GEOTHERMAL MODELING OF SOIL OR MINE TAILINGS WITH CONCURRENT FREEZINGAND DEPOSITION

A geothermal model is described that simulates simultaneous deposition , freezing, and thawing of mine tailings or sequentially placed layers of embankment soil. When layers of soil or mine tailings are placed d uring winter subfreezing conditions, frozen layers are formed in the s oil profile that may persist with time. The following summer, warmer s oil placement may not be sufficient to thaw out layers from the preced ing winter. Remnant frozen soil layers may persist for many years or d ecades. The analysis is unique, as it involves a moving upper boundary and different surface snow cover functions applied in winter time. Th e model is calibrated based on two uranium mines in northern Saskatche wan. The Rabbit Lake scenario involves tailings growth to a height of 120 m over a period of 24 years. At Key Lake, tailings increase in hei ght at a rate of 1.3 m/year. Good agreement between the observed posit ion of frozen layers and those predicted by the model is obtained. Lon g-term predictions indicate that from 80 to 200 years would be require d to thaw out the frozen layers formed during placement, assuming 1992 placement conditions continue. Deposition rates of 1.5-3 m/year give the largest amounts of frozen ground. The amount of frozen ground is s ensitive to the assumed snow cover function during winter.