Ninth International Conference on Permafrost ... - IARC Research

Thermal Regime Within an Arctic Waste Rock Pile: Observations and ImplicationsJim W. CassieBGC AVOT Ingenieria Ltda., Santiago, ChileLukas U. ArensonBGC Engineering Inc., Vancouver, BC, CanadaIntroductionAs part of the advanced exploration for diamond miningprojects in Canada’s north, a small waste rock pile of run ofmine materials was constructed at the Snap Lake DiamondProject to investigate its long-term thermal behavior. The siteis located approximately 220 km northeast of Yellowknife(Fig. 1), and it is in continuous permafrost with a meanannual air temperature (MAAT) of -8.3°C (Fig. 2). Themaximum active layer thicknesses at the location of TH 12(Fig. 3) varied between 5.93 m (9/29/2001) and 5.21 m(9/28/2004).Test cell configuration and materialsThe 8 m high “test cell” (Fig. 4) was instrumented with twohorizontal thermistor cables at the base of the rock pile: onecable from west to east (TH 9) and one from south to north(TH 10). The cables were installed to confirm the expectedsubzero nature of the rock pile, along with measurementsof active layer depths within the rockfill material. The testcell consists of clean to sandy gravel. Temperature measuresfrom the base of the rock pile are available for a period of fiveyears. Unfortunately, the rockfill material from the test cellwas later excavated to make room for the full-scale mine.Thermal MonitoringTemperature trends from the south-to-north thermistor arepresented in Figure 6 as one-year moving averages. There isa clear cooling trend for all locations as well as a dampeningof the seasonal variations close to the boundaries of thetest cell. The cooling rates change with distance from theedge and are highest (~0.7–0.8°C/year) at the edge and thecentre of the pile and lowest (~0.3°C/year) half way into the0510Depth (m)15-30 -20 -10 0 10 20Temperature (ºC)Figure 3. Temperature trumpet (2000–2005). The borehole (TH 12)is located approximately 100 m from the shoreline of Snap Lake.20Figure 1. Location of Snap Lake (De Beers Canada).201510Monthly Mean Temperature (°C)50-5-10-15-20Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecDerived Mean Values2006 Mean Values-25-30-35Figure 2. Mean monthly air temperatures for Snap Lake (1998–2006).Figure 4. View of the test cell form the southeast (Photo: J.W.Cassie, 2003).41