An industry research project has been established to examine the issue of rock mass failure in large open pit slopes. The approach began with numerically modelling failure at the laboratory scale with a displacement discontinuity program. The interim results from the program suggest the output failure mode is dependent upon the input parameter cohesion weakening.

Es wurde ein Forschungsproject mit unterstuetzung von Bergbauindustrie bearbeitet. Die Zielsetzung der vorliegenden Arbeit war die Analysierung des Böschungsgefahr im gröβen Festgesteinstagebauen. Das Verfahren besteht aus der numerischer Modelierung mit Ansatz einen sonderen Rechnerprogram. Die erste Ergebnis zeigt, daβ die Aufgabe am Angabendaten (cohesion) abhangig sein wird.

Un project de recherche industriel fût conduit avec le but d'examiner les eboulements de roche dans les mines a ciel ouvert. La premiere etape a ete de modelliser numeriquement les eboulements dans un laboratoire en utilisant un programme de deplacement discontinu. Les resultants preliminaires du programme indiquent que la prediction des ebouliements est directement reliee à la magnitude de la cohesion.

Introduction

In many open pit mines, overall project economics is driven by the stripping ratio. Lilly (2000) comments that, in most cases, as pit slope angle increases, stripping ratio decreases. This in turn results in a decrease in total operating cost per tonne of ore mined. In ore bodies that are open at depth, the decrease in cost per tonne of ore mined usually means that pit depth can increase until such time as the break-even stripping ratio is again achieved. Thus, gross revenue is increased. As a consequence of this, the net value of many pits is very sensitive to stripping ratio.

When assessing overall stability against rock mass failures in large slopes, the Hoek-Brown failure criterion is a widely used (and widely respected) method for assessing the rock mass shear strength and deformation parameters. It therefore follows from the discussion above that relatively small changes in the Hoek-Brown criterion parameters can potentially have large financial impacts on these operations.

By way of example, consider the simple case of a 400m high slope to be constructed within a blocky siltstone rock mass with a material UCS of 30 MPa. Assume that the design factor of safety for the slope (based on mean input values) is 1.3. For a GSI value of 50, the design slope angle would be 38°. If the design engineer selects a GSI value only 5% greater (52.5), for example, then the Hoek-Brown parameters mb and s increase by almost 15% and almost 43%, respectively. As a consequence, a design slope angle of slightly over 40° would be achievable and this, in turn, could convert to several tens of millions of dollars in project value depending on the length or perimeter of the pit wall involved.

In summary, the selection of slope angle in most large open pit mining projects is one of massive economic importance. Relatively small changes in some of the key input parameters can be valued in the tens of millions of dollars.

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