These ground falls vary from footwall and hanging wall overbreak to crown pillar failure. AGC Obuasi underground operations in Ghana, frequently experience hanging wall graphitic shears in the stope, marking the beginning of mineralization. Minimizing instability in the production process is usually accomplished through empirical methods or by understanding the mechanics of failure through numerical modeling and implementing the needed requirements. Laboratory testing of rock samples, collection of geomechanics input data, rock mass classification and numerical modeling to understand the mechanics of failure and the parameters needed for more accurate numerical modeling were the basis for this analysis. Laboratory tests indicate variability in strength with the ore zone having the lowest strength.
Stereographic analysis indicates two major joint sets plus random joints.
Sensitivity analysis performed in the numerical modeling indicates that the Poisson's ratio of the graphite does not affect stability. However, the Young's modulus of the graphite did affect stability. It may be expensive if not impossible to determine the Poisson's ratio and Young's modulus due to the nature of this graphite. It is therefore necessary to consider its contribution on instability before investing into the determination of these properties. Two modes of failure have been observed depending upon the stress ratio and the strength of the rock mass.