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Colorado Department of Transportation <br />28 March 2001 <br />Page 2 <br />3. Memorandum regarding Greeley Feedlot Reserve Analysis, prepared by CAMAS, 06 <br />July 1999. <br />Based on our review of these documents, Haley & Aldrich has determined that subsurface <br />conditions near the northern mining limits generally consist of up to 6 feet of manure and silty <br />topsoil overburden, underlain by up to 72 feet of sand and gravel. Weathered claystone and <br />siltstone bedrock exists below the sand and gravel. The depth to groundwater is <br />approximately 10 feet below ground surface (bgs). Geotechnical parameters for these <br />materials are discuss and justified in the attached calculations. <br />Aggregate Industries mining and reclamation plans are contain as Exhibits D and E, <br />respectively, of the Regular (112) Reclamation Permit Application (June 200). Aggregate <br />Industries intends to mine using dry mining methods to a depth of approximately 30 feet, and <br />wet mining methods to the bottom of the sand and gravel deposit. Topsoil will be stripped at <br />3:1 (horizontal to vertical) slope and stockpiled. Sand and gravel will be mined at a 3:1 <br />slope. Reclaimed side slopes will remain at the 3:1 mined slope and submerged underwater <br />or have 6 to 12 inches of topsoil placed on it and revegetated. The mined/reclaimed slope <br />geometry is shown in the attached calculations. <br />The purpose of our slope stability analysis was to determine if slope failures were likely to <br />occur beyond the designated 25 feet setback, possibly affecting structures located in the ROW <br />of Highway 263. A typical factor of safety used for slope stability analyses ranges from 1.1 <br />to 1.4. However, because the geotechnical parameters used in the analysis were all based on <br />assumed properties, Haley & Aldrich used a target factor of safety of 1.5 as a threshold value <br />for determining slope stability. <br />Haley & Aldrich used a slope stability analysis computer program, XSTABL ver. 5.2, to <br />analyze the proposed mining/reclamation slope. Haley & Aldrich performed the analysis <br />based on the "worst case" scenario, that being a dry pit to 30 feet bgs with groundwater in the <br />adjacent soil at 10 feet bgs. The program analyzed various slope failure surfaces initiating <br />along the entire slope face at lateral intervals of 10 feet and terminating from 20 feet from the <br />slope crest to 100 feet beyond the crest and calculated factor a safety for each surface. The <br />program identifies the ten most critical surfaces based on factor of safety. Results of this <br />analysis show that no failure surface terminating 20 feet beyond the crest of the slope has a <br />factor of safety of less than 1.5. An analysis of the slope failures terminating at the 25 feet <br />setback, indicated a minimum factor of safety of 1.7. Calculations are attached. <br />Based on the stability analysis, Haley & Aldrich does not anticipate any slope stability <br />problems occurring beyond the minimum offset of 25 feet into the ROW of concern.