Chapter 6 - Rock Mass Rating

Abstract

The geomechanics classification or the rock mass rating (RMR) system was initially developed at the South African Council of Scientific and Industrial Research (CSIR) on the basis of experiences in shallow tunnels in sedimentary rocks. This chapter provides an overview on RMR with discussion on estimation, application, and precaution of RMR. To apply the geomechanics classification system, a given site is divided into a number of geological structural units in such a way that each type of rock mass is represented by a separate geological structural unit. The following six parameters are determined for each structural unit: Uniaxial compressive strength (UCS) of intact rock material, rock quality designation (RQD), joint or discontinuity spacing, joint condition, groundwater condition, and joint orientation. RQD is determined from rock cores or volumetric joint count; it is the percentage of rock cores in one meter of drill run. RMR is determined as an algebraic sum of ratings for all of the parameters. On the basis of RMR values for a given engineering structure, the rock mass is sorted into five classes: very good (RMR 100–81), good (80–61), fair (60–41), poor (40–21), and very poor (<20). It must be ensured that double accounting for a parameter is not done in the analysis of rock structures or in estimating the rating of a rock mass.