Application of rock mass classification systems as a tool for rock mass strength determination

Abstract

The determination of the in situ rock mass strength can be found among the most difficult and challenging investigations in underground mining. The focus of a research project carried out in an underground magnesite mine was the assessment of the rock mass strength using the conventional approaches of rock mass classification, namely Rock Quality Designation (RQD) value after Deere and Deere (1988), Geological Strength Index (GSI) after Hoek et al. (2002), Rock Mass Rating (RMR) after Bieniawski (1989), Q-system after Barton (1990) and Mining Rock Mass Rating (MRMR) after Laubscher (1990) and information on laboratory rock strength. These methods were applied in an underground drift in the vicinity of a stoping panel in order to determine the rock mass strength as well as the influence of the approaching mining activities on the rock mass conditions. Supplementary, core drilling was conducted to provide samples for rock strength determination as well as for the observation of the rock mass conditions using a borehole camera. Based on the results of the rock mass classification systems and uniaxial rock strength tests, the rock mass strength was determined using different approaches. Further on the rock mass strength results were compared to the estimated in situ state of stress, calculated by the weight of the overlying rock strata, assuming a hydrostatic state of stress and physical conditions of the drift. The results from the rock mass strength determination were in the range of 15–450% of the estimated in situ pre-mining state of stress. The results of borehole observations and RQD measurements show considerable variations in rock conditions in the immediate vicinity of drift ranging from extensive fracturing to fairly competent rock. However, with further distance from the drift, the condition of the rock mass changes to that of a massive and undisturbed rock mass. This highlights the difficulties of arriving at realistic rock mass strength values for the deep section of the magnesite mine. The aspects concerning this problem are discussed in detail.