Implications of Joint Properties on the Strength of a Jointed Rock Mass

In order to minimize the environmental and physical hazards of rock slope failures in deep open cut mines, evaluation of rock mass strength with a reasonable degree of accuracy is required. The task is made much more difficult due to the presence of discontinuities or joints, as the strength of a rock mass depends on both the strength of intact rock and of the discontinuities. The main objective of this study was to review the applicability and accuracy of existing models for the prediction of the strength of a jointed rock mass. Therefore, some experimentally-determined strength results with corresponding outcomes derived from five different existing models were analysed. The experimental results reviewed in this paper were obtained from physical model tests. In the empirical models, the influence of joint properties was aggregated into one parameter, called the "joint factor". The analysis shows that the effect of joint orientation on the strength of a rock mass is overestimated by existing empirical models for joint angles smaller than 20° and larger than 40°, measured with direction of the major principal stress. Therefore, a modification is suggested to determine the joint factor, and subsequently the unconfined compressive strength, more precisely.