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Slope Stability Analysis <br />Raptor Pit 125 <br />Weld County, Colorado <br />Page 2 <br /> <br /> <br /> <br />Table 1 – Soil Strength Properties <br /> <br />Material Wet Unit Weight <br />(lbs/cu ft.) <br />Saturated Unit <br />Weight <br /> (lbs/cu ft.) <br />Cohesive <br />Intercept (PSF) <br />Friction <br />Angle <br />Overburden <br />Clay* 114 126 50-150 28 <br />Sand, occasional <br />gravel 132** <br /> 138** <br /> <br />0 <br /> <br /> 35.4*** <br />Gravelly Sand <br /> <br />132** 138** <br /> <br />0 <br /> <br />38.2 <br />Bedrock* 124 134 500 22 <br /> <br />Note: * Unit weight values reported by DRMS <br /> ** Remolded unit weight values reported by Engineering Analytics, Inc. <br /> *** Average of the direct shear testing results <br /> <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 mine floor (seepage face) during steady state dewatering; <br />• The pit depth will vary between 23 and 50 feet below grade; <br />• The termination zone for the 23 foot simulation was placed 1.5 feet back from the <br />crest of the mine wall as the model predicted sheet failure (raveling) for any <br />termination zone beginning down the mine slope; <br />• During extraction activities the pit bank slope will vary between 1.25H:1V and 3 <br />H:1V. <br /> <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 23 foot mine depth with a bank cut of 1.25H:1V, and a 50 foot <br />mine depth with a bank cut of 3H:1V for the bottom 20 feet and 1.25H:1V for the remaining <br />slope. It was assumed that a safety factor of 1.3 or greater will meet regulatory approval. <br /> <br />Discussion <br />As the stratigraphy, other than depth to bedrock, did not vary greatly across the study area <br />two model runs were made to simulate the conditions encountered during drilling. A <br />review of the model results indicate that a safety factor of slightly greater than 1.3 is