Application of modified Hoek-Brown transition relationships for assessing strength and post yield behaviour at both ends of the rock competence scale

Support design for mining or civil purposes relies heavily on use of rock mass classification procedures, not just as a tool for empirical support assessment but also for characterizing rock mass strength. However, for many deep mines and deep tunnels where high stress states can be problematic, it frequently becomes quite difficult to accurately characterize rock mass strength and develop appropriate support designs through the use of conventional rock mass classification based support charts or through application of the generalized Hoek-Brown criteria relationships, (as originally introduced in 1980, with various updates through to Hoek et al., 2002). In general, characterizing rock mass strength through use of the Hoek-Brown relationships as the basis for design of support systems is fundamentally based on the principle that structure within a rock mass acts to reduce both the cohesion and frictional properties, represented by downgrading ‘s’ and ‘m’ respectively in the Hoek-Brown criterion. Within rock mass classification systems the premise is also made that structure exerts most control on rock mass behaviour, and hence worst case classification parameter values and overall lowest rock mass strengths are typically associated with the most heavily fractured rock masses. Although the four most commonly used classification systems, RMR, Q, RMi and GSI (Bieniawski, 1973, 1976, Barton et al., 1974 and Barton, 1976, Palmström, 1995 and Marinos and Hoek, 2000) have some input parameter relationships that directly or indirectly reflect intact rock strength, the importance given to rock strength in all of these classifications is generally limited. For the mid-range of the rock mass competency scale, where block size and incipient strength are such that rockmass behaviour tends to be controlled by interblock shear strength rather than by material strength, the empirical Q-system and Mathews-Potvin type support design charts (Grimstad and Barton, 1993, Potvin et al., 1989) and most numerical modelling tools function well. However, towards the two ends of the rock competence scale (i.e., for very low strength rocks and also, for spall-prone, high GSI rock masses) difficulties can be experienced in not only properly classifying such rock masses, but also in application of the Hoek-Brown criterion for determining rock mass strength. Application of modified Hoek-Brown transition relationships for assessing strength and post yield behaviour at both ends of the rock competence scale