ABSTRACT:

Rock slopes with heights approaching 1000 m are now being designed and excavated in various open pit mining and civil engineering projects around the world. The economic impact of excessively conservative designs or of failures in these slopes can be very large and every effort has to be made to optimise their design. Their size means that they will almost always contain a number of significant structural features and a variety of geological materials. The input required for these models includes a comprehensive geological data base that contains both structural geology and lithological information, an hydrogeological model that permits water pressures to be estimated throughout the slope, estimates of rock mass and discontinuity strength and deformation properties and an understanding of external forces, such as those due to earthquakes, that may be imposed on the slope. Site investigation techniques that can be used to obtain this information and the methods, that can be used to estimate rock discontinuity and mass properties, are reviewed in this paper. Developments in limit equilibrium and numerical modelling techniques are reviewed and their applicability to the design of these large slopes is presented. Practical examples from large projects around the world are presented.

1.0 THE GEOLOGICAL MODEL

A comprehensive geological model is absolutely fundamental to any slope design. Without such a model, slope designers have to resort to crude empiricism and the usefulness of such designs, except possibly for very simple pre-feasibility evaluations, is highly questionable. In open pit mining a great deal of effort is devoted to defining a reliable geological model in order to quantify ore reserves. Therefore a reliable geological model exists and, provided that there is good cooperation between the geological and geotechnical departments of the mine, a small amount of additional effort will produce a sound geotechnical model.

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