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Slope Stability Analysis <br /> Pit 124—Varra 112 Permit Application <br /> Weld County,Colorado <br /> Page 2 <br /> Table 1—Soil Strength Properties <br /> Material Wet Unit Weight Saturated Unit Cohesive Friction <br /> (Ibs/cu ft.) <br /> Weight Intercept(PSF) Angle <br /> Ibs/cu ft. <br /> Overburden <br /> Clay* 114 126 50-150 28 <br /> Sand,occasional <br /> gravel 114.4***/122** 132*** 0 35.6 <br /> Sand,with <br /> gravel 114.4***/122** 132*** 0 33.2 <br /> Sand,gravelly 114.4***/122** 132*** 0 45.1 <br /> Bedrock* 124 134 500 22 <br /> Note: * Unit weight values reported by DRMS <br /> ** Unit weight values reported by Terracon Consultants <br /> *** Remolded unit weight values reported by Knight Piesold Consulting <br /> The assumptions used in the bank stability analysis include the following: <br /> • The static depth to groundwater at the distance to no pumping influence is 6 <br /> feet below ground surface and the water table will intersect the pit bank just <br /> above the minefloor(seepageface)during steady state dewatering. <br /> • The pit depth will vary between 35 and 47 feet below grade. <br /> • During extraction activities the pit bank slope will vary between 2H:1V and <br /> 1.25H:1V. <br /> The software package PC-STABL was used to evaluate slope stability. Simulations using <br /> Spencer, and Modified Bishop methods were ran to determine the most conservative safety <br /> factor. The soil strength properties used in the analysis are presented on computer <br /> generated data sheets which are presented in Attachment B. Stability analyses were ran for <br /> the following scenarios: a 40 foot mine depth with a bank cut of 1.25H:1V, a 47 foot mine <br /> depth with a bank cut of 2H:1V for the bottom 20 feet and 1.25H:1V for the remaining slope <br /> and a 37 foot mine depth with a lower 20 foot bank cut of 21-1:1V with the remaining slope of <br /> 1.25H:1V. It was assumed that a safety factor of 1.3 or greater will meet regulatory <br /> approval. <br />