Mine-scale rock mass behaviour at the Kiirunavaara Mine

Sammanfattning: The interaction of the geological and mining environments leads to a variety of forms of rock mass behaviour, including seismicity and falls of ground. A precise understanding, however, of the role of geology in rock mass behaviour experienced by Luossavaara-Kiirunavaara Aktiebolag’s (LKAB) Kiirunavaara Mine remains unknown.Since late 2008, the sublevel caving mine regularly experiences induced seismicity (Dahnér et al., 2012). Seismic events occur in the footwall, orebody, and hangingwall. Instabilities, sometimes related to specific seismic events, are unevenly distributed throughout the rock mass. Failure mechanisms of these instabilities include structurally controlled failure (sometimes as shake down), strainbursting and spalling, which are typically a result of local stress changes. Occasionally, these falls of ground are rockbursts; violent ejections of rock causing damage to infrastructure and/or personnel that are caused by remote seismic events.Some previous work has been done at the Kiirunavaara Mine for both specific events and specific volumes to better understand the rock mass behaviour (see e.g., Sjöberg et al., 2011, 2012). However, the causes of the uneven distribution of both seismicity and instabilities at the mine are not understood, particularly at the mine-scale. As part of a larger Ph.D. project, this study explores the role of geology in the mine-scale behaviour at the Kiirunavaara Mine. This is done through two approaches: 1) exploratory numerical stress modelling, and 2) development of a geomechanical model of the rock mass.The exploratory numerical modelling of the mine evaluated common assumptions made by researchers and consultants when completing numerical stress modelling of this orebody. A previously estimated virgin in situ stress state was applied in a 3-D model developed of the nearly 5 km long orebody and surrounding host rock. The model had definition between footwall, ore and hangingwall materials. Run as a continuum for this analysis, the stresses from the elastic and perfectly plastic models corresponded to stresses recently measured in situ at two sites using overcoring, indicating that the estimated virgin stress state is consistent at depth. Alternating two commonly used perfectly plastic material properties for the footwall significantly influenced the location of plastic failure throughout the rock mass, including in the hangingwall. A physical alignment of plastic failure from the models and mine seismicity for the entire rock mass was not found for the individual cases. Large magnitude shear events tended to be external to plastic failure. The difficulties relating plastic failure to seismicity can be associated with a number of causes, including that the rock mass characteristics were too simplified (for example, no discontinuities were included, the only geological units included were the footwall, hangingwall and orebody, etc.) to represent the rock mass behaviour.A geomechanical model of the rock mass is needed to better understand characteristics of the rock mass, in addition to those included in the stress models, which may be of importance to behaviour. Due to a complex, heterogeneous and clay-altered rock mass, a new methodology was developed to create a geomechanical model. The methodology is based upon standard statistics, geostatistics, and an extension of previous quantitative domaining work. Clay volumes (represented by a model based on borehole data calibrated to underground mapping) correlated to the geomechanical characteristics and behaviour of the rock mass. The rock mass in the immediate vicinity of the volumes of clay alteration had lower RQD values, more random jointing, and a higher concentration of falls of ground than the surrounding rock mass. The correlation between the geomechanical model and the falls of ground lead to the development of a new conceptual model of some of the mine-scale rock mass behaviour, in which the clay volumes play a significant role in stress redistribution.The understanding developed through this study has laid the framework for future analysis of a more advanced and complex nature. Numerical stress analysis will be used to test the conceptual model developed and further analyze the relationship between geology and mining, with the intention of improving the understanding of the causes of rock mass behaviour. This improved understanding has the potential to aid with selection of production planning alternatives for risk mitigation, not only for the Kiirunavaara Mine, but for other highly stressed, hard rock environments.