INTRODUCTION
Most commonly used methods for assessing the shear strength of large rock masses are based on empirical criteria, with the Hock-Brown criterion having received widespread interest and use over the last 15 years. As a general rule such criteriare based on laboratory scale specimens with very little, and often no, field validation. This paper sets out to provide limited field verification of existing shear strength criteria. The paper critically reviews the Hock-Brown criterion with particular reference to the shear strength of large rock masses. The review indicates that existing criteria have several severe limitations and are unlikely to predict rock mass behaviour over a broad range of conditions. For this reason, it is important that any criterion be validated for the intended use. Several large slopes have been examined with a view to establishing field bounds on shear strength criteria for very large rock masses.
As mine slopes become higher and longer, the necessity to account for very large mass strength in design increases, and thus there is an increasing use of empirical criteria in slope design. Methods used for assessing the shear strength of large rock masses are based on empirical criteria, with the Hoek-Brown criterion having received widespread interest and use over the last 15 years. As a general rule such criteria are based on laboratory scale specimens with very little, and often no, field validation.
The need for a most commonly used strength criterion is the Hock-Brown empirical rock mass failure criterion, the most general form of which is given in Equation 1. Hock & Brown (1980) developed this criterion due to the lack of any available empirical strength criterion. The equation, which has subsequently been updated by Hock & Brown (1988) and Hock et al. (1992), was based on correlations between brittle fracture of intact rock cores and the original Griffith theory (Griffith, 1921 & 1924). The only 'rock mass' tested and used in the development of the Hock-Brown criterion was 152mm core samples of Pangnna Andesitc from Bougainville in Papua New Guinea (Hock & Brown, 1980). Brown & Hock (1988) later noted that it was likely this material was in fact 'disturbed'. The validation of the updates of the Hock-Brown criterion have been based on experience gained whilst using this criterion. To the authors' knowledge the data supporting this experience has not been published.
