Determination of rock mass strength by rock mass classification—Some experiences and questions from Boliden mines

Abstract

In connection with the development of new deposits and with mine planning in existing mines various classification systems have been used for estimating rock mass strength. Some experiences and problems encountered during this work are reported. Rock mass strength obtained from several classification systems is compared with rock mass strength derived from case studies. Case studies from some Boliden mines confirm that the strength of rock masses is highly dependent on the volume that is subjected to critical loading. The scale factor for strength (the ratio between strength of laboratory specimens and rock mass strength) varies for these cases between 4 and 220. Yet, none of the classification systems used takes into account the size dependence of rock mass strength. Several classification systems were examined. Rock mass strength determined by these classifications may differ by a factor of 30. Experience obtained from these case studies is not sufficient to allow recommendations regarding choice of classification systems. The range of validity of the classification systems is usually not well documented. There is a strong need for reliable methods for estimating rock mass strength in the planning and development stage. These should be developed by systematically gathering experience and evaluating feedback from case studies and thereby creating a better understanding of the phenomena that determine the strength of rock masses.