Sublevel caving is a highly mechanizable mass mining method normally utilized in large, steeply dipping orebodies. The fragmented ore flows freely, aided by gravity, down to the drawpoint while the surrounding waste rock caves in due to induced stresses and gravity. Fragmentation of the blasted ore is a vital component in any mining operation and directly affects productivity and efficiency of the following production steps (Nielsen et. al, 1996). In an attempt to reduce mining induced seismicity in Malmberget, LKAB is initiating various trials. One of these trials involves a reduction in blasthole dimension and an increase in the number of blastholes utilized in each ring. A reduction in blasthole dimension is undertaken to achieve a less impactful mining operation in terms of disturbances to surface populated areas, particularly addressed to ground vibrations. Therefore, it is of utmost importance to analyse if fragmentation and production is affected as a consequence of this change. This thesis sets out to evaluate how fragmentation and the LHD operation is affected by variations in blasthole dimension. The evaluation is carried out through analysis of logged production data, on-site filming of the loading sequence and interviews with the LHD operators. The discoveries will be presented chronologically to illustrate the complexities related to compiling a viable dataset to rely on for a credible analysis. The initial theory did not hold up properly and therefore the project was reshaped along the course of progression to provide further information and clarify uncertainties. Unfortunate, major production delays inhibited a quantitative comparison of two parallel drifts with different blasthole dimensions. Hence, no final answer can be provided in this thesis whether a change in blasthole dimension causes any differences in loadability and/or fragmentation or not. However, an analysis of how cycle times vary depending on causes such as operator induced differences, machine induced differences and road conditions will be provided. The field test also provides information on various loading scenarios and the difficulties connected to these. The result obtained in this project mainly addresses the significant operator difference in terms of cycle times which can extend to, on average, 60% depending on experience, road conditions and, most likely, preferences amongst operators. Time differences amongst seemingly experienced operators can reach more than, on average, 30% in hauling time alone. Roughly 96% of the operators state that road conditions in the production area is the controlling factor for hauling speed. Many of the operators further states that the risk of injuries is directly related to road conditions and this is a likely cause to why cycle times vary in this magnitude. Fragmentation was found to affect loadability but not to the same extent as shape and looseness of the muck pile. Compaction of the muck pile and flow disturbances where normally found to be connected to one another. Hence, good loadability would indicate a low occurrence of flow disturbances and a continuous flow of material into the drawpoint. This thesis is written as a part of the final stage of the civil engineering program at Luleå University of Technology (LTU) and represents 30 credits in the field of Soil and Rock Construction. The thesis is a part of a larger project, Face to surface, which sets out to analyse the impact of fragmentation on different stages in the production chain.

Comprehensive and reliable seismic analysis techniques can aid in achieving successful inference of rockmass behaviour in different stages of the caving process. This case study is based on field data from Telfer sublevel caving mine in Western Australia. A seismic monitoring database was collected during cave progression and breaking into an open pit 550 m above the first caving lift. Five seismic analyses were used for interpreting the seismic events. Interpretation of the seismic data identifies the main effects of the geological features on the rockmass behaviour and the cave evolution. Three spatial zones and four important time periods are defined through seismic data analysis. This thesis also investigates correlations between the seismic event rate, the rate of the seismogenic zone migration, mucking rate, Apparent Stress History, Cumulative Apparent Volume rate and cave behaviour, in order to determine failure mechanisms that control cave evolution at Telfer Gold mine.

[pt] O método de exploração Sublevel Caving é um dos métodos de extração massiva mais usados na indústria mundial de exploração subterrânea, sendo considerado pela indústria de mineração, num futuro próximo dentre os substitutos naturais das atuais minas a céu aberto. Uma operação Sublevel Caving requer que o maciço rochoso circundante ao minério rompa continuamente e se movimente para dentro do espaço criado pela extração do minério. Análises existentes na literatura consideram apenas configurações parciais do processo Sublevel Caving sem considerar o processo evolutivo da extração, e o dano induzido ao maciço rochoso decorrente deste processo. Esta dissertação desenvolve uma modelagem numérica utilizando o método dos elementos discretos para simular o mecanismo de ruptura e a subsidência causada pelo método de exploração Sublevel Caving, analisando o referido efeito e suas consequências na evolução do mecanismo de ruptura e subsidência no Sublevel Caving. O software comercial Particle Flow Code (PFC2D) foi selecionado para esta modelagem devido à capacidade de simular, em um evento de excesso de tensão, o fraturamento do maciço rochoso e sua desintegração, desta forma, originam-se o fluxo do material e os deslocamentos em grande escala, os quais são considerados fenômenos físicos dominantes que regem a formação da subsidência e fraturamento num processo Sublevel Caving. Os resultados obtidos nesse estudo mostraram-se satisfatórios, reproduzindo adequadamente a superfície de subsidência induzida por Sublevel Caving, conseguindo-se uma simulação física realista da sua evolução desde o início do fraturamento até à subsidência final. / [en] The Sublevel Caving Method is one of the most massive extraction methods used in underground mining industry worldwide and is considered by the mining industry as one of the natural replacements of the current open cut mines in the near future. A Sublevel Caving operation requires that the rockmass surrounding the orebody continually fails and moves into the void created by ore extraction. This dissertation develops a modeling using the discrete element method to simulate the failure mechanism and subsidence caused by Sublevel Caving method. Analyses reported in the literature consider only partial configurations of process Sublevel Caving, without taking into consideration the excavation evolution process, and damage induced to the rock mass resulting from this process. This dissertation analyzes this effect and its consequences on the evolution of failure mechanism and subsidence in Sublevel Caving. Particle Flow Code (PFC2D) was selected for modeling because of its ability to simulate, if the event of excess stress, fracturing and disintegration of the rock mass and large-scale deformation and material flow, to be simulated, which are believed to be the dominant physical phenomena governing the formation of subsidence and fracturing of Sublevel Caving. The results obtained in this study were satisfactory, reproducing properly the surface subsidence induced by Sublevel Caving, allowing physically realistic simulation of its evolution since the beginning of the fracturing to final subsidence.

Sublevel caving is an underground mass mining method used for extracting different types of ores from the earth crust. Mines using sublevel caving (SLC) as the primary mining method are generally highly mechanized with standardized and independent unit operations. Different unit operations (drilling, blasting, loading and transportation) are performed in isolation with each other which leads to standardized procedures and safe operation. Loading of the material from the production face in sublevel caving is facilitated by the flow of material under gravity into the production face. A large amount of material is loaded from a limited opening termed as the draw point which creates challenges for the mining method.   Material flow in SLC has been studied extensively in the past five decades and different methods have been used to simulate material flow in caving operations. Physical models of different scales has been designed for simulating material flow by using sand, gravel or rocks and studying the movement of material inside the model. Initial physical models showed an ellipsoidal zone above the draw point from which material flowed into the draw point. However, subsequent physical modelling results disagreed with this notion of material flow. Numerical modelling techniques have also been applied to simulate material flow. Currently, marker trials are being used to understand material flow in SLC. Markers (numbered steel rods, RFID enabled markers) are installed in boreholes drilled inside the burden of a production ring and based on the recovery sequence of markers, material flow is predicted. Results from physical models, numerical models and marker trials along with mine experience have been used in the past to optimize mine design and draw control for SLC operation. The results from latest marker trials highlight the chaotic nature of material flow and the unpredictability associated with material flow simulation.   In caving operations, draw control deals with the question of when to stop loading and regulates the loading process by providing the information on when to stop loading. The decision to stop loading a blasted ring and proceed to blasting the subsequent ring is a critical decision made in a SLC operation. If a draw point is closed early then ore is lost in the draw point which cannot be conclusively recovered at the lower levels and if delayed the mine faces greater dilution and increased mining costs. A study of the various draw control strategies used in sublevel caving operations globally has also been done to describe the present state-of-art. An analysis of the draw control and loading operations at the Malmberget and Kiirunavaara mines is summarized in the thesis using information collected through interviews, internal documents, meetings, and manuals. An optimized draw control strategy is vital for improving ore recovery and reducing dilution in SLC. Based on the literature review and baseline mapping study, a set of guidelines for designing a new draw control strategy has been listed.    In the present scenario of fluctuating metal prices and increasing operational cost a new draw control strategy is needed which is probabilistic in nature and can handle the uncertainties associated with caving operations. A draw control model which is probabilistic in nature provide a scenario based solution and can be used to test different draw control strategy before performing mine test. A framework for a probabilistic draw control model along with its application for draw control optimization has been discussed here. An effective draw control requires a constant monitoring system and a constant calibration of the loading criteria’s through draw point monitoring for reducing dilution and improving ore recovery. / Improved resource efficiency through dynamic loading control

