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TETRA TECH RMC <br /> Mr. Jeff Gregg <br /> March 21,2003 <br /> Page 3 <br /> ► A farm/gas well access road approximately 95 feet(at its closest point)west,northwest, <br /> and north of the mine. <br /> ► Weld County Road 58 approximately 165 feet southwest of the mine. <br /> ► A gas well approximately 160 feet northwest of the mine. <br /> ► A tank battery approximately 130 feet northwest of the mine. <br /> ► A tank battery approximately 130 feet north of the mine. <br /> ► Farm buildings approximately I50 feet(at the closest point)southwest of the mine. <br /> ► An un-named ditch approximately 90 feet(at its closest point)south and east of the mine. <br /> STABILITY ANALYSES <br /> We performed stability analyses in order to evaluate potential for damage to existing permanent <br /> structures due to mine highwall slope failures. The scenarios modeled were along the mine perimeter <br /> with a slurry wall and on the mine interior berms(near Phases 1,2,3,and 4)without a slurry wall. <br /> Analyses were performed with the XSTABL computer program. We analyzed the most critical soil <br /> profile based on exploratory boring data and the mining plan. For this critical slope,we modeled five <br /> feet of sandy clay overlying 41 feet of sand and gravel on top of claystone. As required by the <br /> Colorado Division of Minerals and Geology(DMG)a vertical perimeter slope was modeled in the <br /> analysis. At the mine perimeter,slurry walls were modeled 55 feet from the top of the perimeter <br /> mine slope. In addition,a 20-foot construction buffer was incorporated between the slurry wall and <br /> nearby structures to allow sufficient space for slurry wall construction. For the interior berm profiles <br /> where no slurry wail is present,we modeled a 55-foot setback between the structure and the <br /> perimeter mine slope. Groundwater levels on the profiles were inputted to portray site dewatering <br /> when nearby ditches carry water. Both of the above-mentioned profiles were analyzed with and <br /> without a simulated stockpile surcharge. The surcharge was modeled 10 feet from the top of the <br /> mine highwall over a width of 20 feet with a surcharge pressure of 1,200 psf. <br /> No laboratory strength tests were performed on soils at the site. The soil strength parameters used <br /> were based on typical values for the anticipated soils and our experience at other sites in the area. <br /> Weathered bedrock was analyzed at residual strength as required by the DMG. The inputted <br /> parameters were: <br /> Material Moist Unit Saturated Unit Effective Cohesion Effective Friction <br /> Wei ht a Wel ht c (P e Angle(degrees) <br /> Overburden silty sand/ 114 126 50 26 <br /> sandy clay <br /> Sl walls 110 122 0 26 <br /> Sand and gravel 130 137 0 35 <br /> Bedrock 124 134 100 26 <br /> Weathered bedrock 1 110 122 0 14 <br />