Rock masses are often heavily jointed and instability involving falling or sliding of rock block may arise upon excavation. As such, supports or reinforcements are required to stabilize the unstable rock blocks. Accurate prediction of mode of rock block failure and volume of unstable rock block are essential for the design of underground excavations in jointed rocks. Traditionally stability analysis involves a tetrahedral block formed by an excavation free face and the mean orientation of three rock joints and this is termed as the deterministic analysis. However, the orientation of rock joint may be widely scattered and Fisher distribution is commonly employed to evaluate the scatter of joint orientation. In the present study, joint orientation surveys obtained from the field are evaluated. It is found that Fisher distribution may not represent the true dispersion of the rock joint orientation for many cases. Goodness to fit test is employed to evaluate the fitting of the field data. The results show that Kent distribution is more appropriate to represent the dispersion of rock joint orientations. A probabilistic simulation is then employed to generate the distribution of volume of unstable tetrahedral rock block based on the mean orientation of the rock joint and its orientation dispersion obtained from Kent distribution. The simulation results are then compared with that determined from deterministic approach which only employs the mean rock joint orientations.

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