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Mr. Allen Sorenson February 28, 2002 Page 3 of 4 from damage resulting from mining activities. In addition, based on the very conservative nature of the soil properties input to the worst case analysis, the lower factor of safety values indicated through the analysis, should be acceptable as proof for the protection of structures. Given this information, an offset distance from the utility poles to the top of slope is recommended to be 90 feet. South SSA The SSA for the southern utility pole structures looked at two separate scenarios: 1) an excavation to 43 feet encountering the weathered shale, and 2) an excavation to 45 feet where the depth to weathered shale measured 56 feet. The results of the South SSA are presented in the following three tables. SOUTH SSA -Minimum Factor of Safety with Varying Excavation Offets 43 Foot Excavation - 43 Feet to Shale Excavation Offset feet 60 70 80 90 Factor of Safe 1.006 1.087 1.165 1.231 Note: Sli surface taken throw h weathered shale la er. SOUTH SSA -Minimum Factor of Safety with Varying Excavation Offets 45 Foot Excavation - 56 Feet to Shale Excavation Offset (feet 60 70 80 90 Factor of Safe 1.017 1.099 1.183 1.266 Note: Sli surface taken throw h sand and ravel la er. SOUTH SSA -Minimum Factor of Safety with Varying Excavation Offets 45 Foot Excavation - 56 Feet to Shale Excavation Offset (feet 60 70 80 90 Factor of Safe 0.765 0.825 0.913 0.970 Note: Sli surface taken throw h weathered shale I er. As in the West SSA, the slip surface through the weathered shale again governs the stability of the slope. For the same reasons identified in the West SSA, the recommended offset for protection of the southem utility poles is again twice the excavation depth, or 90 feet, measured from top of slope. Conclusion When soil properties input to an analysis are known to a relative high degree of accuracy, standard engineering practice calls for safety factors to protect structures on the order of 1.5 for permanent slopes and 1.3 for temporary slopes, such as the working face of a gravel pit. For the analyses presented here, worst case assumptions were input, thus making a concession for lower factors of safety to be acceptable.