2019-12-13_REVISION - M1980244 (62)

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The following narrative on the slope stability analyses was extracted from the body of the CNI Report: OVERALL ANALYSIS Overall slope stability analyses were performed using the slope stability computer program SLOPEW0, which implements the limit-equilibrium method of slices. Ten cross sections were selected for analysis based on wall height, overall slope angle, and RQD. These ten cross sections are presented on plan maps of the mines with projected exposed geology on Figures 7-1 and 7-2, and with projected exposed RQD on Figures 7-3 and 7-4. The analysis methodology and results are presented below. 7.1 Discussion of Overall Stability Stress levels in slopes can locally exceed rock-mass strengths. The strength of the rock mass must be evaluated and compared to the predicted stresses based on geotechnical, geological, and geomechanical parameters. Overall slope failures are generally associated with one or more of the following characteristics: • Major through-going structures that form daylighted and non-daylighted geometries in the mine wall • Low rock-mass strength in the toe •A ubiquitous pitward-dipping joint set • High-angle faults or continuous joints that form back and side releases for slope movement • Saturated toe, excess hydraulic gradients, and localized high pore pressures • High in situ horizontal stresses These factors, alone or in combination with high mine slopes, can create conditions that lead to instability in the intermediate to ultimate walls. 7.2 SLOPEIW Limit-Equilibrium Analysis SLOPE/W is an overall slope stability computer program that implements conventional limit-equilibrium slope stability analysis. This is the most common slope stability method in geotechnical practice and investigates the equilibrium of a rock or soil mass tending to move down slope under the influence of gravity. Two-dimensional cross sections are analyzed assuming a condition of plane strain. It is assumed that the shear strengths of the materials along a potential failure surface follow a linear (Mohr-Coulomb) relationship between shear strength and the normal stress on the failure surface. A safety factor is derived from the ratio of the resisting forces and driving forces for many potential failure surfaces. The lowest factor of safety (FOS) obtained from the potential failure surfaces is the FOS assigned to the slope. Spencer's Method of Slices approach to solving the slice equilibrium equations was used to conduct the overall slope stability analyses. Spencer's method is preferred because it satisfies both force and moment equilibrium conditions, as opposed to some of the simpler algorithms which only satisfy subsets of the force and moment equilibrium. The probability offailure (POF) is calculated using the mean FOS and the FOS calculated when analyzing the slope with minus one standard deviation rock-mass strengths. The probability of failure is calculated using a closed form solution which assumes that the distribution offactors of safety is Gaussian (normal). 25