Instability in production blast holes causes poor fragmentation, lower ore recovery, delay in production and higher costs in crushing and grinding. In this investigation, three large ore bodies named Alliansen, Fabian and Vi-Riwere selected. Avideo camera was used for mapping production blast holes so as to find borehole stability problems. A total of 298 blast holes were filmed. There were eight types of stability problems identified in the blast holes. These are shear zone, loose rock, cave, crack, deformation, stone jammed, spalling and rock breakage. Among these problems, deformation and crack were the most common ones in all the ore bodies. In addition, instability problem in the blast holes were higher around the rings which were blasted last. Further it has been found that the area which is affected by more seismic responses is the same as the one where several boreholes were re-drilled or sheared.
The Sublevel Caving method is generally preferred for large ore bodies with steep slope. The method is based on the gravity flow of the ore for optimal re-covery. LKAB's Malmberget mine is a large scale underground iron mine. It consists of approximately 20 large and small ore bodies in which 9 are currently being mined. The total length and width of the ore bodies are around 5 km and 2.5 km respectively. At first, open pit mining was done and underground operation has been started since the 1950s. Sublevel caving has been the principal mining method since the 1960s. The annual production of planned crude ore at Malmberget is about 16 million tones [2]. For sublevel caving mines, excavations near the ore body will be subjected to changing stresses as mining progresses downward [1]. In addition, as the mining goes deeper, fractures of hanging wall occur due to the continuous caving. A likely result of changes in rock stresses is the movement of geological structures and thus causes seismicity and rock burst. Hence, it is required to understand instability problems in various ore bodies in the mine as the mining advances deeper and deeper. In 2007, borehole stability had been investigated by Joel Kangas [3]. Investigation of boreholes by filming is a direct method for inspection of the overall stability in an ore body.
