The prediction of the stability of very high rock cuts is a complex interactive subject involving many fundamental disciplines of rock mechanics. This paper provides a general overview of the procedures and processes currently used. In addition it attempts to examine in more detail complex mechanisms involving stress and structural interactions that lead to failure initiation. The role of progressive failure is also reviewed and emphasised. Brief reference is given to the issue of precedent behaviour. The principal conclusion reached is that a more formal reporting and evaluation of major slope design methodology and failure response are required to allow the subject to be addressed more rationally in the future.
The invitation to provide this paper was made by Professor Giovanni Barla who showed great insight in the provision of the title. Of note were the omissions of the word analysis and also the slight limitation on the enormous subject matter allowed by the qualification Very High Rock Cuts. The subject of rock slope stability is a vast area of rock mechanics that embodies most of the fundamental disciplines in terms of the geological characterisation and strength - deformability of rock masses, the understanding of external forces due to such factors as groundwater, seismicity and the important areas of control relating to excavation or cutting methodology as well as stabilisation techniques. In this presentation a survey has been carried out of the critical factors influencing stability conditions and an attempt has been made to assess how these factors can be addressed in terms of engineering design methodology. Above all, the potential limitations of such methods have been carefully examined. It would be impossible in this paper to trace in any meaningful way the development of rock slope engineering up to the present time although the major contributions of many well known people in this field such as Stini, Hoek, Bray, Londe, Goodman, Barton, Patton, Jennings, Voight and many others need to be recognised. The major and fundamental issue with all rock slopes, whether natural or excavated, is the understanding of the geological characteristics that constitute the rock mass. The entire design and predictive process is closely linked in terms of scope and justification to this intrinsic understanding. Particular to this issue is the large scale distribution of natural discontinuities in the rock mass that provide the definition and framework for both the mechanical characterisation of the rock mass. In terms of the subject matter under consideration, it is convenient to firstly classify the rock slopes in question. To do this certain key words (phrases) have been applied as follows:
Implying the use of excavation methods to form an initial rock cut in slightly weathered or fresh rock in which discontinuities will have a major and controlling influence. This may also include undercutting of existing natural slopes to form a composite slope.
Classified here as rock cuts with an overall height in excess of about 100 m.