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Greg Vaughn 2 24 July 1995 Specific Comments: .1 - Vag of Geg=mb= in Final Cgver: ' Cove proposes to install a flexible geosynthetic cover on the closed waste management unit, and to cover the geosynthetic with a one -foot thick vegetative layer.. The foundation layer, already in place, consists of compacted soil and a waste -soil mixture. This will be compacted to 90%. As noted, the geosynthetic is an engineered alternative approach to the prescriptive standards of Chapter 15. Chapter 15 requires economic analysis justifying use of an engineered alternative. The analysis must show that use of the prescriptive methods is unreasonably burdensome and that the proposed alternative is substantially less expensive as well as being both feasible and equally protective as the prescriptive alternative. In order to justify the use of a geosynthetic, Cove must supply information showing that the elastic, tensile, and puncture -resistant properties of the proposed material (whether HDPE or PVC) are appropriate to withstand the expected forces of installation on slopes and the expected settlement of the wastes. In particular if PVC is used, it will be necessary to protect the material from photo degradation after installation. Imm7m W profilsea oY ancnonng Ue geomembrane and lie layout of the panels will be required. Some of the slopes on which the membrane is proposed for placement are quite steep. We are concerned about the feasibility and stability of such placement, especially on the north side of the landfill, where some of the slopes are intended to be left very steep. A cut wall on the north side of the former mine pit is described (p. 19 of the CPCMP) as vertical. The discharger does not propose to stabilize this slope, but does propose to extend the geosynthetic approximately 6 feet up the side walls of the area. (Further discussion of this deep area follows below). As presented, this placement is not recommended, and will require demonstration that it can be effectively done. Additionally, as discussed below, I have separate concerns about the stability of the side slopes and the practicability of installing a cover of any type on them. . 2. Construction Methods and Quality AssuranctQQA I- The plan now contains only a general outline (Table 1) of the CQA program. The final closure plan must contain detailed information on the method of field seaming and field -checking the geomembrane, factory CQA records for the geomembrane, and discussion of any steps that will be taken to accommodate the particular problems that might occur during installation of a flexible membrane layer over spongy fluffy wastes. Methods of foundation preparation and CQA must be described in detail. Design drawings must include details showing methods of anchoring the geomembrane. The cover design, construction drawings, and CQA must be overseen and certified by a California -registered civil engineer (P.E.) or Certified Engineering Geologist (C.E.G.) 3. Ponding on the Final Cover , Section 2546 of Chapter 15 imposes requirements on the ability of the closed installation to handle runoff volume, erosion, and drainage associated with a 100 -year, 24-hour storm event. Specifically, Section 2546 (a) and (f) requires that the final cover be designed to limit ponding, infiltration, inundation and slope failure under the conditions of the 100 - year, 24-hour storm event. Section 2546 (e) requires that surface drainage be diverted away from the waste management unit. These requirements are intended to reduce infiltration into the cover