CASE STUDY
Evaluation of Mining Sequences and Barrier Pillar Design at Kiirunavaara Mine
Industry:
Mining, Underground
Client:
LKAB
FLAC3D
MassFlow
Project Background
The Kiirunavaara Mine, located in northern Sweden and operated by LKAB, is the largest underground iron ore mine in the world. The orebody extends approximately 4 km long and between 80 and 200 m wide. It is mined using the sublevel caving (SLC) method, with production reaching depths of approximately 1,000 m.
In May 2020, a seismic event caused significant damage within the mine, leading to the shutdown of production in the affected area in Block 22. A permanent barrier pillar was established, effectively dividing the mine into northern and southern sections.
Initial design of the barrier pillar prioritized minimizing ore loss. Further evaluation was required to refine its placement with respect to local geology and to adjust ongoing mining sequences to consider the pillar.
Objectives
LKAB sought to support long-term mine planning decisions by evaluating how alternative mining sequences would influence rock mass behavior in the presence of the barrier pillar. Key objectives included:
- Identifying the optimal mining sequence from a rock mechanics perspective.
- Analyze the effect of future mining on the barrier pillar and surrounding fault zones.
- Assessing potential impacts of future mining on critical infrastructure.
ITASCA’s Role
ITASCA developed mine-scale models coupling FLAC3D and MassFlow to evaluate multiple mining sequence scenarios. The work focused on improving understanding of the interaction between mining geometry, geological structures, and stress redistribution to support selection of a sequence that balances operational efficiency and geomechanical stability.
Approach and Methodology
The study was carried out in two stages.
Stage 1: Comparative Evaluation of Mining Sequences
A mine-scale FLAC3D model was developed to simulate six potential mining sequences, including the current SLC mining sequence as a base case and variations of chevron sequences with different geometries and fronts. Modeling at this stage did not include coupled flow. Instead, caving was conceptually simulated by removing rock in the ore and hangingwall above mined areas and replacing it with material with parameters corresponding to caved material. The models were evaluated with regards to stresses, state (plasticity), and potential for sliding along geological structures.
The simulations demonstrated that mining sequence geometry has a significant influence on stress redistribution:
- Steeper chevron sequences created high stress concentrations at the most advanced excavation front of each level.
- Flatter sequences redistributed stress mainly below the mined levels, which can lead to a larger footprint 2-3 levels below the current mining level.
- A chevron sequence with two mining fronts resulted in significantly less slipping and shear deformation of the diabase dikes compared to sequences with a single front.
Stage 2: Coupled Analysis of Selected Sequence
The most promising mining sequence identified in Stage 1 was further analyzed using a coupled FLAC3D-MassFlow model to better capture hangingwall caving behavior and material flow. Results were evaluated based on stress state, material yielding, and potential for sliding along structures, with the focus on the effects on the barrier pillar and the main haulage level located at 1,365 m.
A stress-based criterion derived from previous back-analyses was used to identify zones with elevated risk of stress-induced damage. The chevron sequence will lead to a more rapid downward advancement compared to earlier plans, affecting areas with critical infrastructure on level 1,365 differently depending on their location relative to the chevron tip. Additional studies are recommended for early identification of the more critical areas and to create targeted support plans.
Results & Impact
Based on the initial analysis, a mild (flatter), two-front chevron sequence was identified as the most favorable option. Such a design provides more favorable stress conditions as new levels open and when mining through geological structures with slipping potential.
The coupled analysis indicates that the selected two-front chevron mining sequence effectively reduces adverse behavior of the South and North diabase dikes, critical areas due to their historical association with seismic events, which influences the barrier pillar design. No significant shear displacement is observed as mining advances, and only a small area near the mining front exhibits minor slip.
By evaluating alternative mining strategies in the context of a permanent barrier pillar, ITASCA provided LKAB with a defensible basis for selecting a sequence that improves stability while maintaining operational continuity.
References
- Svartsjaern, M., Rentzelos, T., Shekhar, G., Swedberg, E., Boskovic, M. & Hebert, Y. (2022). Controlled reopening of the Kiirunavaara production block 22 after a 4.2 magnitude event. In Caving 2022: Proceedings of the Fifth International Conference on Block and Sublevel Caving, pp. 685–698.
- Svartsjaern, M., Andersson, J., Rentzelos, T., Sekhar, G., Boskovic, M., Swedberg, E. & Töyrä, J. (2023). Options for continued mining in Kiirunavaara mine. In Proceedings of Bergdagarna 2023, Swedish Rock Engineering and Rock Mechanics Associations (in Swedish).