Cryohydrogeology of a covered waste rock pile in a permafrost environment : large scale field experiment and freeze-thaw numerical investigations

In order to prevent acid mine drainage, a number of cover concepts are used to limit rainwater infiltration through waste rock. A large scale covered waste rock test pile (120x80x14m high) was constructed and instrumented in 2006 at the Diavik Diamond Mine in the Northwest Territories, Canada, in a region of continuous permafrost. The thermal cover of the pile consists of a 1.5-m thick loose till layer capped with 3 m of non-acid generating waste rock. The thermo-hydrologic behaviour of the core and the cover was assessed through the integration of meteorological, moisture content (from TDR and ECH2O probes), thermistor and outflow data collected over a 10-year period. Heating cables located at the base of the pile significantly affected the thermal regime before they were turned off in 2011. The results considered over the 10 year monitoring period show that the cover provides insulation and promotes the onset of a frozen barrier to moisture flow at the base of the active layer, between 2.5 and 3.5 m depth. At some locations below the crest and within the batters, data show evidence of moisture build-up over time on top of the frozen zone. One-dimensional freeze-thaw numerical simulations of coupled heat transfer and moisture flow were carried out, using SoilVision SVFlux and SVHeat packages. The results indicate that the active layer is contained within the upper waste rock layer, where moisture builds up over time. Air temperature and material thermal properties are the key factors controlling the cover behaviour. The initial moisture conditions and hydraulic properties of the cover materials are important in the prediction of ice and moisture content within the till. Simulations show drainage of moisture from the till to the waste rock in the early stage of the pile, suggesting that the till layer in the test pile may not be functioning as a high moisture and latent heat layer. A long term simulation suggests that evaporation, precipitation, moisture content and the active layer thickness eventually reach a dynamic equilibrium after over 30 years if air temperature increase is neglected.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat