Authors: Lamoureux, SC; O'Kane, MA

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Lamoureux, SC & O'Kane, MA 2019, 'Harnessing risk to guide mine rock stockpile closure and long-term cost reduction', in AB Fourie & M Tibbett (eds), Mine Closure 2019: Proceedings of the 13th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 839-854,

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Identifying, understanding, communicating, and managing potential and actual risks associated with mine rock stockpiles (MRSs) closure are effective tools to guide technical studies, inform management decisions and facilitate cost minimisation for closure. Okane Consultants (Okane) Pty. Ltd. have implemented these tools at an iconic Pilbara mine site for nearly three decades. Historic dumping practices (< 1990s) at the site resulted in MRS structures that did not limit sulphide mineral oxidation, as interactions between its principal constituents, mainly oxygen and to a lesser extent water were not controlled. Evidence of potentially acid forming (PAF) material reactivity was recognised early resulting in the identification of potential risks of high internal temperatures and acid metalliferous drainage (AMD) thus guiding preliminary technical studies. Preliminary studies included MRS drilling programs to quantify the structure, reactivity and hydrology of MRSs. During these programs MRSs were instrumented with gas and moisture sensors to quantify MRS reactivity and moisture conditions in response to environmental factors over time. Instrumented cover systems were also tested as a means of limiting oxygen ingress and net percolation (NP). These investigations led to significant advancements in the understanding of the MRS conditions, construction, sulphide oxidation, and associated risks. With closure of a potentially reactive MRS scheduled within five years (year 2023) learnings from preliminary investigations were used to inform managers during a risk workshop focusing on MRS reshaping and the construction of a cover system to limit internal reactivity and AMD. Key workshop outcomes were the need to further quantify the moisture status of the MRS to determine its propensity toward AMD, and the identification that vegetation will be key to limiting NP into the final landform through transpiration thus limiting AMD. In this case study risk was effectively identified and communicated to provide a MRS closure solution specific to the company’s closure criteria and risk profile ultimately resulting in long-term cost reduction for closure.

Keywords: waste rock dump, risk, mine closure

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