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N <br />a TETRA TECH <br />June 24, 2008 <br />Technical Memorandum <br />a el <br />Tt #181736 <br />Subject: Denison Mines, Topaz Mine Slope Stability for Operating and Closure Conditions <br />Slope stability analyses were performed for Cross Section A-A' through the waste rock area of <br />the Topaz mine. The proposed waste rock surface during operation conditions reaches a height <br />greater than 60 feet without a bench. This condition prompted a slope stability analysis to <br />determine the safety factor of the slope. The proposed reclaimed waste rock surface was also <br />evaluated to determine the long term factor of safety. Additionally, each slope was evaluated <br />for seismic loading using peak ground acceleration (PGA) for Cortez, Colorado of 2.8%g (USGS <br />Hazmaps). <br />The slope stability analyses were performed using limit equilibrium methods with the computer <br />program SLOPE/W (GEO-STUDIO, 2007). SLOPE/W solves limit equilibrium slope problems <br />by several different methods. Spencer's method was chosen for this study because it considers <br />both force and moment equilibrium. Interactive searches were performed to determine the most <br />critical failure surface for each section analyzed. <br />Material Properties <br />Material properties used for the waste rock in these analyses are presented in Table 1. The <br />waste rock is comprised of slab sandstone that is angular and well-graded. Given these <br />characteristics, a conservative friction angle of 39 degrees was chosen for the slope stability <br />analyses corresponding to the placement of the waste rock (Lambe and Whitman, 1969). <br />Table 1 Material Shear Strength Parameters Used in Stability Analyses <br />Material Type Total Unit Weight, y Internal Friction <br />Angle, ip Cohesion, c <br /> (pcf) (deg) (psf) <br />Waste Rock 115 39 0 <br />Tetra Tech I <br />3801 Automation Way, Suite 100, Fort Collins CO 80525 <br />Tel 970.223.9600 Fax 970.223.7171 www.tetratech.com <br />PA 181736 Denison USA\Slope Stability\Slope stability tech memo.doc