The sublevel caving used in Kiirunavaara mine induces failure and subsidence of the hangingwall. Two sections of the mine were studied by means of numerical analyses. Numerical models were developed using finite element and discrete element codes. The former was applied to calculate the location of new failure surfaces in the hangingwall and to estimate the break angle when mining advances downwards. The latter was used to analyse the displacement path of the caved rock during draw and to determine its effect on the stability of the hangingwall and footwall. The models were calibrated using displacement monitoring data. The finite element analyses indicated that the break angle is almost constant for deeper mining levels but may change if the geometry of the orebody changes. The discrete element model showed the formation of a stationary zone along the footwall that reduces the magnitude of the shear forces during draw, increasing its stability.

The hangingwall deformation behaviour at the Kiirunavaara mine has been characterised after several years of collecting surveying data from the ground surface. The monitoring system was implemented to track surface subsidence because the city of Kiruna and the railway are situated on the hangingwall. Data time–displacement and time–velocity curves were used and different stages of deformation behaviour were identified – regressive, progressive and steady state. The movement starts with a regressive behaviour for which subsidence is characterised by continuous deformations. At the end of this stage the movement accelerates, marking the beginning of the progressive behaviour where subsidence becomes more discontinuous. A steady state stage is reached when the strength of the failure surface decreases to the residual value. To predict displacement in the vicinity of the railway, accumulated displacements per year were analysed for several stations for which data for the full regressive stage were available. Displacement tendencies were fitted with quadratic polynomial functions. Therefore, the rate of movement follows a linear trend with a constant acceleration. Finally, a critical horizontal strain limit was determined based on the estimated displacement.

Large scale surface subsidence has been experienced at the Kiirunavaara mine since sublevel caving was implemented as a mining method. Surface disturbances are affecting part of the city of Kiruna, the railway, and the power station. Continuous and discontinuous subsidences characterize the hangingwall deformation, which is periodically monitored using surveying techniques and mapping of surface cracks. A historic review of subsidence prognoses was carried out and the results were compared with the actual condition of the hangingwall. The review showed discrepancies between different prognoses. In addition, limit equilibrium analyses indicated that break angles flatten while the mining depth increases. However, this tendency is not clear in the field where break angles show a large dispersion of values. On the other hand, using surveying data, two different analyses were performed. The time-dependent movements of the hangingwall were described using time-displacement curves and strain analysis was performed for different sections of the hangingwall. Three different stages of the time-displacement behaviour were identified and described. Finally, it was concluded that extension strain can reach values which may damage civil structures before surface crack can be observed.

Sub-Level Caving (SLC) is an important mass mining method, involving blasting of ore against granular material in the form of caving debris. The debris compaction due to blasting influences the caving process. Blasting tests were made on cylinders of magnetic mortar placed inside plastic cylinders and confined by packed granular material. By introducing the acoustic impedance between the mortar and the confining granular material, the compaction is found to depend on material, specific charge and physical properties of the debris with statistical analysis. The tests have shown to be a good input for numerical modelling of blast compaction.

Mining with a sublevel caving induces the progressive failure of the hangingwall and in minor proportion the failure of the footwall. Surface subsidence is a consequence of the large deformation experienced by the rock mass. Monitoring displacement and fracture mapping are normally carried out in a regular basis especially when the mine is surrounded by civil and mining infrastructure. Since 1967, at Kiirunavaara mine, subsidence monitoring information has been obtained and used to calibrate predictive models. In this paper the monitoring data collected in Kiirunavaara mine is presented as time-displacement curves to analyse the pathway of the hangingwall and to predict displacements in short term. The results influenced by the quality of the rock mass, structural geology and stress conditions, shows different time-displacement behaviour along the hangingwall.

The current mining method, sublevel caving at the Kiirunavaara iron ore mine, has induced large scale subsidence to the hangingwall. The orebody dips, on average, 60° eastward. Therefore the subsidence is developing toward the city of Kiruna, the railway and other infrastructure. One of the most important factors which can affect the hangingwall subsidence is the existence of large-scale geological structures in the hangingwall. The 2D distinct element code UDEC was used in this work to evaluate the effect of geological structures on the hangingwall subsidence. One so called "basic model" was run without any geological structures. One structure set with the dip in the interval 50° - 90° to the east has been then analyzed. The failure surface at each mining level was defined for the different models through failure indicators e.g. yielded elements in shear and tension and surface critical vertical displacement. To evaluate the effect of each orientation on the hangingwall subsidence, the break angle was calculated at each mining level. The results showed that inclined structures had an obvious effect on the extension of the failure surface on the hangingwall. All models with large-scale structures showed a decrease of the break angle compared to the basic model. The structure orientations showed a tendency to govern the direction of shear and tensile failure in the models. The results indicate that it is important to identify the dominating structures and their orientation and the structural geological domains. Keywords: Numerical analysis, Geological structures, failure surface, Break angle.

