ABSTRACT

INTRODUCTION

In many branches of rock mechanics there is a gulf between theoretical considerations on the one hand, and design and practice on the other hand. Practical solutions to rock mechanics problems often predate theoretical investigations and many design methods were established, often as a result of painstaking trial and error processes, on the basis of field observations and field measurements. Once established, these methods tend to enjoy widespread utilization with only minor alterations. Eventually a method is used in a context that is markedly different from the one in which it was developed, leading to gross over-design if the method is conservative in the new context, or to design 'failure' if it is not. One example of this can be found in tunnel support design, where techniques developed to cope essentially with problems of roof stability in shallow tunnels were found to be inadequate as tunnels became deeper and the nature of the stability problem changed. Another example is that of shaft pillar design (Fairhurst, 1978).

Theoretical developments are often relatively recent, and while soundly based on mechanical laws, inevitably require a particular practical problem to be idealized in order to obtain a solution. The way in which the theoretical problem is posed often makes it difficult to interpret the results in a design context. Moreover, the way in which site investigations are carried out is based on traditional techniques, and the modelers have not yet attacked in a systematic way the problem of what field data they really require. This has led to a situation in which the theoretician or modeler is unable to provide all (or any) answers to specific practial problems, but is nevertheless skeptical of empirical methods, while the empiricist is quick to see the restrictive nature of the assumptions admitted, explicitly, by the modeler, but never comes to grips with the assumptions and restrictions implicit in his own empirical techniques. Neither side has made a determined effort to investigate what the other has to offer.

The reason for this situation is to be found in the inherent complexity of the problems to be solved in rock engineering. Only rarely does a single, all-embracing theory apply to the full range of field conditions; more often there are a number of different geological or mechanical conditions (depending on one?s viewpoint) which can occur, each potentially described by a particular model of limited scope. The problem is that the boundaries or limits of application of these models have not been clearly demarcated. It is worth noting that two of the more successful applications of rock mechanics have been in rock burst and strata control for extraction of deep, tabular orebodies in hard rock and in slope design. The former example is one where the problem has been identified in such a way that a single model is indeed applicable to most situations encountered in practice, while the latter example is one where a successful attempt has been made to relate different modes of rock response to the excavation geometry and specific structural conditions. (See, for example, Hoek and Bray, 1977.)

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