Causes and Impact of Hangingwall Instability in Open Stopes: A Case of Chifupu Project in Zambia

The current design of hangingwall entails stability throughout the extraction cycle of stopes. However, premature collapses are being experienced during extraction in some stopes especially those mined in the upper ore body. This study analyses the causes and impact of hangingwall instability in open stopes for the case of Chifupu project. Empirical and numerical analysis methods have been adopted. Prior to implementation of analysis techniques, a rock mass classification and geotechnical domain definition was performed based on the discontinuity properties. The classification methods used are Rock Mass Rating and Rock Quality Index. Modified stability graph was then adopted to assess the stability of the stope surfaces. Furthermore, due to the presence of a fault in the hangingwall a two-dimensional numerical analysis model was developed to incorporate rock mass heterogeneity. Instability was then evaluated based on depth of relaxation zone and reduction in strength factor. From the foregoing, it was discovered that when the orebody is overlain by prominent geological features within appreciable depth from the immediate hangingwall surface, empirical analysis must be substantiated with numerical analysis. The identified failure mechanism based on the two analysis methods are mainly due to gravity and sliding which also have the potential to introduce significant unplanned dilution. Therefore, the main conclusions for the cause of hangingwall instability has been poor ground condition in the hangingwall, shallow dipping orebody, presence of a fault, breach of the hangingwall leading to exposure of sandstone. It is therefore recommended that leaving a ‘skin’ of ore about 1 m to avoid exposure of hangingwall rockmass may provide temporary confinement to the immediate hangingwall during extraction.