Fall of ground characterization: A case study of BCL Mine, Botswana

Ground control has been and continues to be a major problem facing underground mines around the world. Unplanned fall of ground (FoG) can lead to ore loss, accidents, fatalities and production delay. Hence, FoG is of critical importance to the design of underground mine excavations. Ground conditions, geological discontinuities, excavation geometry, support design measures and mining methods are fundamental factors influencing FoGs. Despite the advances in underground excavation design, there is still a need for developing tools capable of characterizing ground stability which is essential in estimating, controlling, monitoring damage and in the design of ground control procedures. Hence, this study focuses on FoG characterization and a FoG database has been compiled for this purpose. In order to quantify the rock mass behavior around underground excavations and fully characterize the FoG events, empirical methods and numerical methods were used. The Bamangwato Concessions Limited (BCL), an underground mine located in Selibe-Phikwe, Botswana, was used as a case study where FoG events that occurred in various stopes and other excavations were recorded. Overall, two aspects of the FoG characterization were investigated. Firstly, a FoG chart was proposed based on the Mathew’s stability graph method with the purpose of predicting the ground class associated with FoG hazard. The stability of excavations was defined on the basis of the potential severity of FoG hazard as minor, moderate and major. Next, numerical modelling was implemented to analyze the modes of rock failure around the underground openings which had led to FoG. Rocscience Software Package was used to simulate the stress-induced and structure-induced failure modes or a combination of both that had been observed in the field. In general, the results were in agreement with the field data. It is concluded that this study has enhanced the understanding of ground conditions and FoG characteristics; and therefore could be used as a basis to provide recommendation for ground control improvements in BCL mines.