Investigation of Borehole Stability in Malmberget

Sublevel caving method is generally selected for large ore bodies with steep slope. This method is based on the gravity flow of the ore to get better recovery. Due to the continuous caving as the mining goes deeper, fractures of hanging wall occur consequently and thus it collapses into the cave. The local stress field is also affected by various mining activities and it initiates the movement of geological structures in the rock mass and causes seismic event frequently. Thus, it is required to investigate instability problems in various ore bodies in the mine as mining advances deeper and deeper. Investigation of boreholes by filming is a technical way to inspect the overall stability in an ore body.This project work mainly consists of three parts such as Pre-investigation based on production measurement by collecting data from GIRON (Internal production database in LKAB), Field work by filming and Analysis of the data collected from the field. Three ore bodies named Alliansen, Fabian and Vi-Ri were selected for this work.In the pre-investigation, different problems in the borehole are categorized such as re-drilling, wet holes, stones and concrete in the boreholes in different sublevels where the production was completed or about to complete. Re-drilling maps are drawn in different ore bodies as well. The mine co-ordinates of the ring have been used to locate the approximate area of re-drilling. This pre-investigation enabled us to know which areas in the sublevels are more prone to instabilities in different ore bodies. Thus, drifts were selected around this area in the next sublevels or in the same level for filming. These drifts are classified into three criteria; ongoing production, open cut is finished and open cut has not been done.In Alliansen, level 992 m and 1022 m have been selected for field work. In addition, level 855m and 880 m in Fabian and 1026 m in Vi-Ri are investigated by filming. It was not possible to get sufficient filming from the level 880 m in Fabian and 1022 m in Alliansen due to the lack of time and instrument problems.The analysis is done based on production records and field work. In the production records, the percentage of re-drilling has increased to 6.67% from the level 932 m to 962 m in Alliansen and 3.72% from the level 830 m to 855 m in Fabian. Further, the fluctuation of percentage of re-drilling is observed in sublevels in Vi-Ri. Moreover, the percentage of stones found in the boreholes has also increased considerably in different sublevels in this ore body. This analysis is performed based on the total number of holes (which are blasted or charged) in each sublevel.In the field work, a total of 298 boreholes have been analyzed from the collar to the bottom. Typical modes of instability problems such as shearing, cracking, deformation or breakages inside the boreholes are confirmed. In Alliansen, boreholes are sheared off significantly in a certain part in the sublevels. Besides this, in Fabian, boreholes are caved notably in different drifts. In Vi-Ri, boreholes are broken considerably as well. According to the results from filming, borehole instability problems are affected due to the production sequence is analyzed. It is proved that deformation and cracks are most common problems in boreholes around the ring which was blasted last. However, the causes of deformation or cracks are not only the production activities but also the movement of geological structures and fractures of hanging wall due to the continuous cave as the mining progresses downward change local stress field significantly.In Alliansen, geologic structures have been analyzed by considering a common shearing plane which could cross subsequent sublevels. The mine co-ordinates of the borehole where it has been sheared or re-drilled are used to get the co-relations among the sublevels. A statistical regression towards the width of the ore body was sufficient to predict that the same shearing plane are crossing several subsequent sublevels. It is also proved that the area which is influenced by more seismic events is the same as the one where several boreholes were re-drilled or sheared. At last, several causes behind the instability problems are discussed and possible recommendations have been given to ensure a safer environment.Validerat; 20121106 (anonymous