A discussion on the Hoek-Brown failure criterion and suggested modifications to the criterion verified by slope stability case studies

Estimation of the mechanical behavior of closely jointed rock masses is one of the fundamental problems in rock mechanics since the size of representative specimens is too large for laboratory testing. Among the empirical strength criteria suggested for intact rocks and rock masses, the Hoek-Brown criterion has become highly popular. Since its introduction in 1980, the criterion has been refined and expanded over the years, particularly due to some limitations in its application to poor-very poor quality rock masses. In 199,7 the Geological Strength Index (GSI) was introduced into the criterion by its originators as a scaling parameter. In addition, some modifications to the GSI System to provide a more quantitative estimate of GSI and methods of parameter estimation have also been previously suggested by the authors of this paper in 1999. However, the authors considered that some improvements seem to be necessary in order to avoid the gap between failure envelopes of the rock masses with the GSI values between 25 and 26, intersection between failure envelopes of the rock masses of high and low strengths at a certain normal stress level, and a uniaxial compressive strength of zero when s=0 for GSI<25. In this study, some improvements to the equations providing the rock mass parameters OsO and OaO for the criterion were proposed. Further, a modification to the quantitative GSI chart, which was adopted from the original GSI chart by the authors, was also suggested by considering intact or massive rock mass. The validity of the proposed improvements was verified by applying the criterion to a hypothetical slope and to failed open pit mine and spoil pile slope case studies. The results particularly indicated that the switch GSI value of 25 between poor and good to reasonable quality rock masses in the criterion should be replaced by 30 and the parameter OsO should not be assumed as zero for poor quality rock masses. In addition, the improvements suggested in this study were also compared with the new equations in the 2002 version of the criterion, which was published by Hoek and his co-workers after this study has been completed, in conjunction with the use of case history examples.