Azevedo, FD & Lima, JC 2024, 'Design aspects that may be considered for the decharacterisation of mining geotechnical structures with a focus on hazardous tailings dams', in AB Fourie, M Tibbett & G Boggs (eds), Mine Closure 2024: Proceedings of the 17th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1185-1198, https://doi.org/10.36487/ACG_repo/2415_85
(https://papers.acg.uwa.edu.au/p/2415_85_Azevedo/)
Abstract: After the failure events that occurred in the Brazilian mining sector between 2014 and 2022, the existing legislation was revised and new regulations were published, most notably after 2019. The prohibition of the upstream raising of dams is noteworthy as well as the establishment of deadlines for the decharacterisation of structures raised by this method. From the perspective of the decharacterisation designs, according to the regulations of Agência Nacional de Mineração (ANM), the Brazilian national mining agency, four main stages of execution must be addressed, aiming to guarantee the long-term physical, chemical and biological stabilisation of the area impacted by the construction of the dam. The final stage is the monitoring, which begins as soon as the decharacterisation works are completed and must be performed for two years or more. At least for existing hazardous tailings dams not raised by the upstream method, which are generally not removed from their construction site, the paper proposes that this stage of monitoring should be carried out in perpetuity since these dams indefinitely pose a threat to human health and the environment, and the detection of failures becomes virtually impossible after the end of any pre-established monitoring period. Risk analyses conducted concurrently with the decharacterisation designs may show that certain failure modes will eventually remain unacceptable after completion of works and monitoring, from a socioenvironmental point of view. In this sense, along with the presentation of a case study, this paper discusses that such designs should be developed under a ‘risk-based’ perspective and should aim at minimising the severities and probabilities of occurrence of the identified failure modes to values ‘as low as reasonably practicable’ (ALARP), as required by current legislation. The paper also presents brief comments and suggestions regarding decharacterisation designs of existing geotechnical structures, designs of new structures and mine closure.

Keywords: decharacterisation, perpetual monitoring, hazardous tailings dams, risk analysis, designs of geotechnical structures