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Third Party Oversight ' 15 Water, Waste & Land, Inc. San Luis Mine June 9, 1994 1 1 LJ 5.0 DRAINAGE LAYER This section discusses design issues, materials and their placement, permmeability, and final construction observations associated with the drainage layer (Type 2 material). Observed drain pipe spacing is also discussed. 5.1 DESIGN ISSUES A properly designed and constructed drainage layer within a thin lift tailings disposal facility serves two purposes. First, it protects the synthetic liner 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 and provides for drainage of fluids from the tailings (SRK, 1989). Drain pipes, placed at specific intervals at the base of the drainage layer and on top of the VLDPE synthetic liner, 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. 5.2 MATERIAL PLACEMENT DMG became concerned with the material being utilized for the drainage layer after the October 1 site inspection. The quantity of both the oversized (> 1 'h inches) materials and the percent passing the fines (< No. 200 sieve size) appeared to be greater than that specified in the Design Report (SRK, 1993a1. Oversized, angular materials in contact withh the VLDPE liner can potentially puncture the liner. Drainage material with too high a fines content can decrease the effectiveness of the drainage layer by decreasing the permeability of the material, thereby significantly increasing the head on and potential seepage through the liner system. The drainage layer/liner cover borrow area did produced some oversized amd high fines content (> 35 percent) materials (SRK, 1993c). This material had been placed on acres 1 through 25 within the impoundment prior to identification of these out-of-slpecification materials within the borrow area. Prior to the initial site visit on October 28 and subsequent to placement of materials on acres 1 through 25, a screening plant was installed at the borrow area and a different portion of the borrow area (containing less fines) was developed. At the time of the initial site visit and during the remainder of the oversight period, the plant effectively removed oversized materials. More rigorous QA inspection at the borrow area was effective in delineating areas containing materials with the specified gradation (lower fines content). Prior to the initial site visit by WWL, BMRI tested and delineated areas within the expansion area which contained 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. Of the approximately 24 acres of drainage blanket/liner cover material placed before the incorporation of the screening plant in the borrow area, only 0.75 acres contained materials which were out of specification (too high a percentage of fines) (BMRI, 1993s1.