Gas modelling was completed to inform stability upgrade designs for a tailings storage facility (TSF). Significant iron-rich precipitates have been accumulating at the toe of a 40+ year-old tailings storage facility (TSF). From a geotechnical perspective, the concern is that precipitation at the toe result in increased piezometric levels in the dam, which might compromise the long-term dam stability. A buttress with an integrated High-Density Polyethylene (HDPE) geomembrane liner above the drain elevation is being designed, which would limit the ingress of oxygen. The aim of this design is that reduced mineral precipitation will prevent the further rise of piezometric levels in the dam.
A 2D gas model was constructed to model the performance of the liner to limit oxygen ingress with cognisance of the buttress geometry. The following critical aspects were required for the modelling of the gas migration into the proposed sealed buttress:
• The Oxygen Consumption Rate (OCR) of the buttress waste rock.
• The Oxygen Diffusion Coefficient (ODC) of the HDPE liner.
• The degree of saturation in the layers overlying the HDPE.
The gas modelling indicated that below the liner, the air phase reaches sub-oxic conditions within a couple of years. Saturated parts of the buttress reach sub-oxic conditions within a couple of months as gas diffusion into the water phase is too slow to keep up with the consumption of oxygen.
It is anticipated that the sub-oxic conditions will create a geochemical environment that would not favour mineral precipitation and ideally, may even promote mineral dissolution. The results of the gas modelling were further used in a follow up study in a Reactive Transport Model, to calculate the potential for precipitation in the buttress.

This paper explores the considerations and constraints in the implementation of a tailings storage expansion project in Indonesia. An existing tailings storage is being operated at the site. The existing TSF is a valley fill type storage with upslope catchments. Currently the TSF has a main embankment with a height of around 80 m. The site has a tropical climate with high rainfall exceeding evaporation, and medium to high seismicity.

An options study was conducted to assess the future storage needs for the life-of-mine. This study considered a variety of site options including the existing site. At the time of the study, the site had a mine life nine years with 45 Mt of projected ore production. The existing TSF at the time of the study had 13 Mt of storage capacity. The existing TSF has a consequence rating of ‘High B’ in accordance with ANCOLD (2019), due to potential loss of life at the plant and ‘major’ consequence of an embankment failure.

Five site options were considered for future tailings storage including raising the existing facility and in-pit storage. A ranking of the site options was performed and considered project location/cost, engineering risk, environmental risk and social risk. Each option was scored based on outcomes for the criteria. From these scores the options were ranked under each category. Each category was assumed to have equal weighting. From the ranking of the options for each category, an overall ranking was assessed. Based on the assessment the best ranking, was raising the existing facility however an alternative site which had the next ranking was also investigated. Following investigations it was decided that the option of raising of the existing facility was preferred.

The study concluded that the additional 32 Mt can be stored in the existing TSF by raising the embankment between 20 and 30 m. The paper presents the design adopted and presents a discussion of the design constraints, and design and operational controls adopted.

The project is in the process of being approved by government regulators.

The mine discussed in this paper is an open cut coalmine located in Bowen Basin, Queensland. It has been co-disposing mixed plant rejects (MPR) and dewatered tailings (DT) within spoil dumps for over nine years since commencement of operations. MPR is loaded onto trucks at the Coal Handling and Processing Plant (CHPP), transported to prestrip waste dumps and then co-disposed on dump bench through either paddock dumping or at active tipheads along with waste truck tipping. DT is disposed in tailings cells constructed in prestrip truck dumps due to wet weather events and other operational constraints, and then capped with spoil. Physical and mechanical properties, including total moisture contents, densities and shear strengths of the co-disposed MPR and DT, were unknown until the Closure Planning team, Principal Environmental Geochemist, organised a study program to assess geochemical properties on in situ waste. As part of the program, the geotechnical team collected MPR and DT samples from sonic core drilling, which were then sent to the University of Newcastle for laboratory testing. This paper presents the findings from sonic core drilling and laboratory geotechnical testing, and validating and optimising outcomes from reviewing of the existing co-disposal strategies at the mine through dump stability modelling and by using the shear strength results from triaxial compression testing conducted on the samples collected during the fieldwork.

Calibration of a water balance model usually requires measured pond water levels of sufficient duration and frequency. However, such data is not always available for tailings storage facilities, especially for inactive facilities. Satellite remote sensing products may be used as the alternative data set in water balance model calibration when in situ monitoring data is not available. This paper presents the water balance model calibration case study for an inactive tailings storage facility located in Northern Australia. The calibration used the Sentinel-2 open-access remote sensing data which provides high-resolution historical satellite imageries captured every five days for the study site. The historical satellite imageries were used to develop a long-term historical supernatant pond level record for the facility. The process involves using the Normalized Difference Water Index (NDWI) to calculate the supernatant pond size and shape. The identified pond boundary was then converted to a pond water level using the available topographic survey of the tailings beach. This approach developed a three year historical pond water level record which was then used for calibration of the water balance model. The calibrated water balance model was used to conduct a probabilistic water balance assessment which established a more reliable facility flood risk profile for both the dam owner and the local authority. This case study reveals the potential application of the Sentinel-2 Satellite remote sensing data set for the calibration of tailings storage facility water balance models. The paper also discussed the precondition and limitations of this application.

