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Understanding the effect of freezing on rock mass behaviour as applied to the Cigar Lake mining method Roworth, Megan Rose


The objective of this research is to determine how ground freezing affects weak rockmass behaviour with application to the Cigar Lake mine. Cigar Lake mine is a prospective high grade uranium property in northern Saskatchewan where artificial ground freezing will be implemented to support the weak rock associated with the orebody and minimize the potential for a significant water inflow while mining the ore. The deposit comprises a mixture of massive pitchblende, clay and sand and is overlain by thick zones of sandy clay, unconsolidated sand, and altered sandstone. Above and below the orebody, the rockmass shows variations in porosity and permeability due to fracturing and alteration. Artificial ground freezing can be an effective approach to successfully manage and control underground excavations in weak rock mass conditions. Numerous mining and civil projects use artificial freezing worldwide; however, uncertainties remain with respect to understanding and predicting the behavior of frozen rock mass. Previous studies of frozen ground have largely focussed on the behaviour of soil, or in the few studies involving rock, the rock matrix. Of particular interest here is the behaviour of frozen discontinuities present in the weak rock mass and its influence in combination with the matrix on the overall frozen rock mass strength. A comparison of the Cigar Lake mine rockmass and mining operations with that of the McArthur River mine, an unconformity uranium deposit in northern Saskatchewan also utilizing artificial ground freezing will provide the basis for the increase in rockmass quality from unfrozen to frozen conditions. Improving in situ and laboratory characterization methods and developing a better understanding of rock behaviour at sub-zero temperatures is the key focus of this research. A material testing program including unconfined compressive strength, direct shear, and four-point beam experiments was completed using frozen Cigar Lake rock samples. These results are then discussed with respect to the behaviour of the frozen material encompassing the mined out cavities in order to ensure cavity stability during mining. The influence of freezing on the rockmass quality is found to be significant for very weak rocks and decreases exponentially with increasing rockmass strength.

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