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Mr Allen Sorenson <br />Febroary28, 2002 <br />Page 3 of 4 <br />from damage resulting from mining activities. In addition, based on the very conservative nature <br />of the soil properties input to the worst case analysis, the lower factor of safety values indicated <br />through the analysis, should be acceptable as proof for the protection of structures. Given this <br />information, an offset distance from the utility poles to the top of slope is recommended. to be 90 <br />feet. <br />South SSA <br />The SSA for the southern utility pole structures looked at two separate scenarios: 1) an <br />excavation to 43 feet encountering the weathered shale, and 2) an excavation to 45 feet where <br />the depth to weathered shale measured 56 feet. The results of the South SSA are presented in <br />the following three tables. <br />SOUTH SSA -Minimum Factor of Safety with Varying Excavation Offets <br />43 Foot Excavation - 43 Feet to Shale <br /> Excavation Offset feet <br /> 60 70 80 90 <br />Factor of Safe 1.006 1.087 1.165 1.231 <br />Note: Sli surface taken throw h weathered shale la er, <br /> <br />SOUTH SSA -Minimum Factor of Safety with Varying Excavation Offets <br />45 Foot Excavation - 56 Feet to Shale <br /> Excavation Offset feet <br /> 60 70 80 90 <br />Factor of Safe 1.017 1.099 1.183 1.266 <br />Note: Sli surface taken throw h sand and ravel la er, <br />SOUTH SSA -Minimum Factor of Safety with Varying Excavation Offets <br />45 Foot Excavation - 56 Feet to Shale <br /> Excavation Offset feet) <br /> 60 70 80 90 <br />Factor of Safe 0.765 0.825 0.913 0.970 <br />Note: Sli surface taken throw h weathered shale la er. <br />As in the West SSA, the slip surface through the weathered shale again governs the stability of <br />the slope. For the same reasons identified in the West SSA, the recommended offset for <br />protection of the southem utility poles is again twice the excavation depth, or 90 feet, measured <br />from top of slope. <br />Conclusion <br />When soil properties input to an analysis are known to a relative high degree of accuracy, <br />standard engineering practice calls for safety factors to protect structures on the order of 1.5 for <br />permanent slopes and 1.3 for temporary slopes, such as the working face of a gravel pit. For <br />the analyses presented here, worst case assumptions were input, thus making a concession for <br />lower factors of safety to be acceptable. <br />