D. A. Beck Beck Engineering, Sydney, Australia
J. Mathis Zostrich Geotechnical, Ellensburg, USA
J. Heimbach BHP Billiton Canada, Yellowknife, Canada
F. Reusch Beck Engineering, Berlin, Germany
Presented at Slope Stability 2011, Vancouver, Canada

Hydrogeology is an important factor in slope stability. However, parameters governing hydrogeology may change as a function of deformation in, and behind, the slopes. Thus, the coupling of deformation and groundwater may be critical when assessing slope stability. The situation is further complicated in arctic environments where variations in the depth of permafrost, especially around lakes, create irregular and complex storage and flow geometries.
Pits at BHP Billiton Canada’s EKATI diamond mine, Northwest Territories, Canada, are located in the aforementioned arctic conditions. The mine is surrounded by a number of lakes and some pits comparatively large for an Arctic Diamond Mine. While the country rock is relatively stiff granite, cut by very persistent structures, the lower pit walls will be composed of a much softer, and potentially plastically deforming, kimberlite. The interaction of the stiffer country rock, with included persistent structures as potential dilating hydrogeologic pathways, and the much softer kimberlite, with both physical and hydrogeologic properties changing as a function of displacement, was deemed critical to slope stability evaluation. As such, the ultimate slope design for one pit was recently assessed using a fully coupled, 3D, discontinuum finite element hydromechanical model. Full coupling in this case refers to the simultaneous solution of the coupled equations for hydrogeological flow, stress and strain.
Using past slope stability analyses as a guide to calibration, the 3D model was used to test hypotheses about the interaction between subsurface water, structure and the slope and then to test successive iterations of the design to aid the optimisation process. The project highlighted the importance of structurewater interaction in slope stability even for modestly sized pits. Key learnings for data collection and design in similar circumstances are presented.

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