Abstract

Rockfall mitigation designs based on previous experience or vendor assurances may not prove amenable to cost-benefit based design. A purely conservative approach to rockfall mitigation may prove very costly in large pit slopes and major construction projects in deep valleys or other geological hazard zones. On the other hand, safety needs to remain the top priority. In such cases, solutions should be tailored to account for variable block dynamics, slope geometry and ground conditions by applying a flexible initial strategy tailored for optimizing costs while safeguarding personnel and infrastructure and updating the design during construction. This paper presents a methodology for a semi-quantitative assessment, developed for a large mining project, supporting a strategy for selecting rockfall risk mitigation measures via cost-benefit analysis. This is achieved by treating rockfall as a stochastic process, estimating the inherent or background rockfall risk of the site and comparing it with the residual risk once mitigations are in place.

1 Introduction

This work sets upon developing a probabilistic and semi-quantitative method for managing the risk posed by rockfall under the premise that a) the risk to personnel, equipment and infrastructure needs to be adequately managed throughout the project life, b) rockfall hazards at such large areal scales are difficult to evaluate quantitatively and c) that a purely conservative approach to rockfall mitigation consisting of eliminating the entirety of the hazard and/or placing barriers wherever infrastructure may be affected during the project life proves prohibitive. The latter may be the case of open pit slopes and large-scale construction projects (such as tailings facilities or linear works in geological hazard zones), where the scale of the project and capital costs involved in managing rockfall necessitate a reliability-based design.

The performance of rockfall barriers is location-specific and many systems are difficult to implement or require significant capital cost. At very large scales, rockfall mitigation designs that are conservatively designed based on previous experience and vendor assurances are not amenable to cost-benefit based optimization. In such cases, solutions should ideally be tailored to local ground conditions throughout the site – despite the scale. At the same time, multiple complex options or combinations of mitigation measures (controls) may be available and it will be difficult to choose one over the other without additional information regarding their effectiveness. A well-informed strategy is required to minimize the initial overall capital costs while meeting risk management expectations – in essence, implementing enough mitigation to reduce to the expected risk level. In turn, this strategy needs to consider how the rockfall risk (not just the hazard) is distributed throughout the site.

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