EGL Ref: 9215 23 June 2022 Page 52 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. 9.1.4. Dry stacks Dry stacking requires tailings to be dewatered using thickeners and filter presses. Filter presses squeeze the water out of the tailings so they are a wet soil and can be transported by truck or on a conveyor. The soil is then compacted in its final location as a stockpile or stack. Dry stacks require good compaction and placement during winter, which would be difficult in Waihi. Dry stacks also require good underdrainage to ensure the tailings do not become saturated as under high vertical stresses from the overlying tailings soils can become normally consolidated again and prone to contraction under shearing or liquefaction. Like an upstream TSF they require consideration of water shedding and internal drainage. A toe buttress constructed out of rockfill can improve long term seismic stability. The tailings at Waihi would achieve a low permeability however a NAF capping would likely be required for rehabilitation. Management of water runoff is required. Due to the high rainfall a large collection pond is required to manage the contaminated water, a function that a tailings impoundment provides for downstream constructed facilities. Dry stacking is an option; however, this technology is not ideally suited to the climatic and earthquake hazards that exist at the site. At the Waihi Operation there are significant operational issues and risks with the use of filtered tailings due to the high rainfall. The high rainfall combined with the potential for earthquake shaking would likely require construction of perimeter embankments. These factors combined with the high capital and operating cost, are major constraints for the adoption of this technology at the Waihi Operation. 9.1.4.1. Storage 1A dry stack capping Dry stacks of tailings could be constructed on top of Storage 1A with the surface runoff managed in Storage 2. This would require the Storage 2 not to be closed after raising to its currently consented elevation of RL160.7 until a dry stack on Storage 1A is completed. On its own this option would not be economically viable. 9.1.4.2. Storage 2 dry stack capping Dry stacks of tailings could be constructed on top of Storage 2. However, surface water management would require large collection ponds formed using earth embankment dams to be constructed to the north of Storage 2 during operation. This would compromise the proposal to develop a rock stack in this area which is required to meet project scheduling requirements. For the above reasons it is not considered a feasible option. 9.1.4.3. Dry stack north of Storage 2 or east of Storage 1A The areas both North of Storage 2 or East of Storage 1A are on land owned by OGNZL and could be used as dry stack locations. However, for the reasons explained in section 9.1.4 this technology is not ideally suited to the Waihi Operation as there are significant operational issues and risks because of the high rainfall and earthquake hazard.
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