work then finishes with a geotechnical and geomechanical software application, which will determines and previews slope stability problems for a mining pit. This thesis includes two stability analysis procedures: the first method applies the limit equilibrium theory, which looks for the critical failure surface that depends on depends on a factor of safety. The second one applies a numerical model that uses distinct element method, which through a constitutive model, describes how the materials and discontinuities behave in a rock mass, obtaining as a result, the displacement and velocity vectors for each excavation stage.

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