The principal objective of this project is to give prognoses of future subsidence at Kiirunavaara mine located below the city of Kiruna. To reach this objective, it will be developed different models (e.g. geological, geostructural, geomechanical, and geohidrological conditions) and then through numerical analysis identify future failure modes. Through these analysis it will be increased the understanding of how plastic and brittle deformation zones are interrelated, and the relation between the regional stress state and the structural geology.

Two types of deformation zones have been identified on the hangingwall at the Kiirunavaara mine - continuous and discontinuous. The continuous deformation zone is characterized by a smooth lowering of ground surface accompanied by horizontal displacement moving in the direction of the mine to the cavity created by the mine extraction. The discontinuous deformation zone is characterized by large displacement over limited surface areas forming chimneys, steps and large tension cracks. Since the deformation of the hangingwall is time dependent, the time-displacement behaviour was independently analyzed in two directions - X (perpendicular to the ore body) and Z (vertical) using surveying data from a network of stations from twelve surveying profiles along the hangingwall. These data cover a period of time of 13 years starting from 1996 and ending in 2008. The movements along the Y-direction (parallel to the ore body) are smaller and erratic therefore they were not considered. In addition, extension strain and slope were calculated for some areas where the limit between deformation zones was not clear. Curves of time versus displacement were analyzed. In the curves three stages were identified -regressive, progressive and steady state. During the regressive stage only continuous deformation is observed in the field using monitoring systems. The progressive stage is an intermediate stage between the regressive and steady state where the first cracks appear and develop. The limit between the first two stages is defined by the on-set of failure point in the curves. The point of failure (critical vertical displacement, CVD, or critical horizontal displacement, CHD) in the hangingwall was defined using the change of stage between the progressive and steady state stages. This point also defined the limit between the continuous and discontinuous deformations zones. Although the hangingwall subsidence is extending with deeper mining levels, the results show a general tendency of decreasing rate of movement due to the increasing level of caved rock that confines and constrains the movement. This fact means that it will take a longer period of time for a point farther from the mine to reach the same displacement magnitude compared to the points closer to the mine. Today, the railway is located in the outer limit of the continuous deformation zone and therefore in the regressive stage. The ground in this area is experiencing about 2 to 5 cm of vertical and horizontal accumulated displacement. To predict further displacements and rates of movements, polynomial functions were fitted to the timedisplacement data during the regressive and progressive stages. The most representative functions for each surveying line were selected to do the forecast for nine stations located close to the railway. The result indicates that the vertical and horizontal displacements could increase to about 10 to 30 cm in the following 4 years (until 2012). This prediction was also extended to cover the whole regressive stage until the year 2018 (see Appendix C). This is a conservative forecast because it was estimated with data from points closer to the mine. No large movement or catastrophic collapse is likely to occur during the regressive phase. However, the deformation may concentrate at large geological structures and its effect needs to be evaluated to adjust the forecast. There is a documented case by Herdocia (1991) where a regional fault in Gränsgesberg mine was reactivated, thus changing the overall deformation behaviour of the hangingwall.

The mining method used in the Kiirunavaara Mine induces failure in the hangingwall which caves and subsides by the effect of stress relaxation and gravity. Since the orebody is dipping towards the city of Kiruna, surface disturbances are approaching the town as mining proceeds to a greater depth. The aim of this thesis is to contribute to the understanding of the hangingwall failure process and its relationship with the ground surface subsidence. To achieve this objective two conference papers related to numerical analyses and one journal paper regarding ground surface deformation are compiled. Results from numerical analysis using continuous methods indicate that the first tension cracks are formed by extension strain on the ground surface, followed by shear failure along an almost planar failure surface between the mining level and the tension crack. The break angle calculated from the model showed good agreement with those calculated using field data. The break angle varied while the orebody width changed. A discrete element method was also used to analyze the hangingwall - caved rock - footwall interaction. The model showed that the caved rock constrains the movement of the hangingwall and footwall. Additionally, the model for the mine section Y1500 indicated that without the backfill the footwall will fail. The time-dependent behaviour of the hangingwall was analyzed using time- displacement curves obtained from surveying stations. Three different phases were found - regressive phase, steady state, and progressive phase. The rate of movement was estimated for the first two phases. Additionally, strain analysis was carried out using subsidence data from the hangingwall. The results indicated that critical values of tensile strain can be reached before visible cracks can be found on the surface. Moreover, large geological structures concentrate the strains, creating abnormal subsidence behaviour at the hangingwall.