Mining companies are increasingly challenged with water treatment obligations that may endure in- perpetuity.  Methods for conventional treatment of mine effluent is costly and, in some cases, ineffective at meeting environmental water quality objectives. Due to these challenges, mining companies are undergoing a transformation from providing conventional to innovative environmental management solutions; implemented during initial mine planning through source control and water management, to operational and post closure water treatment and reclamation. Technologies that manage and treat mine influenced water through stimulation of biogeochemical processes support this transformation, and may minimise compliance issues and operational requirements, and assist in meeting sustainability goals. This presentation will present case studies to exhibit the development, design and implementation of alternative technologies that have been successfully applied at mine sites for the treatment and management of mine influenced water. Applications of technologies including in situ and ex situ treatment reactors such as Gravel Bed Reactors™ and bioreactors, phytotechnologies, constructed and engineered wetlands, pit lake in-pit treatment, and permeable reactive barriers will be presented. The deployment of mobile treatment systems to mine sites, such as containerised treatment pilot systems, will be discussed as an important stage to facilitate treatability studies, regulatory approval, and advancement of technology application to full- scale. Advantages of using alternative water treatment and management tools will be presented along with new challenges and opportunities to apply these technologies at Australian mine sites.

The Global Industry Standard on Tailings Management (GISTM; Global Tailings Review (GTR), 2020) requires the use of a multi-criteria alternatives assessment (MAA) for a new TSF with the goal to select an alternative that: (1) minimises risks to people and the environment throughout the tailings facility life cycle; and (2) minimises the volume of tailings and water placed in external tailings facilities. The use of TSF alternatives trade-off studies in one form or another is well-established; however, in the context of GISTM roll-out, the widespread application of the often more rigorous MAA process is a significant emerging trend in tailings management.

Application of the MAA process can be a complex undertaking with requirement for engagement with stakeholders from technical and non-technical backgrounds, and consideration for a broad range of alternatives including tailings dewatering and alternative disposal methods such as commingling waste. This paper provides an overview of applying the MAA process and lessons learned from six recent applications of the MAA process covering a diverse range of ranging of commodities (coal, copper, gold, iron ore, and poly-metallic), project statuses (proposed, operational, and closed), and locations (Australia, Africa, and the Americas).

The typical MAA process applied in the reference projects included confirmation of the ‘must haves’ (or design basis), the ‘can’t haves’ (or exclusions/‘fatal flaws’), and ‘nice-to-haves’ (or objectives) for identification and assessment of TSF alternatives. The nice-to-haves/objectives are the foundation for the MAA framework and, in line with guidance provided from Environment Canada (Environment Canada, 2013), these were typically grouped under a set of ‘accounts’ covering technical, environmental, and social (or socio-economic) considerations alongside project economics. The comparison of MAA frameworks from the reference projects provides useful insights into commonalities between projects as well as the site-specific objectives driven by nuances in the site’s physical (climate, topography, geology etc) and social settings.

Alternative tailings disposal methods have gained much attention due to their advantages over conventional slurry tailings storage facilities. Co-disposal of tailings and mine waste rock is one of the leading methods of alternative disposal methods. However, application of this technology has been mostly limited to small-scale and research projects until recently. This paper discusses the use of co-disposal technology in a large-scale mining project in Western Australia.

An options assessment was performed to evaluate alternative tailings disposal methods against conventional slurry deposition, and co-disposal of tailings and mine waste rock was selected as the preferred option for tailings management. A feasibility study was completed for the selected co- disposal option that involved co-mingling of tailings with mine waste rock and construction of alternating layers of waste rock and tailings. Geotechnical and geochemical characteristics of tailings and waste rock, site topography and subsurface conditions, climatic conditions, construction and operational benefits, life cycle costs and safety in design were evaluated during the study.

Suitable mixing ratios of waste rock and tailings were assessed relative to the mine waste production schedule to maintain rock to rock contact ensuring improved static and dynamic stability of the co- disposal facility. Containment of potentially acid forming waste rock was incorporated in the design by including tailings in the waste rock dump.

Preliminary designs were completed for the co-disposal waste rock dump facility and slope stability and deformation analyses were performed for the design. The co-disposal facility will be developed to a detailed design during the next phase of the project.