Leading practice store and release cover trials for a tailings storage facility at Century mine

Abstract

A store and release cover that is designed (the cover) to limit the release of sulphate salts and metals (the potential contamination) from acid forming tailings is site specific; being a function, among other factors, of the borrow materials available, tailings deposition method, and the climatic setting. Three experimental cover trials, each a 75 × 75 m square were developed and constructed on the tailings storage facility at MMG (Minerals and Metals Group) Century mine in the semi-arid Northwest Queensland. The covers rely on the storage of rainfall during the wet season and its release during the dry season through evapotranspiration. Covers typically comprises of a compacted fine-grained reduced permeability layer (RPL) of compacted clay overlain by a significant thickness of loose rock and soil (rock mulch). At the mine there is a paucity of fine-grained material for the RPL, this may be overcome by using mixtures of coarse and fined grained soil and rock (the mixtures). The cover trials have been designed to test three potential RPL mixtures which include minus 10 mm crusher dust conditioned with bentonite, a geo-synthetic clay liner placed between an upper and lower layer of minus 10 mm crusher dust, and minus 10 mm crusher dust conditioned with an extra 15 to 30% fines (passing 0.075 mm). All three cover trials have been enhanced by the addition of a capillary break (CB) layer directly above the tailings. The purpose of the CB is to stop the vertical rise of potential contamination from reaching the top layer of the cover, which is rock mulch. The performance of the cover trials is being monitored by instrumentation, including volumetric water content and matric suction sensors, a weather station capable of estimating evapotranspiration and lysimeters to measure percolation (deep infiltration) through the base of the cover. The instrumentation allows for direct measurement of infiltration into the cover, storage of rainfall and percolation through the base of the cover. The paper describes the cover trials construction, instrumentation and monitoring data from the past two years.