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 (e.g. 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...