Thermal Design Considerations for a Seasonally Frozen Capillary Barrier Diversion Cover System

Abstract

Soil covers on mining waste are typically designed for temperate climates and often rely on fine-grained soils to limit net percolation. Mine sites in cold regions, such as Northern Canada, have limited fine-grained materials and have climates that reduce the effectiveness of designs utilized in more temperate climates. A new cover system that harnesses the cold climate and available coarse textured materials has been proposed. The cover system, a seasonally frozen capillary barrier diversion cover system, relies on the ability of frozen soils with high levels of water saturation, to divert infiltrating meltwater downslope and away from the underlying mine waste. The thermal effects of flowing water on heat transfer in the frozen soil were studied in this research. The potential for early thaw due to the convection and conduction associated with water flow was examined through the use of a numerical model to simulate several case studies and cover geometries. Vertical forced convection was investigated through a case study of frozen column experiments. Lateral convection was investigated through a case study of a natural analogue at Wolf Creek, Yukon, while conduction was investigated through a case study of ponding water and increased conduction. Illustrative cover design models incorporated climate data and idealized materials from a representative mine site into several different geometries, each representing a potential thermal failure mode. These simulations reveal that no one thermal process dominates in frozen soil. Lateral convection can dominate in sloped high hydraulic conductivity soil, given enough water is available to generate large lateral water flows. Under low flow conditions, the influence of lateral convection drops. Vertical convection will result when large amounts of water percolate vertically, causing greater thaw rates throughout the slope than by conduction alone. Conduction will occur regardless of water flow, but increased rates of conductive heat transfer can occur when water ponds on the ground surface, which results in increased rates of thaw leading to preferential infiltration of water below ponded areas. In finer layers of low hydraulic conductivity, conduction will always dominate, as water cannot achieve the flow rates necessary for convection to dominate. This research has shown that the proposed cover design is a viable alternative to the current practice. Further modelling, laboratory and field studies are recommended for future research.