EGL Ref: 8983 23 June 2022 Page 35 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. 13.5. Geomembrane liner performance OGNZL has undertaken site specific trials of geomembrane performance when exposed to tailings liquor and overburden rock leachate from the existing TSFs (Ref. 21). The trials involved testing of enhanced HDPE and Linear Low Density Polyethylene (LLDPE) liners. LLDPE is more flexible than HDPE and this can be an advantage where large strains are expected. The trials indicated that the enhanced HDPE membrane performed much better in oxidative stress tests when exposed to leachate and tailings liquor samples from Waihi and would have a much longer effective design life. A HDPE liner provides more resistance to chemicals breakdown and has sufficient flexibility to withstand expected differential settlements and expected strains. Consequently, a HDPE geomembrane is recommended. The use of a geomembrane liner provides additional protection against seepage from the tailings into the environment above an earthfill liner alone. Over time the geomembrane becomes redundant to the design performance. Seepage control is provided by the Zone A and B earthfill liners and the consolidation of tailings. 13.6. Groundwater and leachate seepage estimation Estimates of groundwater and leachate seepage, including flows to the various subsurface drains, are based on seepage models with permeabilities of the different materials determined from insitu and laboratory testing, and also the historic performance of the subsurface drain flow and piezometric levels measured at the existing TSFs. 13.7. Uphill diversion drain sizing The uphill diversion drains will be sized in accordance with the design criteria in Section 10.2. Design will include freeboard above the design flow to allow for sedimentation, and waves and unusual flow conditions that may occur. The fill and cut slopes associated with the drain will be designed to meet conventional factors of safety and performance for different load conditions. The drain will be constructed from materials that have inherent long-term durability (earth and rock). Armour rock will be used in locations where velocities could result in erosion of bare earth surfaces. 13.8. Perimeter drain sizing The perimeter drain will be sized in accordance with the design criteria in section 10.3. Design concepts and drain geometry will be similar to the existing perimeter drains. Design will include freeboard above the design flow to allow for sedimentation, and waves and unusual flow conditions that may occur. 13.9. Collection pond sizing The collection ponds will be sized in accordance with the design criteria in section 10.5. Design concepts and geometry will be similar to the existing collection ponds. The fill and cut slopes associated with the collection ponds will be designed to meet conventional factors of safety and performance for different load conditions. The ponds will likely have a volume greater than 20,000m3 and be higher than 4 m and will be classified as large dams by the Building Act, and therefore will require design as per the NZDSG. A dam breach assessment will be required to assign the PIC, which
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