Rock mass
72 classification
In earlier chapters there was discussion about the influence of the rock mass
structure on the rock mass properties necessary for both the theory
and practice of rock engineering. In Chapter 20, we will refer to block
theory and to the fact that there is now a complete topological solution
to the rock block geometry. This validity of the theory depends critically
on the persistence of the discontinuities. We also mentioned that,
given the discontinuity geometry and all the associated stiffnesses, the
deformability of a rock mass can be calculated. But the ability to make this
calculation depends on the availability of data on the discontinuity
geometry and stiffnesses.
It is evident that even with the most generous resources available for site
investigation, there remain problems in applying the theories in practical
engineering circumstances. As a consequence, several engineers have
developed rock mass classification schemes-which are essentially a
compromise between the use of a complete theory and ignoring the rock
properties entirely. All the classification schemes consider a few of the key
rock mass parameters, and assign numerical values to the classes within
which these parameters lie for a given rock type. As we will see, the
schemes provide a short-cut to the rock mass properties that are more
difficult to assess (eg. the prediction of rock mass deformability) and
provide direct guidance for engineering design (e.g. in predicting the
amount of support required for a tunnel). One of the pioneers of rock mass
classification, Professor Z. T. Bienawski, has recently written a book
Engineering Rock Muss Clussifications (1989) on the subject.
Here we will provide a brief review of the two main classification
schemes which have been widely used. We wiIl also be discussing the
classifications within the overall philosophy of this book, noting their
strengths and deficiencies.
12.1 Rock Mass Rating (RMR) system
The advantage of the Rock Mass Rating (RMR) system, in common with
the Q-system described in the next section, is that only a few basic