EGL Ref: 8983 23 June 2022 Page 18 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. 11.0 DEVELOPMENT OF STORAGE 3 TSF 11.1. Design Concepts The RL155 crest height provides a total storage volume of approximately 7,000,000 m3. Allowing for freeboard requirements to store a Probable Maximum Precipitation (PMP) inflow design flood with 1.0m freeboard, the volume available for tailings storage is estimated at approximately 5,700,000 m3. Depending on the processing rate, discharge regime between TSFs, tailings grind size, and tailings underdrainage, average dry densities in the order of 1.0 to 1.28 t/m3 can be expected based on experience at Waihi with the existing TSFs. Considering that no underdrainage is proposed for Storage 3 a final average dry density of 1.2 t/m3 is recommended for design. On this basis the potential tailings storage in Storage 3 is approximately 6.8 Mt (million tonnes). The tailings impoundment is to be fully lined with an earthfill liner. The earthfill liner on the upstream face of the embankment is part of the embankment zoning. Additionally, a 1.5 mm HDPE geomembrane liner is proposed within the tailing’s impoundment and up to the initial embankment height (RL135) to further minimise tailings seepage. The Storage 3 embankment is zoned with low permeability liners and capping to provide for secure disposal of PAF overburden material obtained from mining of GOP or MOP4. These liners and capping limit oxygen and water ingress and along with the addition of lime into the material being placed minimises any acid generation potential. This approach has been successful for Storage 1A and 2. Groundwater beneath the facility will be collected beneath the base earth liner of the impoundment and the embankment through a series of subsurface drains. Leachate from the material within the embankment will be collected via a series of leachate drains. Groundwater and leachate collected in the drains will be pumped back to the Water Treatment Plant via the perimeter ring main system around Storage 1A and 2, which will be extended to Storage 3. Power will be supplied by extending the existing power line at the Development Site. Tailings delivery pipelines, water return pipes and power for the decant return will be extended around the back of Storage 1A and Storage 3. Clean run-on water from the hills above the TSF will be diverted around the facility to the Ruahorehore Stream. This diversion will be an extension to the existing Southern Uphill Diversion Drain which currently starts behind Storage 1A and runs behind the East Stockpile. This drain is set at a level which allows for future potential raising of Storage 3 to a crest of RL177. The length of the new section of the Southern Uphill Diversion Drain is approximately 2950m. 11.2. Collection Ponds Dirty surface water run-off prior to the placement of PAF can be managed with typical earthworks erosion controls and sediment controls including Sediment Retention
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