Ninth International Conference on Permafrost ... - IARC Research

Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s ttemperature at pile base (ºC)20-2-4-6-80.7 m2.7 m4.7 m8.7 m13.7 m18.7 m28.7 m38.7 m48.7 mEstimated active layer thickness (m)121086428.7 m13.7 m18.7 m28.7 m38.7 m48.7 m02000 2001 2002 2003 2004 2005Jan 01Jul 01Jan 02Jul 02Jan 03Jul 03Jan 04Jul 04Jan 05Figure 5. One-year moving average temperature trends for TH 10.Figure 6. Estimated active layer thickness at different distancesfrom the edge in the waste rock test cell calculated from thetemperatures at the base.pile. This difference can be explained by the formation ofconvective cells within the pile that force cold air to funneldown the centre of the pile, therefore, generating increasedcooling.Active Layer Depths/Permafrost AggradationChanges in the active layer depth underneath the testcell had to be estimated from the horizontal temperaturereadings at the base of the waste rock. At a distance of 2.7 mfrom the edge, the temperatures stayed below 0° during thewhole summer after construction. TH 9 showed subzerotemperatures even at a distance of only 0.7 m from the edge,whereas TH 10 seemed to stay at a zero curtain. No positiveground temperatures, however, were recorded on this side.These readings show that, at the edge of the waste rock pile,the active layer thickness is in the order of 2 m, assuminga slope angle of 37°, which is the angle of repose for therockfill material.In order to estimate the active layer of the waste rock testcell, temperatures at the edge and the base were compared.It was assumed that the ground surface temperature at thetop of an 8 m pile is similar to the temperatures measuredat the edge of the test cell. By linearly comparing thistemperature with the temperature at the base, an active layerthickness can be calculated. The cooling, hence permafrostaggradation in the waste pile, can be followed by the thinningactive layer thickness (Fig. 6). It can be noted that the activelayer thickness also decreases with distance from the edge.In the centre of the test cell (i.e., 28.7–48.7 m), the activelayer thickness is reduced from 8 m to 2–3 m during this5-year period. A thicker active layer was noted closer to theedge. However, the active layer thickness was still reducedby 2 m.ConclusionsThe thermal regime within coarse-grained mine waste rockpiles is of particular importance when designing short- andlong-term mine facilities in cold regions. To investigate thetemperatures at the base of a gravelly rock pile, thermistorswere placed at the base of a test cell at Snap Lake, andtemperatures were recorded between 2000 and 2005 beforethe pile had to be excavated to make room for the full-scalemine. The pile temperatures cooled rapidly, and permafrostunder the pile was reestablished within 5 years of operation.The active layer completely remained within the pile. Thecooling rates differ spatially within the pile confirmingthe development of convective circles during winter. Coldsurface air penetrates down the center of the waste rock pile.The pile excavation further confirmed that ice built; that is,permafrost aggradation occurred at the location of the testcell.These measurements confirm predictions from variousnumerical simulations recently published that suggestaccelerated cooling within coarse-grained waste rock pilesin the Arctic.The composition and internal thermal behavior has to beconsidered in the design of waste rock piles, in particular forpermafrost aggradation/degradation under warming climateconditions. These results provide further evidence as to thewater balance of waste rock piles in cold regions.AcknowledgmentsThe authors would like to thank De Beers Canada for theirsupport and for allowing the publication of these results.Test cell excavation observationsIn summer 2005, the test cell had to be excavated.Observations made during this excavation confirmed thetemperature readings, in that the core of the test cell wasfrozen in only 5 years.42