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Mining in the production area 'Block 19' in the LKAB Kiirunavaara Mine has resulted in extensive seismic activity and several serious rockfalls. This paper describes a modelling study aimed at quantifying the seismic potential for future mining of Block 19, considering several different mining sequences. A 3D discontinuum model was used, in which geological structures were included explicitly to simulate the development of fault slip seismic events. Four different mining sequences were simulated-three of which involved leaving the whole, or part of, Block 19 as an unmined pillar. Mining was simulated from the current active mining level and for a production period of more than 20 years ahead in time. The results were evaluated in terms of stress influences on critical infrastructure in the footwall of Block 19 and, more importantly, the possible extent of fault slip seismic events due to mining (by calculating seismic moments). The results showed that leaving a pillar in Block 19 may be less desirable as higher seismic moments will develop in the final mining stages. However, continued mining without a pillar will also likely result in an increased seismic activity in Block 19, which needs to be addressed appropriately. Block 19 should be mined prior to the neighbouring production areas, but additional and more detailed numerical modelling may be needed to fine-tune the extraction sequence for each level mined. https://papers.acg.uwa.edu.au/p/1201_26_sjoberg/ Numerical analysis of alternative mining sequences to minimise the potential for J. Sjöberg et al. fault slip rockbursting

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ObjectivesTo evaluate the influence of the group and individual education of students on shade matching quality. Secondary study objective included a survey‐assessment of quality and difficulty level of color matching.Materials and MethodsForty dental student participants in Integrated Academic Studies of Dentistry at the Faculty of Medicine, University of Niš (26 female, 14 male) 21–25 years old, signed consent to voluntarily participate in the research. The Ethics Committee of the Clinic of Dentistry, Faculty of Medicine, University of Niš, approved the study under number 14/11–2019‐2EO.They were randomly divided into two groups (n = 20), experimental (IE, individual education) and control (GE, group education). Each group was divided into four subgroups (n = 5). The study was performed in three phases. In Phase I (I) and III (III), students matched the shade of the maxillary central left incisor (#9) using three different shade guides. Each of the students served as a “patient” for shade matching, while the other members of the subgroup, individually (VMI) and as a team (VMT), visually matched the tooth shade. In Phase II (between phases I and III), group IE received standard group and individual education and training, while group GE had only group education. Shade matching results in before and after education were compared with results obtained by an intraoral spectrophotometer. CIELAB (∆E*), and CIDE2000 color differences (∆E00) were calculated. The color differences between VMI or VMT and ES readings were graded. In addition, the students rated the level of difficulty and quality of shade matching from 1 to 5, signifying the most to least difficult, and the worst to best match, respectively. Means and standard deviations were calculated. Differences were compared using the Student t‐test.ResultsMeans (s.d.) of CIEDE2000 color differences for VMI before and after IE were 4.5 (1.5) and 3.3 (0.9), respectively. Corresponding values for group GE were 4.4 (1.4) and 4.0 (1.0), respectively. As compared with GE, group IE achieved 15% better results for the best matches and 27.5% for one of the three best matches in by‐point comparison. Mean (s.d.) results for VMT in the experimental group before and after IE were 3.6 (2.4) and 2.8 (1.6), respectively. Corresponding results for the control group were 4.4 (2.1) and 3.5 (1.7), respectively. Group IE had statistically higher VMI and VMT scores than GE, while, for GE, only VMT scores were statistically higher. Group IE achieved better results in by‐point comparison. After education and training, the students rated the shade matching as more precise and less difficult. Results were significantly better for Quality (IE) (sig = 0.010) and Difficulty (GE) (sig = 0.021).ConclusionsWithin the limitations of the study, the experimental group achieved significantly better results in individual color matching after education and training than the control group, while both groups made progress in team color matching. Both groups rated shade matching more accurate and easier after education.Clinical significanceImproving the quality of shade matching is expected to enhance the esthetics of dental restorations, increase patient satisfaction, and reduce color corrections.

Large magnitude seismic events are a concern for most hard rock underground mines extracting minerals at depth. At the Kiirunavaara Mine, a long-term chain of events was initiated with a 3.0 magnitude event in 2008 in production block 19 on mining level 907 m. In the aftermath of the event, this production block started to trail behind the adjacent mining blocks creating an upward pointing wedge over three blocks, with block 19 at the top. In 2020 another large magnitude event (4.2 Mw) occurred in the same block, now designated block 22, while opening level 1022 m. The event caused significant damage to the mine infrastructure over a large volume affecting overall mine output for months.Because of the significant damage to the infrastructure, it was not possible to resume mining on the topmost levels in block 22. Mining resumed instead from the lower levels, creating a remnant pillar between the existing sublevel and the resumed cave. The behaviour of this pillar has been analysed in detail using a coupled FLAC3D-CAVESIM numerical model. The modelling used a global-local approach where the mine scale stress field from sublevel cave mining until 2020 was superimposed on a local model where the resumption of mining activities in block 22 was explicitly simulated for several alternative layouts and sequences.To alleviate the seismic hazard from an uncontrolled pillar collapse after resumed mining, the best option sequence was sought to facilitate controlled pillar caving. Variation studies were performed to find high impact parameters controlling the pillar behaviour. Parameters studied included looking at rock mass quality, horizontal sequencing and alterations in the footprint of the resumed cave.From the modelling effort, it was concluded that the confinement of the remnant pillar was the major factor controlling the pillar caving and seismic potential. With support from underground damage assessments and the numerical models, it was decided to attempt to resume the mining from level 1079 m (abandoning the damaged levels 1022 and 1051). A new set of footwall drives were excavated inside the orebody to replace the damaged drives at the footwall contact. From the new footwall drives, the existing crosscuts were rehabilitated towards the old footwall drive as far as possible to reduce the confinement of the remnant pillar above. Resuming production at 1079 will attempt to reset the global mining sequence, with the production block no longer lagging compared to adjacent blocks.