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LKAB’s underground mine in Kiirunavaara has experienced an increasing seismic activity the last ten years. This seismic activity is caused by the stress redistribution resulting from the mining method of large-scale sublevel caving. The energy from the seismic events propagate in the rock mass as seismic waves. If one of these waves interacts with an excavation, it will be subjected to dynamic loads, and damage can potentially occur. Damage can be caused by different mechanisms depending on many factors such as pre-existing structures in the rock mass and the state of stress. To prevent these damages, LKAB has installed a rock support system for handling dynamic loads. This thesis has analysed available damage mapping reports, investigations, pictures, seismic data and history, in order to evaluate the function of the support system when subjected to dynamic loads. The conclusion of the analysis is that the support system is well designed, but there are areas of improvement. The main damage mechanisms are bulking without ejection and rockfall due to seismic shaking. Bulking with ejection and ejection due to seismic energy transfer were concluded to not yet be a problem in the Kiirunavaara mine. This result implies that an improved stiffness, static strength and yieldability are to be considered in order to decrease the amount of bulking. For rockfall due to seismic shaking, there are two main areas of improvement. The structural mapping has to be given higher priority, and it should provide direct support recommendations if needed. The second part is to increase the static strength of the system in order to survive rockfall due to seismic shaking. Since bulking with ejection and ejection due to seismic energy transfer are not yet considered significant problems, there is no need to improve the support system with respect to absorption of kinetic energy. The location of the damages in the drift profiles were also analysed, and it was concluded that a majority of the damages that occurred in the footwall drifts were located in the corner of the abutment facing the orebody. In the crosscuts, a majority of the damages occurred in the abutment and roof. Based on this, it is suggested that the support should be improved in the abutment and roof of the crosscuts, and in the abutment facing the ore of the footwall drifts.

Tese de mestrado integrado. Engenharia Civil (Especialização em Geotecnia). Faculdade de Engenharia. Universidade do Porto. 2010

Stability of underground excavations is a prerequisite for the proper functioning of all other systems in a mining environment. From a safety point of view, the lives of people working underground rely on how well the support systems installed underground are performing. The ground control engineer cannot design an effective support system unless the area of the rock mass around the opening, prone to failure, is well identified in advance, even before the excavation of the tunnel. Under high stress conditions, usually experienced at deep mining levels, stress-induced rock failure is the most common type of instability around the underground openings. This thesis focuses firstly on the use of the finite difference numerical tool FLAC to simulate brittle rock failure under static in-situ stresses. Brittle failure of the rock mass around underground openings is a particular type of stress-induced failure, which can result in notch-shaped breakouts around the boundary of the tunnel. Generation of these breakout zones is a discontinuum process and approximating this process using FLAC, which is a continuum tool, requires careful consideration of the stress conditions and the stress related behavior of rock material. Based on plasticity theory, this thesis makes an effort to estimate the breakout formation using an elastic – brittle - plastic material model. Due to seismic challenges that deep mining operations are currently experiencing, rockbursting is a major hazard to the stability of underground structures. Therefore in this research, brittle failure of rock in the vicinity of the underground excavations is approximated also under dynamic loading conditions. The numerically modeled results of two different material models iv are compared with each other along with a previously developed empirical graph. This assessment, when further validated by field observations, may provide a different perspective for underground support design under burst-prone conditions.

