Rotation of principal axes and changes of stress due to mine-induced stresses

Abstract

This research project comprises stress monitoring, primarily of the changes induced by longwall mining or destress blasting, which was realized in a mine of the Ostrava-Karviná Coalfields (Upper Silesian Coal Basin). Verification of the stress field response to stresses induced by different geological or mining factors was the main aim of the research project. Stress change monitoring in the region of the advanced longwall was carried out using seven stress probes. In all, seven boreholes were instrumented with compact conical ended borehole monitoring (CCBM) probes for continuous stress monitoring of the stress–strain changes due to mining and the destress blasting effect. Compact conical-ended borehole overcoring (CCBO) and CCBM probe installation were carried out before the start of the longwall mining. Data of the stress changes in all seven probes were recorded weekly and, after the longwall advanced, daily. Analysis of the stress changes was based on the idea of original stress distribution, which is based on long-term experiences in rock mass behavior in similar geological and mining conditions. For all probes, the magnitude of stress changes and principal axis direction changes were investigated. Analysis results to date indicate that stress changes ahead of the longwall face present not only the changes of stress magnitude, but the rotation of the axes of the stress components as well. Rotations of the principal axes emerged as very important factors of the stress tensor changes and their influence on rock mass deformation and stability of the workings. In addition, results show that the vertical to horizontal stress component ratio converges to 1 and in many cases horizontal stresses are higher than vertical ones. This of course relates to the impact of mine-induced stresses. This knowledge is basic for the purpose of prevention of rock burst because the actual stress state is fundamental to its proper and operative planning.