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<br />i <br />Response: BMR has selected the secondary soil liner as the most appropriate <br />' liner type for the San Luis Project. The native soils in the <br />vicinity of the heap leach pads are of the proper gradation to <br />be easily amended using powdered bentonite to produce an <br />t effective soil liner. Soil liners are less susceptible to <br />manufacturing defects and are not susceptible to seaming defects. <br />Finally, in the unlikely event of a leak, soil liners can provide <br />attenuating characteristics for constituents in i.he leach <br />solutions. <br />BMR does not anticipate erosion damage to the amended soil <br /> secondary liner due to flows in the secondary recovery layer <br />' (SRL} for the following reasons. <br /> 1. The possibility of significant flow into the SRI. through <br />' the primary liner can be anticipated only after loading and <br /> leaching operations are underway. As the liners and SRI. will be <br /> confined by the weight of the overlying ore, flow through the SRL <br /> must occur through soil voids, not as sheet flow along ether the <br />' synthetic liner/SRL interface or the SRL/soil liner interface. <br /> Therefore, erosion of the soil liner is possible only if the <br />' eroded particles can be transported or piped into and through the <br />SRL <br /> . <br />2. Transportation of eroded soil liner particles into the SRL <br /> is not anticipated as the maximum piping ratio between the SRL <br /> soil and the liner soi l is approximately 3.5 to 4.0. The piping <br /> ratio is calculated as follows: <br />.`~ D75 of the filter soil <br /> D85 of the liner soil <br />D is the particle size of the SRL, in this case for wh?ch 15% <br />o~ the soil particles by weight are fines. D8 is the particle <br />size of the soil liner, in this case for whic~r 85% of the so~l <br />particles by weight are fines. <br />A number of investigators have determined that piping ratios of <br />5 or less will not allow significant piping. Seeoage. Drainaq_e <br />and Flow Nets by Harry R. Cedergren published in 1977 by Wiley- <br />Interscience includes a very good discussion and summary of <br />filter requirements to prevent piping. <br />3. Flow into the SRL from the liner burial trenches around the <br />heap leach pads will not occur due to the location of all burial <br />trenches on top of the perimeter berms. All upgradient runoff <br />flows are expected to be diverted around the pads by the berms <br />and no ponding of water in the burial trenches is anticipated. <br />' No application change necessary. <br />29. The permit application states that the "target permeability" of the <br />' compacted soil liner is 1x10-6 cm/sec. Although we do not recommend the <br /> <br />