Unconventional support assessment for mines in sedimentary bedrock - Applicability of rock classification systems in horizontally bedded sandstone

Abstract

Coal mining has been a huge industry ever since the industrial revolution but has in more recent years started to be phased out as renewable energy sources have been further prioritised by our society. In order to not let the vast knowledge and experience of the coal mining industry fade, this study seeks to evaluate the applicability of Coal Mine Roof Rating, CMRR, in other settings. The assessment of the classification system is performed through a comparison to Rock Mass Rating, RMR. Both systems are evaluated through a case study of the Kvarntorp mine, a sedimentary sandstone mine in Sweden. The mine has been excavated through room and pillar mining, resulting in horizontal rooms with pillars of rock between them. The geological properties of the bedrock were partly gathered from older investigations from the mine and partly through extrapolations and estimations based on properties of rock in similar conditions. The Geological strength index, GSI, along with the Generalized Hoek-Brown failure criterion was utilised to characterize the rock quality. Two-dimensional finite element analysis was utilised to assess the system numerically. Two models with differing approaches to evaluate joint sets and weakness zones were created in the software RS2. The models were used to assess the stability of 13 different scenarios, covering both the different classification systems and the prospect of heightening the roof of the mine. The behaviour of the two model types was very similar, showing no major discrepancies. Their output indicates that the support recommendations from the classification systems don’t affect the vertical displacement of the roof or the general behaviour of the model. However, the roof suffers from yielded elements and bolting is necessary to control these loose blocks. The CMRR support recommendation is deemed to be the most efficient alternative in the Kvarntorp mine but it requires thorough assessment of any weakness zones in the mine. If the weakness zones are hard to assess, the RMR support recommendation is deemed the safer alternative even though it is potentially too conservative. As the study was only conducted for the one case of Kvarntorp, these findings are not directly applicable in other settings but may instead be considered