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1 <br />' San Luis Project - Phue R Rsiae 1 Expansion CortttuMion Shwa Report <br />' occurred during the initial deposition demonstrates that placement of this material has not <br />prohibited dewatering or consolidation of the tailings adjacent to the initial embankment. As <br />discussed herein, the initial deposition and subsequent dewatering is more difficult to achieve <br />than under current conditions due to the rate of tailings accumulation. The difference is related <br />to the narrowness of the valley during initial deposition which promotes a faster rate of rise and, <br />therefore, allows less time for evaporation to play a role in dewatering. The drainage in the <br />initially deposited tailings is primarily through the drain layer and, in the case of tailings <br />adjacent to the embankment, the drainage into the embankment materials. Wtth the expansion <br />' of the disposal azea and the increased aerial span, true sub-aerial deposition is occurring which <br />is promoting higher evaporation and lower rates of infiltration. Given the proven ability of the <br />' Type 2 material to allow adequately dewatering of the tailings during the initial deposition, use <br />of similaz material for the embankment raise will not affect the dewatering of future tailings <br />deposition. <br />' In addition, use of more permeable material would ultimately promote some movement of <br />' tailings into the embankment. The change in material specifications is consistent with preventing <br />that movement in the same manner as it was prevented during the initial embankment <br />construction. <br />1 Further, the shear strength testing conducted on the embankment raise material indicates that it <br />' has a higher friction angle and cohesion factor than that assumed in the stability analyses <br />presented in the Phase II, Raise 1 Design Report. These higher values increase the factors of <br />safety and overall stability from that presented in the Design Report. <br />' A number of other construction related changes were outlined in this report. These changes <br />' included liner placement to the ultimate elevation of the disposal area to minimize the need for <br />seaming, modification to the preparation of the subgrade to allow for additional compaction, in- <br />place permeabilities of the secondary soil liner which are lower than the specifications require, <br />' alteration of the liner tie-in between phases to minimize the potential for seepage collection, <br />placement of underliner beneath the pipe to prevent abrasion to the primary liner submittal of <br />additional liner samples for independent testing, and cover of the liner in the solution collection <br />ditches to provide additional protection. <br />' All of these field engineering changes completed on the construction to date have been designed <br />to facilitate the construction activities and improve the performance of the facility. None of the <br />' modifications will result in changes in the performance of the facility which could negatively <br />impact the operations or compromise the environmental protection measures inherent in the <br />design. <br />' October, 1993 Zq SRK Project No. 16725 <br />1 <br />