Five years of cover performance monitoring and modelling for a store and release cover at Century Mine

Mine waste facility liner and cover systems


Pascal Defferrard


New Century Resources

In 2010 a store and release cover (the cover) was constructed on the south waste rock dump (SWRD) at Century mine. The purpose of the cover is to reduce percolation into the potentially acid-forming (PAF) waste rock by maintaining a compacted reduced permeability layer (RPL) at near-saturated conditions. This is achieved by overlaying the RPL with a 1.5 to 2 m thickness of soil-mulch. The soil-mulch has a hummocky final surface that captures rainfall so it does not runoff, allowing it in part to infiltrate, with excess water removed by evaporation and transpiration.

The RPL is the trafficked surface of the SWRD.

The unsaturated behaviour of the cover is being monitored by instrumentation including volumetric water content and matric suction sensors and a weather station capable of estimating evapotranspiration. 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 (SWCC).

The key functions used to describe the unsaturated behaviour of the cover materials is the SWCC and the unsaturated hydraulic conductivity function. The SWCC is a plot of soil water versus soil suction. It provides the loci of moisture states of a soil from the saturated to the dry state.

The paper presents the results of five years of monitoring. It shows that total infiltration into the top of the trial dump was about 1.34% of the total five years of rainfall, with the majority of the infiltration going into storage within the cover. Runoff from the top of the dump is not allowed so that the remaining 95% of the total rainfall was lost to evaporation or as lateral flow when infiltration reached the RPL. The paper presents some simple analyses of the wetting by rainfall infiltration of the cover, leading to breakthrough at the base. Field data are used to support the analyses.

Presentation of the waste rock dump water balance is also included in the paper using SVFLUXâ„¢GE which is a finite element model that simulates water transport through a saturated or unsaturated medium.

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