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Memo to Berhan Keffelew <br />CRESSON AM#7 Geotechoical Findings <br />page 4 <br />minimum static slope safety factor of 1.4 is acceptable for the Lower Squaw Gulch Overburden <br />Storage area. This finding is limited to this specific facility at the CRESSON site, only. <br />As a portion of its design, CC&VG proposed to permanently convey surface drainage from the <br />Upper Squaw Gulch Overburden Storage Area beneath the Lower Squaw Gulch Overburden <br />Storage Area in a rock underdrain placed by gravity segregation with a 24" diameter AD N-12 <br />pipe and riser (details depicted on Drawings 3, 4 and 5 in Appendix 5). This proposal is <br />acceptable to the Division during the operational life of the storage area, during which frequent <br />observation will allow maintenance if the trash rack or pipe inflow were to clog. Following <br />completion of the Lower Squaw Gulch Overburden Storage area, however, observation and <br />maintenance will be less intense. For this reason, the Division will require that the storm water <br />surface diversion system for the Lower Squaw Gulch Overburden Storage Area be properly sized <br />to accommodate the entire projected runoff from the up gradient Squaw Gulch drainage. <br />The Division informed CC&V that the structure will have to demonstrated to be stable in the <br />event the redundant underdrain is compromised, in order for CC&V to retain the Lower Squaw <br />Gulch Overburden Storage azea as a permanent cross-valley fill. To provide this demonstration, <br />CC&V would need to complete a stability evaluation, including consideration of an appropriate <br />dam embankment phreatic surface reflecting impoundment of a pool above the fill. Further, <br />CC&V would need to revise the surface drainage facility designs to provide an appropriately <br />sized emergency spillway. <br />CC&V has chosen "to construct that portion of the Lower Squaw Gulch Overburden Storage <br />Area only as high as will allow surface water draining from the area upgradient of the upstream <br />toe of that earth fill to continue to drain over and azound the earth fill." CC&V will observe this <br />limitation until it has addressed the progression of construction and the underdrain system to the <br />Division's satisfaction. <br />DI....~:_..- <br />