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~~ 1 1 1 1 Third Party Oversight San Luis Mine 5.1 LOW-PERMEABILITY LAYER 18 5.0 LINER SYSTEM Water, Waste & Land, Inc. January 10, 1993 The San Luis tailings disposal facility was designed with a low permeability layer which would exhibit a maximum permeability of 1 x 10'5 centimeters per second Icm/sec) (SRK, 1993a1. The original specifications for this material type required that the final in-place density be at 95 percent of maximum dry density, compacted at minus 2 per cent to plus 3 percent of optimum moisture content. ~ During Raise I Expansion, the moisture specification was revised to use low moisture contents (up to 6 percent dry of optimum) at which the material was being compacted to stiffen the condition of the compacted material before the installation of the VLDPE liner (SRK, 1993c). Preliminary field permeability (BAT) tests confirmed that the target permeability of 1 x 10-5 cm/sec was being achieved and exceeded regularly at the low moisture content. BAT permeability test results available at the time of the initial site visit ranged from 3.3 x 10'6 to 1.3 x 10-a cm/sec (SRK, 1993c). The average value for all tests was 6.3 x 10' cm/sec. Confirmatory laboratory permeability tests are still in progress. Final field permeability (BAT) tests confirmed that the target permeability of 1 x 10-5 cm/sec was exceeded regularly. Final BAT permeability test results ranged from 7.0 x 10-6 to 1.3 x 10'a cm/sec (BMRI, 1993r). The average value for all tests was 8.7 x 10' cm/sec. The only time that WWL observed the installation and in-situ testinc of the low permeability liner was during the initial site visit on October 28. On November 11, the first day of third-party oversight inspection, the placement of the low permeability material and VLDPE liner had been completed. Preliminary QA test results were reviewed and the BAT permeability test results were consistently lower than the required 1 x i0'S cm/eec specified in the design report (SRK, 1993c1. 5.2 SYNTHETIC LINER Although very little VLDPE liner installation was observed, QA test results were acceptable. Liner repairs were required due to several failed seams and numerous holes created during snow and ice removal. Areas where such activities occurred were inspected by field QA personnel and were patched and tested by the liner installation subcontractor. All activities were observed by the field oversight engineer. Although difficult in cold, wet and dusty weather, results of the seaming and patching operations were acceptable.