EGL Ref: 8983 23 June 2022 Page 14 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. 8.0 POTENTIAL IMPACT CLASSIFICATION (PIC) Potential Impact Classification (PIC) of a large dam sets appropriate design levels for the dam and guides construction and operational requirements. The PIC of a dam reflects the potential impact a hypothetical dam breach could have on people, property, infrastructure, and environment. Storage 3 will be designed and operated in accordance with modern standards which are set out in the NZDSG. Dams that are designed and operated to these standards have a low and acceptable risk of potential failure, and a breach would be unlikely to occur. Assessment of the PIC considers various factors including Population at Risk (PAR), Potential Loss of Life (PLL), damage to houses, infrastructure, and environment as well as community recovery time. The assessed PIC sets the design level so the design is appropriately resilient to extreme conditions brought on by natural hazards or unlikely scenarios which may occur. The PIC of Storage 3 is assessed to be High based on a dam breach assessment (Ref. 19) undertaken in accordance with the NZDSG. The dam breach assessment also provides maps for use in emergency planning that is part of the dam safety management system (refer to Section 16.5). 9.0 TAILINGS CHARACTERISTICS Tailings in Storage 3 will be deposited subaerially via spigots and end pipe discharge on to a tailings beach as is currently undertaken on Storage 1A. Tailings are deposited over short sections on a rotational basis to allow resting and drying. The pond water level is maintained low during operation to expose as large an area of tailings as possible to air-drying. Airdrying has the benefit of achieving higher density and strength. The deposition of tailings onto a beach (subaerial deposition) via spigots promotes segregation of the tailings. The coarsest tailings generally settle out closer to the point of deposition, with the finer fraction (slimes) transported further. The deposition of tailings on a rotational basis results in local variations in tailings characteristics both between spigots and transverse to the embankment crest. Changes in ore characteristics can also affect the characteristics of the tailings. Samples from boreholes within Storage 2 show that the tailings generally comprise of cohesive low plasticity material (sandy silt, clayey silt) with occasional thin lenses of cohesionless (non-plastic) silty sand material. Lenses rather than layers are inferred from comparison of CPTs on similar sections. Samples of the typical cohesive low plasticity tailings and cohesionless lenses were obtained for testing confirming a low plasticity material. Pore water pressures measured from CPT dissipation testing in Storage 2 indicated a sub hydrostatic profile within the tailings, which indicates underdrainage is occurring. This increases the consolidation stress and strength of the tailings. Storage 1A and 2 do not have a Zone A base liner within the impoundment. Storage 3 will have a fully lined impoundment (combination of earth and geomembrane liner) so the pore pressure profile will be hydrostatic (unless tailings underdrains are installed). This means it will take longer for the tailings to fully consolidate and will result in lower tailings densities than in Storage 1A and 2 (for the same discharge rate and tailings characteristics). Lower densities mean less tailings stored and greater potential for liquefaction of tailings in the impoundment. However, the proposed downstream embankment will remain stable as it is not dependent on the strength of the tailings for stability.
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