Rock mass strength by rock mass classification. Robert Hack (1997). South African Rock Engineering Congress (SARES) Johannesburg, RSA. pp. 346-356 Rock mass strength by rock mass classification Robert Hack Section Engineering Geology, Centre for Technical Geosciences, International Institute for Aerospace Survey and Earth Sciences (ITC), Kanaalweg 3, 2628 EB Delft, The Netherlands ABSTRACT: The strength of a rock mass for foundation purposes is for a large part determined by the discontinuities in the rock mass. Numerical calculations of discontinuous rock masses prove often to be cumbersome and unreliable. Rock mass classification may be an equal or more reliable methodology. The Slope Stability Probability Classification (SSPC) system designed for slope stability may be used for this purpose. The system has been developed during four years of research in Falset, province Tarragona, Spain. The rock slope classification scheme assesses orientation dependent and orientation independent stability. The orientation independent stability assessment leads to a rock mass strength criterion based on classification data, e.g. intact rock strength, discontinuity spacing and discontinuity condition. The criterion is developed in the context of a slope stability classification system, however, there is no reason that the criterion is not also valid for the determination of rock mass strength for other purposes, such as foundations on a discontinuous rock mass. The results of the strength criterion are compared to the results of the ’modified Hoek-Brown strength criterion’ and to the rock mass strength as determined by Bieniawski’s classification system. 1 INTRODUCTION In the last decades the study of discontinuous rock mechanics has developed tremendously. For con- structions, such as slopes, foundations and shallow tunnels it has been recognised that discontinuities have a major influence on the mechanical properties of a rock mass. This perception has major conse- quences for the assessment of the engineering behaviour of a rock mass. Calculations for engin- eering structures in or on a rock mass have to include discontinuity properties. Variations in properties, however, can be considerable along the same discontinuity plane. As there may be hundreds of discontinuities in a rock mass, each with its own variable properties, these, taken together with inhomogeneities in the rock material, require that in order to describe or calculate the mechanical behaviour of the rock mass accurately, a large amount of data is required. Laboratory and field tests are available to obtain discontinuity properties. Testing in large quantities is, however, time con- suming and troublesome. Continuum calculations for engineering structures in or on a rock mass, whether analytical or numeri- cal, cannot be appropriate, as the simplifications needed to present the rock mass as a continuum are so substantial that it is nearly always highly ques- tionable to what extent the final calculation model still represents reality. Discontinuous ’distinct block’ numerical calculations can model the discon- tinuities and calculate the behaviour of a rock mass in all detail, provided that property data are avail- able. Apart from the need to have powerful com- puters to do the large number of calculations required by the vast quantity of discontinuities, the test data needed for a detailed numerical discontinu- ous calculation are never available. An often applied practice to avoid these problems is to simplify the discontinuity model, and estimate or 346 Hack, R., 1997. Rock mass strength by rock mass classification. In: Gürtunca, R.G., Hagan, T.O. (Eds) SARES'97 - Implementing Rock Engineering Knowledge; 1st Southern African Rock Engineering Symposium, Johannesburg, South Africa, 15-17 September 1997. South African National Group of the ISRM, Johannesburg, South Africa, pp. 346-356.