International Caving Study 2 and Mass Mining Technology

Sublevel caving operations at great depths are subjected both to large stress concentrations that are redistributed as the mining front progresses and to mining-induced seismicity. This is the case for Kiirunavaara mine, Sweden’s largest underground mine. Since the mine was declared seismically active in 2007 / 2008, large rockfalls controlled by structures have happened in many parts of the mine, despite the use of rock support systems designed for bearing dynamic loads. A novel layout for sublevel caving operations, internally named “fork layout” is being tested at a satellite mine. This layout was conceived to place the ore-parallel longitudinal footwall drifts further away from the contact between the orebody and footwall drifts. That way, the differential stresses that generate stress-related damages are expected to be reduced. However, the effect of implementing the fork layout on the hazard potential for structurally controlled rockfalls has not been studied in detail yet. Large rockfalls that occurred in different parts of the mine were analysed with respect to their structures, location of the damage event and type of excavation. The majority of these occurred at footwall drift intersections. Information from damage mapping and seismic events that triggered these rockfalls was used to generate a conceptual model that illustrates the relative spatial relation between the seismic source and damage location. In addition, the seismic source parameters of the events that triggered these rockfalls were processed using scaling laws to obtain ground motion parameters such as peak particle velocity and acceleration at the damage site. The effect of implementing the fork layout on rockfall hazard was tested in the intersections between footwall drifts and crosscuts (FD-CC), and intersections between access and footwall drifts (AD-FD) in two production blocks, using the traditional layout for sublevel caving mining as a point of comparison. Two different fork layouts were tested, FD-CC at 80° (or AD-FD at 100°) and FD-CC at 70° (or AD-FD at 110°). Structural data available from face mapping and oriented core logging was used to define predominant joint sets at the investigated blocks. Using the structural input, wedge volumes at the intersections were modelled deterministically and probabilistically in Unwedge. The variations in wedge volumes formed at the intersections between layouts were used as a proxy for rockfall potential, meaning that if a layout reduced the wedge size, the smaller the rockfall hazard if triggered by a seismic event, and vice versa. It was concluded that most rockfalls at the FD-CC intersections are controlled by structures from three major joint sets. It was observed that rockfalls at FD-CC intersections occurred more often at certain footwall drift orientations. Many seismic events that triggered these rockfalls are located close to the ore passes and generated ground accelerations between 0.5 to 10 times the gravity acceleration. Implementing fork layouts with FD-CC at 80° intersection angle generates larger wedges than the traditional layout and thus, scenarios with a higher rockfall hazard. On the other hand, using fork layouts with FD-CC at 70° intersection angle reduces wedge size at the southern FD-CC intersections; hence, the rockfall hazard is reduced in these intersections. In the northern FD-CC intersections, the wedge volumes are increased and thus, a higher rockfall potential is generated in these intersections. AD-FD at 110° intersection angle generates also a smaller rockfall hazard than the traditional layout in both production blocks.

The sublevel caving mining method is a mass production method with potentially very low operational costs. The success of this method is dependent on, among other factors, the cavability of the orebody and the overlying rock mass. However, caving of the surrounding rock mass also results in deformations in the cap rock as well as on the ground surface above the orebody being mined. From this follows that any existing infrastructure on the ground surface must be relocated as not to be affected by the mining-induced deformations.This thesis work was undertaken to bring about a better understanding of the rock mass behavior in the cap rock of non-daylighting orebodies, with particular application to the Printzsköld orebody as part of the LKAB Malmberget Mine. Rock testing, field observations and underground mapping was conducted to characterize the rock mass in the caving environment. A methodology for identifying the caving front based on seismic monitoring data was derived by studying the Fabian orebody (which has caved to surface), and using laser scanning data for validation. The methodology was then applied to the Printzsköld orebody to identify the caving front.Numerical modeling was performed for various scenarios of the rock mass as mining proceeded. Modeling included (i) stress analysis to understand stress changes and their effects on the rock mass behavior, (ii) discontinuum numerical modeling to quantify the influence of large-scale geological structures on the cave progression, and (iii) discontinuum cave modeling to simulate possible cave mechanisms in the cap rock more explicitly. Laser scanning together with seismic event data were used to calibrate the numerical models.The numerical simulation results showed that as mining progresses, the cap rock and hangingwall were exposed to stress changes that resulted in yielding. Two failure mechanisms were predominantly at play (i) shear failure (dominant in the cap rock) and (ii) tensile failure (dominant in the hangingwall). The presence of the large-scale structures affected thenearfield stresses through slip along the cave boundaries. The effect of the structures on the far field stresses were less significant.Discontinuum modeling to explicitly simulate failure and caving involved simulating the rock mass as a jointed medium using Voronoi tessellations in 2D, and bonded block modeling (BBM) in 3D. Both the 2D and the 3D modeling results showed fair agreement when comparing the inferred boundary of the seismogenic zone, with that identified from seismic monitoring data. Predictive numerical modeling was conducted for future planned mining to assess future cave development in the cap rock. The results from 3D modeling indicated that cave breakthrough for the Printzsköld orebody is expected when mining the 1023 m level, corresponding to approximately year 2022, as per current mining plans. The 2D model was non-conservative with cave breakthrough predicted to occur when mining the 1109 m level, corresponding to the year 2028.The estimated boundary between the seismogenic and yielded zones, as defined in the Duplancic and Brady conceptual model of caving, was coinciding with, or was close to, the cave boundary in the Printzsköld orebody. This may imply that in some areas the yielded zone was not present and that the Duplancic and Brady model may not be universally applicable. Additional work is required to verify this indication, as well as to fine-tune the modeling methodology.

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