An economic risk evaluation approach for pit slope optimization

L.F Contreras

doi:10.17159/2411-9717/2015/v115n7a7

Abstract

Synopsis In open pit mine design, it is customary for geotechnical engineers to define the appropriate slope design angles within practical limits. The conventional approach to slope angle design is based on the comparison of calculated stability indicators, such as the factor of safety (FS) and the probability of failure (PF), with generic acceptability criteria not directly related to the impacts of failure. A major drawback of this type of approach is related to the difficulty of defining meaningful acceptability criteria. An alternative methodology of pit slope design is proposed, where the economic impacts of potential slope failures are calculated and used as the elements on which to apply the acceptability criteria for design. The methodology is based on the construction of a graph, referred to as a risk map, that relates the probability of exceeding the economic impact of slope failure to the magnitude of the impact measured in monetary terms. The process includes the analysis of a selected number of representative years of the mine plan and slope sections of the pit areas to define the required inputs for the construction of the risk map. The paper discusses the concepts used in interpreting the probability of slope failure, and describes the approach followed for the estimation of the economic impacts of slope failure and the construction of the risk map. Finally, the two main uses of the risk map are discussed, including the comparison with acceptability criteria for the evaluation of a specific open pit design and the comparative analysis of open pit design options in terms of value and risk to identify optimum pit layouts.

Highlights

The open pit mine design process seeks to define the optimum pit limits and sequence of mining, in order to derive the maximum benefit from the exploitation of a mineral resource given its spatial distribution and the particular geological, economic, and mine settings
Pit slope angles are determined using the conventional approach, whereby slope stability indicators such as the factor of safety (FS) or the probability of failure (PF) are calculated and compared with generic acceptability criteria to define the values to be used in the mine design process
The proposed methodology is an evolution of the approach described by Tapia et al (2007) and Steffen et al (2008), where event tree analysis similar to that used for safety risk evaluations was applied to the economic assessment of slope failures

Summary

Introduction

The open pit mine design process seeks to define the optimum pit limits and sequence of mining, in order to derive the maximum benefit from the exploitation of a mineral resource given its spatial distribution and the particular geological, economic, and mine settings. The methodology is based on a quantitative risk evaluation of the slopes, which has as a central element the construction of a risk map that relates the probability of the impact to its magnitude In this process the economic impacts of slope failure are calculated and used as the elements on which to apply the acceptability criteria for design. The proposed methodology is an evolution of the approach described by Tapia et al (2007) and Steffen et al (2008), where event tree analysis similar to that used for safety risk evaluations was applied to the economic assessment of slope failures. The graphs and data used in this paper to present the methodology are derived from these two previous studies

Background

Methodology

Section 3 Notes: PFmodel PFgroundwater PFgeology PF mining PFtotal

Conclusions