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CDA technical bulletin on tailings dam breach analyses V. Martin Knight Piésold

CDA technical bulletin on tailings dam breach analyses V. Martin Knight Piésold, Vancouver, Canada M. Al-Mamun SNC Lavalin, Calgary, Canada A. Small Klohn Crippen Berger, Fredericton, Canada ABSTRACT: Understanding the consequences of a tailings dam breach ultimately leads to de- signing safer dams and properly preparing for emergencies. Guidelines for dam breach studies are available for water dams, but none of these deal with the hydrodynamic and geotechnical is- sues related to tailings flows. Since 2013, the Mining Dams Committee of the Canadian Dam Association (CDA) has been working on developing methodologies to improve the way tailings dam breach analyses (TDBA) are conducted. Workshops were organized in 2014 and 2015 to understand the state of practice at the time. In 2016 a CDA Working Group was established to develop guidelines specific to tailings dams. The Working Group led the development of the TDBA Bulletin and feedback was obtained on several drafts including a workshop in 2017. The CDA Technical Bulletin for TDBA will provide the key steps that should be undertaken. The differences between water retaining and tailings dams will be addressed. The presence of a supernatant pond and the potential of the tailings to liquefy and flow, are the key parameters in- fluencing the runout potential and outflow volume. The physical processes occurring during a TDBA will be discussed with guidance provided on estimating the volume of released tailings during a breach and predicting where the tailings could flow. The TDBA is planned to be issued in 2019. RÉSUMÉ: Pour concevoir des barrages plus sécuritaires et de bien se préparer aux si- tuations d'urgence, il faut comprendre les conséquences d'une brèche de barrage minier. Des lignes directrices pour les études des brèches de barrage sont disponibles pour les barrages hy- drauliques, mais aucun s'appliquer spécifiquement aux problèmes hydrodynamiques et géotech- nique des écoulements des résidus miniers. Depuis 2013, le Comité des barrages miniers de l'Association canadienne des barrages (ACB) développe des méthodes pour améliorer comment les études des brèches de barrages sont menées. Des ateliers ont été organisés en 2014 et 2015 pour comprendre l'état de la pratique à l'époque. En 2016 un groupe de travail de l'ACB était établi pour développer des lignes directrices spécifiques aux barrages miniers. Le groupe de tra- vail a développé le bulletin technique et des commentaires sur les brouillons ont été reçus, com- prenant aussi un atelier en 2017. Le bulletin technique de l'ACB pour les études des brèches de barrages miniers énonce les étapes clés à suivre. Les différences entre les barrages hydrauliques et les barrages miniers se- ront abordées. La présence d'un bassin surnageant et le potentiel de liquéfaction et d'écoulement des résidus sont les paramètres clés qui influencent le potentiel de ruissellement et le volume de sortie. Les processus physiques qui se produisent au cours d'une brèche de barrages miniers se- ront discutés et des conseils sont fournis pour estimer le volume de résidus miniers rejetés lors d'une brèche et la prévision de l'endroit où les résidus pourraient s'écouler. Le Bulletin sera pu- blié en 2019. 1 INTRODUCTION This paper provides an update on a guidance document that is being prepared by the Canadian Dam Association (CDA) for tailings dam breach analyses (TDBA). The CDA provides a forum to gather and distill consensus on what constitutes good practice for dam safety in Canada. The process to develop the Technical Bulletin on Tailings Dam Breach Analyses involved four years (2016-2019) of collaboration by CDA members. The draft Bulletin is currently undergoing re- views by various CDA committees and external reviewers. A Tailings Dam Breach Working Group (WG) was established by the CDA Mining Dams Committee (MDC) to focus on the development of the guidance for TDBA. Discussions and workshops were held at annual CDA conferences and a number of revisions were made to in- corporate the feedback received. The Bulletin is nearing completion and will expand on the 2007 CDA Technical Bulletin Inundation, Consequences and Classification for Dam Safety and on the 2014 CDA Technical Bulletin Application of Dam Safety Guidelines to Mining Dams. The CDA Bulletin on TDBA is intended to provide dam safety professionals with guidance on the general process and scope for conducting these analyses. While the Bulletin will provide a step by step procedure for such analyses, it is up to the dam owners and the professional engi- neers to agree on the scope that meets the objectives and the requirements set by the Regulators. Reliable TDBA and mapping are critical for tailings dam design and safety