The authors have developed the Mine Geotechnical Risk Index (MGRI), which has been advanced to attribute tolerable thresholds of risk in a given transitional mine closure context. This paper presents the philosophy behind the development of the MGRI using a conceptual case study and sets out how the authors propose it could be applied by practitioners in particular scenarios only when assessing tolerable thresholds of risk. The reader should note that this approach is not intended to replace conventional engineering risk assessments, it is merely an alternative method in evaluating the tolerability of elevated risk thresholds.
Tailings storage facilities (TSFs) are an essential infrastructure of mineral processing, but they represent a significant physical, chemical and biological hazard and must, therefore, be strictly and responsibly sited, managed and closed. Tailings can, for example, be dispersed by many processes (such as sinkholes, earthquakes, intense rainfall and flood events, and wind), substandard design and construction, and seepage. The stability and behaviour of TSFs needs to be continuously monitored and one highly effective way of doing this is through satellite Earth observation (EO). The EO industry is witnessing a technological revolution. Large and long-lifespan satellite sensors that have been the staple of national space agencies and commercial satellite manufacturers are now being complemented by constellations of low-cost, short-lifespan cube sats by companies with the ambition to image the whole earth daily. Satellites with synthetic aperture radar (SAR) sensors are also collecting high volumes of data, with the added benefit of being able to do so day or night and in different weather conditions. The range of data options and capabilities these provide open opportunities for novel data analysis techniques for TSFs. One of these is satellite InSAR (interferometric SAR; a technique used to map millimetric-precision changes in ground height over time), which is already used by mining companies to reduce risk in and of their operations. From monitoring the stability of TSFs, through to assessments of impacts of natural hazards, InSAR allows rapid and accurate targeting of high-risk areas and structures to identify those that require subsequent investigation through ground-based methods. To demonstrate the application of EO data and InSAR in identifying pre- and post-failure mine activities and TSF deformation, the authors will present a case study across Cadia mine, New South Wales, Australia, which had a localised TSF failure on 9 March 2018. The InSAR results presented show that low-magnitude subsidence signals were observed across the TSF dam during the year preceding the collapse. In January 2018 a notable change in behaviour was observed, with a concentrated area of subsidence focused on the region which initially failed on 9 March 2018. Furthermore, post-collapse InSAR measurements show an increased rate of subsidence for regions either side of the failure zone. Review of medium- and high-resolution satellite images show that the failure was phased, with an initial failure and then a subsequent failure at least two days after 9 March 2018. It also highlights what might be construction activity associated with a dam raise prior to failure.
The development of the geotechnical risk model for the Chuquicamata mine started in 2005 and included the safety and economic impacts of slope failures at different scales (Tapia et al. 2007; Steffen et al. 2008). The model has been updated recently to include a quantitative evaluation of large economic impacts derived from inter-ramp and overall slope failures using a probabilistic approach (Contreras 2015). This paper describes how the later component of the model was used as a tool for the evaluation of four closure alternatives for the open pit. The methodology included three main tasks: 2. Evaluation of the consequences of slope failure associated with economic losses derived from impacts on production and costs. 3. Generation of risk maps to compare several closure alternatives. The results of these analyses provided information on magnitude of impacts and their likelihood for the four closure alternatives evaluated. The evaluation of these results facilitated the selection of the appropriate closure alternative considering the mine reference criteria for economic risk.
Slope stability acceptance criteria is often applied from standard tables representing industry practice or corporate risk tolerance. While in many cases such standard off-the-shelf solutions are fit for purpose, in the case of slopes with a high value increase per incremental slope angle increase, or where highcost infrastructure or other sensitive locations are nearby, it pays to have a custom Probability of Failure acceptance criteria determined through risk analysis. The value lies in being able to quantify the consequences of hazards that are slope angle driven, and in determining the mining schedule consequences of these hazards through Probability of Failure. This paper demonstrates the concept of the risk-based geotechnical assessment through a practical example of a small saprolite pit in West Africa, how such an analysis was carried out, and how the costappropriate risk controls were put in place. The example is based on a gold mine, and considers the timing and size of potential failure as well as the mining schedule and cash flow schedule. The type and size of appropriate risk controls are also estimated in the calculation. Finally, the paper demonstrates through the model how the value of geotechnical engineering controls can be calculated using the concept of real options.
Ground motion over mine sites, while an everyday occurrence, may represent hazards that need to be identified and monitored over time. An accurate and regularly updated overview of surface movement over mining operations is therefore critical as part of an ongoing risk assessment program. By having a complete picture of ground stability, movement patterns which represent potential geotechnical hazards to safety and mine operations can be identified and tracked over time. From routine monitoring to highfrequency updates, interferometric synthetic aperture radar (InSAR) technology is increasingly being used to identify a wide range of movement patterns which may be of concern to mine operators and geotechnical engineers. Recent advances in radar image processing algorithms, combined with an increase in the number of satellite systems launched into orbit, have resulted in improvements in the ability of this technology to capture complex and rapid displacement. In particular, the ability to characterise rapid and sudden motion (metres of movement) has increased the utility of InSAR from a practical standpoint in characterising geotechnical hazards. Further exploitation of 2D monitoring approaches in capturing vertical and horizontal movement, particularly for producing displacement vectors along cross sections, can also provide additional insights into hazard characterisation. Key differences between the ability of highresolution imagery to capture complex and rapid deformation in comparison to low-resolution (but freely available) Sentinel imagery are also touched upon. This paper will focus on the practical application of InSAR technology to monitor mine sites around the world, illustrating how new processing approaches and data sources are used in the identification of geomechanical risks that are typically of greatest concern from both an operational and safety standpoint. Examples of results over an active mine site will be shared and a particular emphasis will be placed on selecting the right InSAR tool for helping geotechnical engineers best manage risk due to movement.