Authors: Yuan, PH; Zhan, G; Jones, A; Hufford, R

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Yuan, PH, Zhan, G, Jones, A & Hufford, R 2023, 'Conversion of an evapotranspiration soil cover to a geosynthetic cover for a waste rock facility closure', in B Abbasi, J Parshley, A Fourie & M Tibbett (eds), Mine Closure 2023: Proceedings of the 16th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth,

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Earthen water balance covers (e.g., ET covers) are used widely throughout the world to minimize or eliminate meteoric percolation through mine waste facilities. ET covers promote sustainability because they are constructed with natural materials generally available on or near a mine site, and function harmoniously with local hydrological processes. However, percolation from ET covers varies widely, and is dependent on total annual precipitation and precipitation occurrence (rain or snow). At semiarid and arid sites having low annual precipitation, percolation rates through ET covers are typically negligible (i.e., less than 5 mm/yr). In contrast, at humid sites with high annual precipitation, percolation rates can be high (i.e., greater than 100 mm/yr). In 2002, an ET cover was constructed on a waste rock facility (WRF) at Rain Mine in northern Nevada. The WRF is located at high altitude (2,020 m above mean sea level) where annual precipitation, primarily in the form of snow, is estimated at 442 mm. Mature vegetation was established by 2011 and estimated percolation through the WRF was about 12% of annual average precipitation (2011–2019). The high percolation rate from the WRF occurs because accumulated snow is difficult to manage with an ET cover in a snowmelt hydrology setting. To achieve low percolation rates, a geosynthetic cover is required. This paper presents a case study, where parts of an existing ET cover are replaced with a geomembrane that is overlain by a geocomposite drainage layer. Existing ET cover soils were salvaged, and then placed on top of the geosynthetic cover. The geosynthetic cover system was constructed on the part of the WRF that receives most of wind-drifted snowfall deposits. This case study summarizes a performance evaluation of the ET soil cover, characterization data collected, and geotechnical analyses performed in support of the new cover design. Construction experience learned from the execution of the project and performance data of the new cover system collected to date are also presented. Preliminary results indicate that the seepage rate from the WRF has been significantly reduced after the new cover installation.

Keywords: Waste rock facility closure, earthen water balance cover, geosynthetic cover, cover performance evaluation, geotechnical analyses

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