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<br />' San Luis Projcct - Phue II Raise 1 I?xpansion Conawction Status Report <br /> 6.0 EVALUATION OF CURRENT CONSTRUCTION CONDITIONS <br /> 6.1 Extent of Liner Placement <br />t <br /> Increasing the extent of the area which will be lined during this construction campaign will have <br />' a minor beneficial effect on the operation of the facility. As the ultimate impoundment area will <br /> be lined, a second phase of expansion is eliminated. This will reduce the number of seams <br /> required in the geomembrane liner and thereby reduce the potential for seam defects which could <br /> remain undetected and contribute to seepage from the facility. <br /> 6.2 Subgrade Preparation <br /> <br /> The subgrade preparation procedure which has been adopted will provide a better bond between <br />' the insitu and fill materials. This has little or no affect on the function of the disposal area, but <br /> represents goad construction practices. <br />' 6.3 Dam Raise Fill <br />' Revisions of the specifications for the raise fill, material will not alter the intent of the design <br />or performance of the facility. <br />The basic premise of tailings deposition in the San Luis tailings disposal azea is managed, thin- <br />lift, sub-aerial deposition. By depositing tailings in a thin layer from a series of discharge points <br />i positioned around the perimeter of the disposal area, several key situations are created which <br />are intrinsic to this type of operation. The tailings particles begin to settle from the slurry <br />immediately upon discharge to the disposal area. The particles settle from the suspension and <br />form a beach while the supernatant and remaining tailings continue to flow down the beach to <br />' the water pool in the interior of the facility. Thus, most of the water in the slurry flows by <br />gravity across the surface of previously deposited tailings to the free water pool. <br />' As the tailings on the beach consolidate following initial deposition, much of the entrained water <br />bleeds to the surface and trickles down to the free water pool With tailings deposition moved <br />' to another portion of the disposal area, additional water entrained within the beach tailings is <br />removed through evaporation and moisture uptake {although relatively minor) by previously <br />' deposited and dried beach tailings. As the beach tailings continue to loose moisture through <br />evaporation and desiccate, capillary tension between tailings particles causes the tailings to <br />October, 1993 15 s[tK ProjeM No. 14725 <br />