The Colomac gold mine, located at 64° latitude in northern Canada, operated intermittently from 1989 to 1997. The main tailings dam (Dam 1) was built in a narrow, flat-bottomed valley at the original, drainage outlet from a natural lake. The dam foundation consisted of glacial soils overlying bedrock in a discontinuous, warm permafrost setting, with an unfrozen zone (talik) in the valley thalweg. Possibly due to the growth of the unfrozen part of the foundation, seepage from Dam 1 increased from 6 gpm in 1996 to about 300 gpm in mid-1999. For environmental reasons, a seepage pumpback system (SPS) was installed in 1996.
In 1999, after the mine’s owner went into receivership, the federal government assumed control of the mine site. SRK subsequently worked with government agencies to develop a site remediation plan, including the development of a new dam (Dam 1B) 80m downstream of Dam 1 with the intention of limiting seepage to acceptable levels and enabling SPS decommissioning.
SRK’s Dam 1B design called for the construction of a geomembrane-lined rockfill dam over an area with no unfrozen zones. Thermosyphons, which use passive heat exchange, would chill the foundation for a period of 20 years after dam construction. Numerical analyses undertaken by another firm (EBA Consultants) indicated the foundation would remain frozen for a period of 150 to 200 years, depending on which global warming scenario was assumed.
During the winter of 06/07, SRK supervised the construction of Dam 1B. In 2008, the SPS was removed. Subsequent inspections have confirmed the thermosyphons are working well and the dam is performing according to its original design objectives.
