This paper describes three-dimensional numerical stress modeling of the LKAB Malmberget mine and stress calibration of the model, using the results from newly conducted stress measurements at the site. A detailed model of the mine, comprising 18 orebodies, was setup for analysis in the three-dimensional finite difference program FLAC3D. The model includes the currently known orebodies, some reaching down to mining level 1600 m. The model also includes extensive biotite zones near two of the orebodies. A unit stress tensor approach was used for model stress calibration, in which unit stresses of each tensor component was applied to the model, and the stress response calculated. These response tensors were combined (using superposition for linear-elastic analysis) and the results fitted to stress measurement data. This approach provided an unbiased methodology for stress calibration, which accounts for influences from mining and variations in material properties. The end result of this work was a calibrated three-dimensional model including all actively mined orebodies at the mine. The results from this model provided knowledge about stress distribution effects globally and locally in the mine, as well as input to local model of drifts, shafts, and other critical underground excavations. The model is also used for the planning and design work for potential continued mining at depth in the Malmberget mine. Some example applications are also presented in the paper.

Problem description

Underground mining causes stress redistribution around mined areas, which are important to quantify, as these constitute the load acting on the underground excavations. Numerical modeling is the preferred tool for such analysis. The complex geometry of many large underground mines, necessitate a three-dimensional modeling approach, to correctly assess post-mining stresses. In addition, representative input data to such analysis is required, including an assessment of the initial (virgin) stress state in the rock mass. Stress measurements are often challenging in themselves, but the results may also be influenced by the mining geometry. Thus, numerical models often need to be calibrated against measurements. This paper presents a case study in which a non- biased stress calibration method is used to derive the boundary stresses to a three-dimensional numerical model of a large underground sublevel caving mine.

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