Dissertação (mestrado)—Universidade de Brasília, Faculdade de Tecnologia, Departamento de Engenharia Civil e Ambiental, 2015. / Submitted by Albânia Cézar de Melo (albania@bce.unb.br) on 2016-03-30T12:54:06Z No. of bitstreams: 1 2015_JaderAlfonsoRuizPerez.pdf: 4717566 bytes, checksum: 692bdeba2a74fcf07ff414bc22c93fe5 (MD5) / Approved for entry into archive by Raquel Viana(raquelviana@bce.unb.br) on 2016-03-31T11:01:33Z (GMT) No. of bitstreams: 1 2015_JaderAlfonsoRuizPerez.pdf: 4717566 bytes, checksum: 692bdeba2a74fcf07ff414bc22c93fe5 (MD5) / Made available in DSpace on 2016-03-31T11:01:33Z (GMT). No. of bitstreams: 1 2015_JaderAlfonsoRuizPerez.pdf: 4717566 bytes, checksum: 692bdeba2a74fcf07ff414bc22c93fe5 (MD5) / O objetivo desta dissertação é estudar os principais critérios empíricos que predizem o potencial de rockburst nos túneis por meio das tensões e as propriedades mecânicas da rocha, analisando a influência das tensões in situ e a geometria da escavação. Para o estudo do desempenho dos critérios na avaliação do potencial, é utilizada a metodologia de análise Característica Operativa do Receptor – Análise ROC, a qual por meio de uma matriz de confusão, compara os valores preditos pelos critérios com os valores obtidos na realidade. Para isto se contou com um banco de dados de 141 eventos ocorridos em diferentes locais do mundo e recopilados de 16 artigos diferentes disponíveis na literatura. Com esta metodologia foi também avaliado o desempenho dos intervalos de classificação das intensidades do potencial de rockburst (Baixo, Moderado e Violento). No estudo da influência das tensões in situ foram propostas quatro etapas de modelagem numérica com o software Examine 3D, o qual utiliza a ferramenta numérica Método dos Elementos de Contorno (MEC). Nas quatro etapas são utilizadas diferentes magnitudes dos estados de tensão e diferentes atitudes. Já no estudo da influência da geometria da escavação, foram utilizadas duas seções, uma circular e uma em arco-retângulo. Nas duas seções foram utilizados os mesmos estados de tensão para assim comparar os diferentes resultados entre as tensões induzidas e sua influência na formação de zonas com potencial de rockburst. Finalmente, é apresentado um procedimento para projeção de sistemas de suporte em tuneis que estão submetidos a condições de rockburst, nele são apresentados os princípios de projeto e critérios de aceitabilidade orientados à determinação da demanda de carga, deslocamento e energia, em função da magnitude de um evento sísmico de projeto. Ao final, é apresentado exemplo no qual é determinado o sistema de suporte para um caso de estudo real onde se tem problemas de rockburst. / The aim of this work is to study the main empirical criterion to predict rockburst potential in tunnels through the stresses and mechanical properties of the rock by analysis of the influence of the in situ stresses and excavation geometry. Receiver Operating Characteristic (ROC) methodology was used to study of the criteria performance in the potential assessment. The ROC analysis uses a matrix confusion to compare the values predicted by the criteria with the values obtained in reality. It was used a dataset with 141 events occurred in different parts of the world and compiled from 16 different paper available in the literature for the ROC analysis. With this methodology was also evaluated the performance of classification ranges intensity of rockburst potential (Low, Moderate and Violent). Four stages of numerical modeling with Examine 3D software were proposed to study the influence of the in situ stresses. This software uses a numerical tool Boundary Element Method (BEM). In the four stages were used different stress states, different dips, and dip directions. Two cross sections were analyzed to study the influence of the excavation geometry, one circular and one in arch-rectangle. It was used the same stress states in both cross sections to compare the different results of induced stress and its influence in the formation of rockburst potential areas. Finally, a design procedure for rock support systems in tunnels under rockburst conditions is presented. In this design procedure are listed the design principles and acceptability criteria to determining the load demand, displacement and energy in function of the seismic event magnitude. At the end, it is shown an example in which the rock support system is determined for a real case with rockburst problems.

