O’Kane, MA & Ayres, B 2012, 'Cover systems that utilise the moisture store-and-release concept – do they work and how can we improve their design and performance?', in AB Fourie & M Tibbett (eds), Proceedings of the Seventh International Conference on Mine Closure
, Australian Centre for Geomechanics, Perth, pp. 407-415, https://doi.org/10.36487/ACG_rep/1208_36_O_Kane
In general, the mining industry has ‘labelled’, or referred to, cover systems for mine waste storage facilities as per the cover system’s primary function. Examples include: i) ‘store-and-release’ type cover systems, ii) ‘water-shedding’ and/or ‘barrier’ type cover systems, or iii) ‘capillary break’ type cover systems. This approach however, has led to a significant misunderstanding in regards to cover system performance expectations. For example, a ‘water-shedding’ cover system will typically include a barrier layer (low permeability layer) within the cover system and then an overlying growth medium layer. In reality, the growth medium layer is simply another label for a store-and-release cover layer because the functionality of the two is the same (i.e. store surface infiltration within the material, and then evapotranspirate moisture to release it back to the atmosphere). The underlying barrier layer is required to promote ‘water-shedding’ for conditions when storage is overwhelmed in the growth medium layer (e.g. periods of high rainfall).
This paper puts forth an approach to cover system design that focuses on developing cover systems that meet site-specific requirements and ‘work’ because a particular cover system has a high probability of meeting the design (or performance) criteria. For example, if it is determined that in order for a particular mine waste storage facility to be closed within the context of meeting site-wide closure objectives, that the average annual net percolation rate for the cover system must be less than 10% of rainfall, then a cover system is ‘working’ if there is a high probability of meeting this criterion for any given year. This would be determined on the basis of monitoring and modelling.
Hence, the fundamental first step in cover system design and performance is to determine, on a case-by-case basis, to what level, or extent, the cover system must ‘work’. Only then can one determine whether a cover system is ‘working’, or not. This paper will first focus on discussion with respect to the need for developing site-specific cover system design criteria. Following this, discussion and examples will be provided on why and when cover systems do not ‘work’, with particular emphasis on cover systems that utilise, as their primary function, the moisture store-and-release concept to control net percolation rates. In addition, the need for developing a quality assurance program and implementing quality control for construction of moisture store-and-release cover layers will be discussed using field examples such that performance criteria can be met in the short-and long-term. In other words, to ensure the cover systems ‘work’.
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