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Third Party Oversight 13 Water, Waste $ Land, Inc. San Luis Mine January 10, 1993 ' 4.0 DRAINAGE LAYER ' 4.1 DESIGN ISSUES A properly designed and constructed drainage layer within a thin lift tailings disposal facility serves two purposes. First, it protects the VLDPE from damage before; tailings are deposited. Second, it allows for increased drainage and densification of the tailings as it has ' a higher permeability than the tailings ISRK, 1989). Drain pipes, placed at specific intervals at the base of the drainage layer and on top of the VLDPE, collect and transport tailings fluid ' out of the impoundment. In a properly constructed system, the combination of interconnected drain pipes and drainage layer material acts to effectively drain the tailings and decrease the head on and potential seepage through the VLDPE liner. 4.2 MATERIAL PLACEMENT ' DMG became concerned with the material being utilized for the drainage layer after its site inspection on October 1. The quantity of both the oversized (> 1 '/: inches) riaterials and the percent passing the No. 200 screen (fines) appeared to be greater than tha~: specified in the Design Report (SRK, 1993a1. Oversized materials in contact with the VLDPE liner can potentially puncture the liner. Drainage material with too high a fines content c;an decrease the effectiveness of the drainage layer and thereby significantly increase the 'head on and I potential seepage through the liner system. The borrow area for the drainage layer/finer cover material did contsin oversized materials and high (>35 percent) fines content (SRK, 1993c1. Drainage layer material had been placed on acres 1 through 25 prior to identification of these out-of-specification materials within the borrow area. Prior to the initial site visit and subsequent to placement of materials on acres 1 through 25, a screening plant was installed at the borrow area and a different part ' of the borrow area (containing less fines) was developed. The plant appeared to be effectively removing oversized materials. More rigorous inspection of the borrow area appeared to be ' effective in delineating areas containing materials within the specified percentage of fines. Prior to the initial site visit, BMRI tested and delineated areas containing out-of- specification materials. Following a review of available QA test results, WWL determined that relatively small areas within the expansion area contained drainage layer materials which did not pass the specifications (SRK, 1993c1. WWL was informed that corrective action being considered by BMRI included the following. 1