Six years of cover performance data for leading practice store and release cover trials at Century Mine

Abstract

A store and release cover is designed to minimise potential contamination from acid forming tailings which 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 75 m by 75 m square have been developed and constructed on the tailings storage facility at New Century Resources Century Mine in the semi arid North West Queensland. In settings such as Century Mine, store and release cover systems rely on the storage of rainfall during the wet season and its release during the dry season through evapotranspiration. They typically comprise a compacted fine-grained reduced permeability layer (RPL) of compacted clay overlain by a significant thickness of loose rock and soil mulch. Where there is a paucity of fine-grained material for the RPL it may be possible to engineer a material from a mixture of coarse and fined grained material. The cover trials have been designed to test three potential engineered RPLs which include minus 10 mm crusher dust conditioned with bentonite, a geosynthetic 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 layer directly above the tailings. The purpose of the capillary break is to stop the vertical rise of potential contamination from the acid forming tailings (metals and salts) back into the loose rock and soil mulch layer of the cover. The performance of the cover trials has been 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, percolation through the base of the cover and in situ soil water characteristic curves for predictive modelling. This paper describes the store and release cover trials construction at Century Mine, the instrumentation of the trial covers and the cover performance monitoring data collected over the past six years.