Development of Rock mass classification systems(V)
3. Applications of rock mass classification systems
Using either of the classification systems described earlier, the engineering quality of a rock mass can be assessed. The RMR system gives a number between 0 and 100, and the Q-system gives a number between 0.001 and 1000. By these approaches, we are able to produce a description of the rock mass based on classes defined by the numbers in the classifications. For example, an RMR value of 62 is a 'good rock: similarly, a Q-value of 20 indicates a 'good rock'. The RMR value provides five such quality classes and the Q-system provides nine
Figure 6 Excavation stand-up time for the RMR system
Both the classifications described were developed for estimating the support necessary for tunnels excavated for civil engineering schemes. The engineer should be careful when using classification schemes for other projects. It is one thing to utilize the rock mass parameters in a taxonomic system for classifying and describing the rock; it is quite another to extrapolate the information to the general design of excavations and their support. Bieniawski (1989) has noted "it is important that the RMR system is used for the purpose for which it was developed and not as the answer to all design problems".
Using the rock mass parameters in each case to provide a quantitative assessment of the rock mass, and utilizing experience gained from previously excavated stable and unstable tunnels, design charts have been constructed, as shown in Figs 6 and 7, for estimating 'stand-up time' or support requirements. For a description of the complete technique for establishing the support requirements, the reader is referred to Bieniawski (1989), which expands on the fundamentals given here of the two systems.
Figure 7 Support requirements for the Q-system
Attempts have been made to extend the classification system to slopes (Romana, 1985). Naturally, the six parameters utilized in the RMR system are relevant to slope stability, but the classification value needs to be adjusted for different engineering circumstances. The way in which Professor Romana numerically adjusted the RMR value was by considering the following factors:
(a) F1 associated with parallelism between the slope and the discontinuity
(b) F2 related to the discontinuity dip for plane failure;
(c) F3 concerning the slope angle compared to the discontinuity dip angle;
(d) F4 relating to the method of excavation.
The classification value is then found from the formula
Table 9 indicates the numerical values of the four factors required to adjust RMRBASIC to RMRSLOPE, together with the SMR classes, the types of failure anticipated, and any remedial measures necessary to improve stability.
Table 9 The SMR rating system (from Romana, 1985 and Bieniawski, 1989)
4. Discussion
It is important to place the value of rock mass classification schemes and the estimations described above within the context of practical rock engineering. It is easy to point to the value of the classifications when, often inexperienced, personnel have to make assessments of rock mass quality and support requirements, especially when faced with no other clear alternative. Similarly, it is easy to say that none of the techniques has any solid scientific foundation and can quite clearly be dangerously misleading if the potential failure mechanism is not identified within the classification system. Stress is not included in the RMR system; the intact strength of rock is not included in the Q-system. Either of these parameters could be a fundamental cause of failure in certain circumstances. Even more severely, a shear or fault zone in the rock could exist which dominates the potential failure mechanism of, say, a cavern or slope.
Because the perceived main governing parameters for rock engineering have been included in the RMR and Q-systems, their use must provide some overall guidance. However, the use of these systems as the sole design tool cannot be supported on scientific grounds. For example, the fact that the measured values of discontinuity frequency and RQD depend on the direction of measurement, yet this is not accounted for in either of the systems described. Similarly, because the rock mass modulus depends on the discontinuity stiffness to a large extent, the modulus is also anisotropic, yet the predictions of E only provide a single (i.e. isotropic) value.
The rock mass classification approach should be supplemented by other methods in due course, as the correct mechanisms are identified and modelled directly. Moreover, it is an unnecessary restriction to use the same classification parameters without reference to either the project or the site. For example, in a hydroelectric scheme pressure tunnel, the in situ stress and proximity of the tunnel to the ground surface are two of the most important parameters. The RMR system cannot help under these circumstances. The Q-system cannot be used for predicting E below a dam if the stratified nature of the rock mass means that there is significant anisotropy of stiffness.
Subscribe to:
Post Comments (Atom)
0 comments:
Post a Comment