Over the last 30 years, the Kiirunavaara mine has experienced a slow but progressive fracturing and movement in the footwall rock mass, which is directly related to the sublevel caving (SLC) method utilized by Luossavaara-Kiirunavaara Aktiebolag (LKAB). As part of an ongoing work, this paper focuses on describing and explaining a likely evolution path of large-scale fracturing in the Kiirunavaara footwall. The trace of this fracturing was based on a series of damage mapping campaigns carried out over the last 2 years, accompanied by numerical modeling. Data collected from the damage mapping between mine levels 320 and 907 m was used to create a 3D surface representing a conceptual boundary for the extent of the damaged volume. The extent boundary surface was used as the basis for calibrating conceptual numerical models created in UDEC. The mapping data, in combination with the numerical models, indicated a plausible evolution path of the footwall fracturing that was subsequently described. Between levels 320 and 740 m, the extent of fracturing into the footwall appears to be controlled by natural pre-existing discontinuities, while below 740 m, there are indications of a curved shear or step-path failure. The step-path is hypothesized to be activated by rock mass heave into the SLC zone above the current extraction level. Above the 320 m level, the fracturing seems to intersect a subvertical structure that daylights in the old open pit slope. Identification of these probable damage mechanisms was an important step in order to determine the requirements for a monitoring system for tracking footwall damage. This paper describes the background work for the design of the system currently being installed.

No abstract

The Kiirunavaara mine is one of the largest sub-level-caving (SLC) mines in the world and has been in underground operation for more than 50 years. The mine has been the focus of several case studies over the years. The previous works have either focused on the caving of the hanging wall, using the footwall as a passive support, or focused on the footwall using the hanging wall to apply a passive load. In this updated study the findings of the previous case studies are combined to study the interaction between the caving hanging wall, the developing cave rock zone and the footwall. The geological data for the rock types in the mine area are used to derive upper and lower limits for the geomechanical parameters calibrated for numerical models in the previous studies. The calibrated parameters are used as inputs to a numerical model constructed using Itasca’s Particle-flow-code (PFC) encompassing a mine-scale 2D section at the mid portion of the mine. The model captures the failure locations well in the footwall underground and indicates damage development without a coherent large-scale failure. The trend in subsidence data on the hanging wall is adequately simulated but the magnitude of deformation is underestimated. The input strength for the hanging wall was lowered to study the impact of hanging wall strength on footwall damage development. It is shown that when the footwall strength is kept constant, while lowering the hanging wall strength, the extent of damage and magnitude of displacements in the footwall increases. From these observations it is argued that the hanging wall and footwall cannot be studied independently for the Kiirunavaara mine since the cave rock zone significantly affects the damage development in both walls.

No abstract

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.