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<br />The mining would include removal of sand and gravel in the PSCo easement up to a perimeter <br />within 50 feet of each tower. Once the reserves aze extracted, the PSCo easement would be <br />backfilled with overburden and clay back to the original grade. The mining would be done at a <br />%z H: 1 V slope. The most critical stage of mining with the lowest safety factor for slope stability <br />_ would exist when all gravel reserves were extracted to bedrock and prior to any backfilling <br />activities. It is assumed at this stage that the sand and gravel alluvium would be dewatered which <br />would eliminate any pore pressures being induced in the soils. However, if the South Platte <br />flooded after excavation occurred, there would be some impact from a flooded pit creating a high <br />groundwater table in the islands mined azound the powerline towers. A point load from the <br />towers was added to the analysis to simulate static loading into the foundation and surrounding <br />ground. <br />The mining operation along the perimeter of the pit will border public right-of--way such as 104`h <br />Avenue, existing utilities such as gas lines, telephone lines, etc. The edge of the proposed <br />excavation would be offset from the Cooley property line a distance of 35 feet and could <br />- possibly use a vertical highwall to max;m;~e resource removal. The setback of 35 feet appeazs to <br />be adequate for the types of structures being protected. Slope stability calculations indicated that <br />safety factors were well above the minimum of 1.5. If specific locations have lazge point loads <br />such as those found at the PSCo towers, then the setback should be increased to 50 feet for those <br />structures. <br />The mining adjacent to the Fulton Ditch on the east side of the river will be done as a wet mining <br />operation. A dragline will be used to extract the gravel reserves with no dewatering being done. <br />A groundwater lake will gradually be Left that is hydraulically connected to the valley fill aquifer. <br />This will prevent the formation of a cone of depression in the groundwater table. The <br />groundwater lake would then be used as a settling pond for washer fines from the plant and <br />would also serve as a water supply for plant operations. Any water use would be done under an <br />approved augmentation plan and would be done in such a manner to prevent injury to water <br />rights. The lake would eventually be refilled with clays and silts from the washing operation <br />such that no groundwater would be exposed. <br />The critical time for analysis on slope stability impacts for the Fulton Ditch would be doting the <br />mining. The Fulton Ditch was assumed to be full with sufficient seepage to maintain a high <br />water table in the vicinity of the ditch. The water table would remain close to historical levels <br />throughout the operation. Based on maintaining the historic level of the groundwater table, there <br />would be minimal seepage losses from the Fulton Ditch. Results of this analysis aze contained in <br />the <br />The soils properties are based on assumed unit weights and properties generally associated with <br />materials of similar nature. No laboratory testing was done on the on-site soils. The soil <br />property assumptions were set at conservative values, which would result in lower calculated <br />safety factors than would be found under actual conditions. The values chosen are based on <br />experience and unit weights obtained by the gravel companies for other deposits located in the <br />immediate vicinity. In all cases the calculations indicated safety factors above 2.0 which is very <br />conservative. <br />Page 2 <br />