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Third Perty Oversight 20 Wetar, Weste & Land, Inc. <br />' San Luis Mine June 9, 1994 <br />' 6.0 LINER SYSTEM <br />This section discusses the materials used and QA testing for the low-permeability layer <br />' (Material Type 3) and the synthetic liner (VLDPE). <br />6.1 LOW-PERMEABILITY LAYER <br />' The San Luis tailings disposal facility was designed with aloes-permeability layer which <br />would, after compaction, exhibit a permeability of no greater than 1 x 10.5 centimeters per <br />' second Icm/sec) (SRK, 1993a). The original specifications for this material type required that <br />the final in-place density be at 95 percent of maximum dry density, compacted at minus 2 per <br />cent to plus 3 percent of optimum moisture content. Optimum was determined in the field <br />' to be approximately 15 percent (SRK, 1993c). <br />The Construction Status Report (SRK, 1993c1 and the Final Construction Report (SRK, <br />' 1994) stated that during construction of the liner system, the moisture specilFication was <br />revised to use low moisture contents lug to 6 percent dry of optimum) during compaction to <br />stiffen the condition of the material before the installation of the VLDPE liner. Field QA test <br />' results presented in the Final Construction Report show that in-situ moisture contents were <br />as low as 4.8 percent (10 percent below optimuml. The low moisture contents do not appear <br />to have adversely impacted the material as field permeability (BAT) tests demorAStrated that <br />the target permeability of 1 x 10-5 cm/sec was achieved and exceeded regularly at these low <br />moisture contents. BAT permeability test results, shown on Figure 4, ranged from 7.0 x 10-e <br />to 1.3 x 10-e cm/sec (SRK, 1993c). The average value for all BAT tests was 8.7 x 10'' <br />' cm/sec. <br />DMG requested laboratory permeability testing frequency of 1 test per acre on the <br />low-permeability material (DMG, 1993k) to confirm the in-situ BAT permeability test results. <br />The low-permeability material was entirely placed and covered by VLDPE liner when the DMG <br />requested laboratory testing. Confirmatory laboratory permeability test results, shown on <br />Figure 4, ranged from 9.4x10'' to 2.6x10_a cm/sec. These results, confirming the~results from <br />the field BAT permeability tests, show that the specified permeability wall exceeded <br />consistently. It is WWL's opinion that the low-permeability layer should perform as designed. <br />The only time that WWL observed the installation and in-situ testing of the <br />low-permeability liner was during the initial site visit on October 28. On November 1 1, the <br />first day of third-party oversight inspection, the placement of the low-permeabiity material <br />' and VLDPE liner had been completed. <br />' 6.2 SYNTHETIC LINER <br />Although very little VLDPE liner installation was observed, OA test results were found <br />to be acceptable. Liner repairs were required due to several failed seams and numerous holes <br />created during snow and ice removal in November 1993. Areas where such activities <br />occurred were inspected by field QA personnel and were patched and tested iby the liner <br />' installation subcontractor. All inspection and repair activities were observed by WWL <br />oversight personnel. Although difficult in cold, wet and dusty weather, results from the <br />seaming and patching operations were acceptable. <br />1 <br />1 <br />