Le phénomène de coup de terrain est une explosion violente de roche qui peut se produire dans les mines souterraines. Dans la présente recherche, nous avons essayé de démontrer les causes qui peuvent influer sur la prédisposition aux coups de terrain en utilisant la modélisation numérique. Cependant, avant tout, l'état de contrainte avant l'exploitation minière et les contraintes induites par les excavations environnantes doivent être étudiés avec précision. La mine de charbon de Provence, qui a subi un phénomène de coup de terrain au niveau de son puits vertical entouré de nombreux panneaux de longue taille, a été choisie comme cas d’étude. Un modèle numérique 3D à grande échelle a été construit pour inclure la zone du puits vertical avec ses piliers et galeries à petite échelle et les panneaux de longue taille à grande échelle avec leurs zones de foudroyage associées. Plusieurs problèmes ont été rencontrés lors du développement de ce modèle numérique à grande échelle. Le premier porte sur l'initialisation de l'état de contrainte à grande échelle, où les contraintes verticales mesurées divergent avec le poids des déblais et les contraintes in situ sont très anisotropes. Le deuxième porte sur la simulation de la zone de foudroyage associée aux panneaux de longue taille. Le troisième concerne l'évaluation de l’instabilité du pilier en fonction de son ratio résistance/contrainte moyenne et de son volume. Le quatrième concerne l'évaluation de la prédisposition aux coups de terrain au niveau du puits vertical en fonction de différents critères. Cinq méthodes ont été développées pour initialiser l’état de contrainte hétérogène dans le modèle numérique à grande échelle avant l’exploitation minière. Elles sont basées sur la méthode de corrélation Simplex, qui consiste à optimiser la différence entre les valeurs de contrainte mesurées in-situ et les valeurs numériques. Le but est de développer des gradients qui soient capables d'exprimer l'hétérogénéité de la contrainte et qui soient compatibles avec les mesures in-situ. La méthode basée sur l’initialisation de l'état de contrainte avec des gradients 3D s’est avérée plus efficace que celle traditionnelle basée sur les ratios de contrainte horizontale à verticale. Concernant la simulation du foudroyage, trois modèles ont été développés et intégrés dans le modèle numérique pour exprimer le comportement mécanique dans la zone de foudroyage au-dessus des panneaux de longue taille. Deux d’entre eux sont basés sur un comportement élastique alors que le troisième est basé sur un comportement elasto-plastique avec écrouissage un phénomène de consolidation. Il a été constaté que la zone de foudroyage au-dessus des panneaux de longue taille peut atteindre 32 fois l'épaisseur de la couche exploitée et que le module d'élasticité de la partie la plus endommagée de la zone foudroyée ne doit pas excéder 220 MPa pour satisfaire la convergence toit-mur. Mais, avec l'avancée de l'exploitation, ce matériau souple se compacte sous la pression des couches supérieures. Dans le cas d'une largeur critique et super-critique, la contrainte verticale dans la zone de foudroyage pourrait dépasser le poids des déblais et pourrait augmenter jusqu'à 4 fois ce poids sur les bords. La contrainte verticale a augmenté dans les piliers au niveau du puit vertical suite à l'exploitation des panneaux de longue taille à proximité. Il a été constaté que le volume du pilier joue un rôle important dans sa stabilité. Le rapport contrainte/résistance a été jugé insuffisant pour expliquer un coup de terrain. Plusieurs critères ont été intégrés au modèle numérique pour évaluer la prédisposition aux coups de terrain. Il a été constaté que les critères basés sur les contraintes et les déformations sont capables d'évaluer la prédisposition aux coups de terrain / Rockburst is a violent explosion of rock that can occur in underground mines. In the current research, the main objective is to demonstrate the causes that may influence the rockburst proneness by using the numerical modeling tool. However, firstly, the pre-mining stress state and the induced stresses due to surrounding excavations have to be studied precisely. The Provence coal mine, where a rockburst took place in its shaft station that is surrounded by many longwall caving panels, has been chosen as a case study. A large-scale 3D numerical model has been constructed to include the shaft station area with its small-scale pillars and galleries, and the large-scale longwall panels with their accompanying goaf area. Many problems appeared while developing such large-scale numerical model, the first problem was the initialization of stress state at a large-scale, where the measured vertical stresses are in disagreement with the overburden weight, and the in-situ stresses are highly anisotropic. The second problem was the simulation of the goaf area accompanying longwall panels. The third problem was the assessment of pillars instability in terms of its strength/average stress ratio, and its volume. The Fourth problem was the assessment of rockburst proneness in the shaft station based on different rockburst criteria. Five methods were developed to initialize the heterogeneous pre-mining stress in the large-scale numerical model. These methods are based on the Simplex Method, which is mainly based on optimizing the difference between the in-situ measured stress values and the numerical stress values to develop stress gradients able to express the stress heterogeneity and compatible with the in-situ measurements. The method that is based on initiating the stress state with 3D stress gradients was found to be more efficient than the traditional method that is based on the horizontal-to-vertical stress ratios. Regarding the goaf simulation, three models were developed and implemented in the numerical model to express the mechanical behavior within the goaf area above longwall panels. Two of these models are based on an elastic behavior, and the third one is based on the strain-hardening elasto-plastic behavior that takes the consolidation phenomenon into consideration. It