Development of Rock mass classification systems(Part-III0
2.5 Rock Mass Rating (RMR) system
The Rock Mass Rating (RMR) system has been developed by Z. T. Bienawski. This system is based on 351 cases histories covering 15 years. The advantage of the Rock Mass Rating (RMR) system is that only a few basic parameters relating to the geometry and mechanical condition of the rock mass are used. In the case of the RMR system, these are:
(a) The uniaxial compressive strength of the intact rock;
(c) Discontinuity spacing;
(d) Condition of discontinuity surfaces;
(e) Groundwater conditions; and
(f) Orientation of discontinuities relative to the engineered structure.
The way in which these parameters are used to provide an overall rating is shown in Table 7.
RMR =∑ (classification parameters) + discontinuity orientation adjustment
In Section A. of Table 7, with the first five of the classification parameters and their ratings, the parameters are grouped in five classes, each one covering a range of values appropriate to that parameter. When assessing a given rock mass, one establishes into which of these groups the parameter lies, and then sums the resulting numerical ratings for the five parameters.
In Section B. of Table 7, there are ratings for discontinuity characteristics. The orientation of the discontinuities becomes progressively more important from tunnels and mines, through foundations, to slopes, Sections C. and D.
Table 7 The rock mass rating system (after Bieniawski, 1989)
B. Guidelines for classification of discontinuity conditions
D. Rating adjustment for discontinuity orientations
In Sections E. and F. of the table, the rock mass classes are given with a description from ‘very good rock’ through to ‘very poor rock‘, with estimates for tunnel stand-up time and the Mohr-Coulomb strength parameters of cohesion and friction angle for the rock mass.
E. Rock mass classes determining from total rating
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