management as they help identify and characterize threats to public safety and the environment. The results of the study are typically presented on inundation and deposition maps (as appropriate) and could be used for various purposes including dam consequence classification, emergency planning, dam safety management, failure mitigation planning in case a failure occurred, and mine closure and dam decommissioning planning. There is little published guidance specific to TDBA currently available. Practitioners often refer to guidelines for dam breach analysis of water retaining dams, such as the CDA (2007), or the Washington State (1992) and FEMA (2013) guidelines. Those guidelines were primarily de- veloped for water retaining dams, and while all those documents provide details on dam beach analyses, none of them addresses the hydrodynamic and geotechnical issues related to tailings flows that are critical to tailings dam breach events. The CDA Bulletin on TDBA will aim to fill this gap in the literature and will offer a basis for discussion between dam owners, dam safety professionals, and tailings dam safety regulators. The physical processes of breaching for tailings dams are complex, as they may include flow of fluids (supernatant water and eroded and/or fluid tailings), combined with a flow of liquefied tailings and/or slumping of solid tailings. The rheological behaviour of the released materials differs from that of water and impacts the total volume of tailings released. Studies of previous tailings dam failures show that the volume of mobilized tailings could range from as low as 1% to as high as 100% of the total storage volume (Lucia et al. 1981; ICOLD 2001; Rico et al. 2008; Azam&Li 2010; Small et al. 2017). A tailings dam failure can be defined as the inability of the dam to meet its design intent, whether in terms of management, operational, structural or environmental function, resulting in loss to the stakeholders and the environment. For the purposes of this Bulletin, a tailings dam failure is a physical breach of the dam followed by an uncontrolled release of stored materials that could include fluids and tailings. The uncontrolled release of contaminated seepage without a physical breach of the dam was not considered for dam breach assessment purposes in the Bulletin. The characteristics of a tailings impoundment or a tailings storage facility (TSF) and its foun- dations, construction method, as well as operations, maintenance and environmental conditions (e.g., rain, wind, earthquake, etc.) inform possible failure modes. Failure modes commonly con- sidered for tailings dams include collapse and overtopping of the dam, or a combination of these two modes that are considered under fair weather and flood induced conditions, as required. Collapse of the dam can occur due to various mechanisms like slope or foundation instability, piping, erosion, seismic event, etc. Overtopping can occur either due to an extreme flood event, or inadequate operation of the facility. This paper provides an overview of the proposed key steps to be undertaken during a TDBA. Various considerations specific to tailings dams are discussed. 2 KEY STEPS FOR TAILINGS DAM BREACH ANALYSES In the CDA Bulletin on TDBA, a step by step process will be provided for conducting these analyses. Figure 1 provides a flow diagram for the different steps to be undertaken in a typical TDBA. A description of these key steps is provided in subsequent sections, focusing on those steps that are specific to tailings dams. 2.1 Objectives and scope The objectives are determined at the onset of the assessment including what the results will be used for (e.g., consequence classification, emergency planning, etc.). This then dictates the re- quired scope including the resolution and accuracy of the analyses (e.g., populated vs. non- populated areas), and determines the level of effort and tools used. Desktop TDBA generally re- ly on simpler, qualitative type analyses and do not result in detailed inundation and/or tailings deposition maps. Detailed or quantitative TDBA rely on complex computer modelling and may include additional hydrologic, slope stability, or tailings liquefaction analyses. The results of a detailed TDBA are used to prepare inundation and/or deposition maps, as required. 2.2 Background information and review The available information is collected and reviewed at the onset of the assessment, and data gaps are identified that are related, but not limited to:  The TSF design and staging of the dams and other relevant facilities, including relevant plans and cross-sections for the dams  The tailings characteristics and susceptibility to flow liquefaction due to various trigger mechanisms including lateral unloading developed as a result of a dam breach  uploads/Management/ icold-2019-cda-tailings-dam-breach-pdf.pdf