was found that the goaf area above longwall panels could reach up to 32 times the seam thickness, and the elastic modulus of caved area (the first few meters in the goaf area) did not exceed 220 MPa to fulfill the roof-floor convergence. But, with advance of the exploitation, this soft material consolidated under the pressure of the overlying strata. In case of critical and super-critical width, the vertical stress in the goaf area exceeded the overburden weight, and it increased up to 4 times the overburden weight on the rib-sides. The vertical stress increased in the shaft station pillars as a result of exploiting the nearby longwall panels. It was found that the pillar volume plays an important role in its stability. And, the strength/stress ratio was found to be insufficient to quantify the rockburst proneness in underground mines. Many rockburst criteria were implemented in the numerical model to assess the rockburst proneness. It was found that the criteria that are based on stress and strain changes were able to assess the rockburst proneness

Mining engineers are tasked with ensuring that underground mining operations be both safe and efficiently productive. Induced stress in deep mines has a significant role in the stability of the underground mines and hence the safety of the mining workplace because the behavior of the rock mass associated with mining-induced seismicity is poorly-understood. Passive seismic tomography is a tool with which the performance of a rock mass can be monitored in a timely manner. Using the tool of passive seismic tomography, the advance rate of operation and mining designs can be updated considering the induced stress level in the abutting rock. Most of our current understanding of rock mass behavior associated with mining-induced seismicity comes from numerical modeling and a limited set of case studies. Therefore, it is critical to continuously monitor the rock mass performance under induced stress. Underground stress changes directly influence the seismic wave velocity of the rock mass, which can be measured by passive seismic tomography. The precise rock mass seismicity can be modeled based on the data recorded by seismic sensors such as geophones of an in-mine microseismic system. The seismic velocity of rock mass, which refers to the propagated P-wave velocity, varies associated with the occurrence of major seismic events (defined as having a local moment magnitude between 2 to 4). Seismic velocity changes in affected areas can be measured before and after a major seismic event in order to determine the highly stressed zones. This study evaluates the seismic velocity trends associated with five major seismic events with moment magnitude of 1.4 at a deep narrow-vein mine in order to recognize reasonable patterns correlated to induced stress redistribution. This pattern may allow recognizing areas and times which are prone to occurrence of a major seismic event and helpful in taking appropriate actions in order to mitigate the risk such as evacuation of the area in abrupt cases and changing the aggressive mine plans in gradual cases. In other words, the high stress zones can be distinguished at their early stage and correspondingly optimizing the mining practices to prevent progression of high stress zones which can be ended to a rock failure. For this purpose a block cave mine was synthetically modeled and numerically analyzed in order to evaluate the capability of the passive seismic tomography in determining the induced stress changes through seismic velocity measurement in block cave mines. Next the same method is used for a narrow vein mine as a case study to determine the velocity patterns corresponding to each major seismic event. / Doctor of Philosophy / Mining activities unbalance the stress distribution underground, which is called mining induced stress. The stability of the underground mines is jeopardized due to accumulation of induced stress thus it is critical for the safety of the miners to prevent excessive induced stress accumulation. Hence it is important to continuously monitor the rock mass performance under the induced stress which can form cracks or slide along the existing discontinuities in rock mass. Cracking or sliding releases energy as the source of the seismic wave propagation in underground rocks, known as a seismic event. The velocity of seismic wave propagation can be recorded and monitored by installing seismic sensors such as geophones underground. The seismic events are similar to earthquakes but on a much smaller scale. The strength of seismic events is measured on a scale of moment magnitude. The strongest earthquakes in the world are around magnitude 9, most destructive earthquakes are magnitude 7 or higher, and earthquakes below magnitude 5 generally do not cause significant damage. The moment magnitude of mining induced seismic events is typically less than 3. In order to monitor mining induced stress variations, the propagated seismic wave velocity in rock mass is measured by a series of mathematical computations on recorded seismic waves called passive seismic tomography, which is similar to the medical CT-scan machine. Seismic wave velocity is like the velocity of the vibrating particles of rock due to the released energy from a seismic event. This study proposes to investigate trends of seismic velocity variations before and after each seismic event. The areas which are highly stressed have higher seismic velocities compared to the average seismic velocity of the entire area. Therefore, early recognition of highly stressed zones, based on the seismic velocity amount prior the occurrence of major seismic events, will be helpful to apply optimization of mining practices to prevent progression of high stress zones which can be ended to rock failures. For this purpose, time-dependent seismic velocity of a synthetic mine was compared to its stress numerically. Then, the seismic data of a narrow vein mine is evaluated to determine the seismic velocity trends prior to the occurrence of at least five major seismic events as the case study.