Building on the empirical research completed in the footwall contact zone of LKAB's Malmberget mine, a method has been developed for predicting damage in the crosscut entries of the mine layout. Three-dimensional stress measurements combined with analyses of measured and observed damage in the crosscuts have allowed new interpretations of the crosscut performance. Stress analysis focused on the relative differential stresses measured in the area of interest. These stress changes were then linked directly to observed changes in the entry condition according to the Entry Condition Rating (ECR) system, a damage mapping system developed for the mine. A bilinear model was used to describe the entry condition rate of change such that the peak measured differential stress corresponds to increased degradation rates. These bilinear degradation rate trends are shown to be directly related to the geotechnical qualities of the rock using the Geologic Strength Index (GSI) of the monitored locations, irrespective of lithology. The final step was to develop an empirical model that allows prediction of a crosscut's future ECR based on changes in the relative differential stress and the GSI of the crosscut. In combination with simple numerical modelling tools the model can be used to predict ahead of time when additional reinforcement will be necessary.

The LKAB's Malmberget mine in Sweden is one of the largest sublevel caving mines in the world, with an annual production rate averaging 18 million tons. This high rate of production at depth (>1,000 m) creates significant mining-induced stress redistribution on a global scale. At a production-level scale, this redistribution results in undesirable amounts of deformation in the entries and typically leads to general degradation in the footwall contact zones. This is exacerbated by highly varied geological and geotechnical characteristics of the lithology often found in the contact zone. To better understand the impact of mining-induced stress on production level entries, a study was conducted to measure stress changes and associated deformation over a two year period, as mining progressed in the vicinity of the instrumentation. Three-dimensional relative stress measurements using digital hollow inclusion stress cells and multiple-point borehole extensometer measurements were combined with convergence and floor heave measurements and regular damage mapping throughout the contact zone to better understand the evolution of damage in these areas. A site-specific Entry Condition Rating (ECR) system was developed to help geomechanics better track and understand the expected performance of the crosscut given the current state of mining. The result of the work is a better understanding of where and when damage is expected to occur, and the ability to properly time the installation of secondary support in a pre-emptive manner.

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As we approach the middle of the 2020s a few things are clear: (1) The world's population is increasing, and along with it, the need for raw materials. (2) The planet is changing, and we need to adapt our processes to the new realities we face. (3) The demands of society require that we develop better techniques that limit our impact on the people living around us. These three challenges combine to create a 'Perfect Storm' for the mining industry -not only should we produce more, which requires faster rates, greater depths, and higher stresses, but we must do so with less waste and fewer emissions, while minimising seismicity and surface subsidence, all at the same time. These challenges can only be met through a total transformation of the way we operate, something that Luossavaara Kiirunavaara AB (LKAB) has accepted with open arms. The first challenge is being addressed through new mining methods and layouts developed and tested in full-scale to find new ways to produce. At expected depths of up to 1,900 m, more than 50% deeper than LKAB's present mines, and production rates expected to be significantly higher than today's, entirely new ideas are necessary. The second challenge is met through total electrification. This is much more than simply changing out diesel for electric, but includes new ideas about ventilation, energy storage, and redefining the very foundations of our processing technologies, a path that will reshape the entire region around the mines. The third challenge is especially important and requires finding new ways to control seismicity and subsidence and minimise the use of tailings dams, while implementing environmentally friendly techniques and holding continuous dialogue with the community to ensure all stakeholders are satisfied from day one. This paper provides a glimpse of the path LKAB is following and how we hope to reshape our industry.

LKAB's Malmberget mine is not only one of the largest SLC mines in the world, but it is also one of the most complex, where between 10 and 15 orebodies are mined simultaneously. The subvertical orebodies are generally stacked, where one orebody is on the footwall or the hangingwall of the other. This results in complex stress interactions and deformation patterns when these orebodies are mined simultaneously. Keeping the entries into the respective orebodies open is a critical part of daily mine planning at theMalmberget mine and the ability to predict stress changes and associated damage to these entries contributes significantly to this planning. To develop an effective empirical tool for predicting mining-induced stress changes and associated entry deformation and damage, a two-year monitoring program was initiated in 2019 to record those stress changes and their consequences. These results are described in other papers written by the authors for this same symposium. This paper, however, focuses on the numerical modelling component of this work, where a series of numerical modelling was done using FLAC3D with monitoring data as the basis for model calibrations. Given the extent of the mining operations at Malmberget mine, the stress changes are global in extent on the scale of kilometres and extend from 100 to 200 m below the active mining front. The field data collected in the entries are local and do not necessarily reflect the global extent of stress changes. Nevertheless, through numerical modelling it is possible to build a global model to observe stress changes and the resulting deformations as neighbouring orebodies are mined.