A brief history of the development of the Hoek-Brown failure criterion

The original Hoek-Brown failure criterion was developed during the preparation of the book Underground Excavations in Rock by E. Hoek and E.T. Brown, published in 1980. The criterion was required in order to provide input information for the design of underground excavations. Since no suitable methods for estimating rock mass strength appeared to be available at that time, the efforts were focussed on developing a dimensionless equation that could be scaled in relation to geological information. The original Hoek-Brown equation was neither new nor unique – an identical equation had been used for describing the failure of concrete as early as 1936. The significant contribution that Hoek and Brown made was to link the equation to geological observations. It was recognised very early in the development of the criterion that it would have no practical value unless the parameters could be estimated from simple geological observations in the field. The idea of developing a ‘classification ’ for this specific purpose was discussed but, since Bieniawski’s RMR had been published in 1974 and had gained popularity with the rock mechanics community, it was decided to use this as the basic vehicle for geological input. By 1995 it had become increasingly obvious that Bieniawski’s RMR is difficult to apply to very poor quality rock masses and it was felt that a system based more heavily on fundamental geological observations and less on ‘numbers ’ was needed. This resulted in the development of the Geological Strength Index, GSI, which continues to evolve as the principal vehicle for geological data input for the Hoek-Brown criterion