Effect of geological structures on the subsidence of the Kiirunavaara hangingwall : Field data collection - Numerical analysis

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 degrees eastward. Therefore the subsidence is developing toward the city of Kiiruna, 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. As a part of ongoing hangingwall subsidence research project, an exploration campaign was carried out to characterize the rock mass and identify the major geological structures using diamond drilling and geophysical methods. Three drill cores (koj1-koj3) were obtained close to the city of Kiiruna along the mine section Y1900. In addition, six drill cores (koj4-koj9) were obtained along the railway as a part of another project related to the deformation of the ground below the railway. All of the 9 drillcores were mapped by the author in January and February 2009. Each drillcore was composed of several core runs. RQD was calculated for all the core runs, point load test was carried out on one core sample at each run, and orientation analysis was carried out on the oriented cores in the field. Later the RQD values were used to estimate the geological strength index (GSI). Finally, GSI values together with the results from point load tests were used as input data in Roclab program to estimate the rock mass strength and deformation modulus. Results indicated to a good quality of rock mass obtained from drillcores koj1 to koj3. Existence of possible geological structures can govern the surface deformation produced by subsidence at this region. Although the carried out core orientation analysis cannot decide the major geological structures orientation, but at least allows the exploration geologists and geophysical engineers to understand the geology of the area and make decision about the future of the site. Results obtained from mapping drillcores koj4 through koj9 along the railway indicated to a poor quality of rock mass in proximity of the railway. Results from this region will be used in further analysis by geologists to identify the possible weak zones at this region and their effects on the subsidence of the railway area. The 2D distinct element method “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. Two different structure sets have been analyzed. One structure set had a dip in the interval 50 – 90 degrees to the east, whereas the other set had the dip 60 degrees to the east and 60 degrees to the west, respectively. 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 and structure set on the hangingwall subsidence, the break angle was calculated at each mining level. The results showed that except the structure inclined 90 degrees, the other inclinations 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 tension failure in the models. The results indicate that it is important to identify the dominating structures and their orientation and the structural geological domains.Validerat; 20101217 (root