Success in achieving a stable face in a rock excavation depends on minimising the levels of both natural instability and the instability induced by the excavation process. The former requires optimisation of the design in terms of the host geotechnical conditions and the latter the selection of an excavation technique which minimises damage to the design slope. This paper is concerned with the issues involved in optimising slope design. It deals with the type of field data required, reviews traditional methods of analysis, and describes new techniques. It also introduces the computer program "ROCKSTAB" which has been developed to assist the presentation, processing, and evaluation of data.
Stability in a rock slope is determined by the combined levels of natural and induced instability in the host rock mass (Matheson, 1986). Natural instability results from the interaction of geotechnical features in the rock mass and the ground surface. This consists of a zone of disturbance in which the natural discontinuities have been dilated by the gases generated after detonation of the explosive charges. The zone of disturbance is particularly extensive when bulk blasting techniques have been used. Unstable rock slopes generally require expensive remedial treatment to render them stable, and may need continual maintenance throughout their design life, stable rock slopes, on the other hand, minimise the requirement for both remedial and maintenance work and reduce whole life costs. In the design of a rock slope one of the main objectives should be to minimise the levels of both natural and induced instability occurring in the final face. The former is accomplished through the collection and evaluation of relevant field data during the site investigation; the latter through the selection of an appropriate excavation technique. The present paper is concerned solely with optimising the design slope and minimising the level of natural instability in the final face, other papers (Matheson, 1983b and 1986) describe methods, such as presplit blasting, capable of minimising the induced stability.
Data representative of the discontinuities present in the host rock mass must be collected to enable stability assessments to be made and an optimum design to be established. This is normally accomplished at an early stage in the site investigation. Techniques of field data collection are well established( Hoek and Bray, 1977; Goodman, 1976) and, at the very least, involve an interpretive classification of discontinuity type and measurement of the discontinuity orientation. Many other discontinuity characteristics can be of significance (Anon, 1977) and it is usually the responsibility of the engineering geologist to decide the relevance of each to the situation and to determine which characteristics require the collection of data, it has been argued (Matheson, 1989) that a measure of the importance of each discontinuity is vital to the evaluation process. An example of the type of field data which can be collected to permit evaluation with or without considering importance, is given in Table. 1.
