ESTIMATING HOEK-BROWN ROCK MASS STRENGTH PARAMETERS FROM ROCK MASS CLASSIFICATIONS

The use of rock mass classifications for designing support of underground excavations in rock has gained acceptance over the past 15 years to the extent that most geotechnical data collection programs now focus on the input parameters to the Norwegian Geotechnical Institute tunneling quality index (Q), the geomechanics classification rock mass rating (RMR), or both. In developing their empirical failure criterion for intact and heavily jointed rock masses, Hoek and Brown turned to rock mass classification schemes for the prediction of rock mass strength. The backgrounds of the two classifications used most frequently are reviewed, and ways in which they may be adapted to derive the Hoek-Brown rock mass strength parameters m, s, and sigma sub c, are suggested. To incorporate the results of practical applications of the failure criterion under real engineering conditions, Hoek and Brown proposed equations to estimate rock mass strength parameters from classifications. These equations relate Bieniawski's RMR to m/m sub i and s (where m sub i is the Hoek-Brown parameter m for intact rock). The Barton et al. Q-index can also be used according to Bieniawski through a relationship between RMR and Q. The use of the complete quantitative rating or index from either classification is not recommended, and it is suggested that some components of the classification schemes are more appropriate than others in estimating the Hoek-Brown parameters. The proposed adaptations of Bieniawski's and Barton's work partially overcome the concern that classifications derived specifically for the estimation of tunnel support may not be appropriate for estimating rock mass strength.