GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 29 . 3.1.2 Summary of proposed works Gladstone Open Pit (GOP) is one of the first scheduled components of the WNP works. The pit will comprise two conjoined pits sited over Gladstone Hill and Winner Hill and will subsequently be developed into a tailings storage facility (TSF). The proposed activity is described in detail in the Gladstone Pit TSF Design Report prepared by GHD (2022c). A high-level plan of the proposed GOP is provided in Figure 3.2. A summary of the activity is provided below, with references to relevant plans in the GHD (2022c) design report provided in italics: – Preparation works: Earthworks (topsoil removal and stockpiling). Establishment of clean water diversion drains. – GOP excavation: Mining of the GOP will disturb an area of approximately 23.2 ha, with the pit itself approximately 18.7 ha. Excavation of the GOP to a maximum depth of 1,005 mRL (GHD GOP DWG 12537997-C002). Groundwater dewatering (as needed) to enable mining excavation. It is understood that the walls of the GOP will be depressurised using horizontal drain holes which will generally be 20 m to 100 m long. – TSF Development (GHD GOP DWG 12537997-C003 – C007): Closure of the pit and installation of a drainage system to capture tailings seepage and reduce pore water pressure on the liner. Backfill with rock (PAF and NAF) to a minimum elevation of 1,060 mRL to form a base upon which a liner subgrade and geosynthetic liner will be placed. All PAF will be lime treated to reduce ARD. Emplacement of tailings to a maximum elevation of 1,103 mRL. Water recovered by the drainage system will be diverted to the WTP. The drainage system will cease to be operated after water quality is considered to be sufficiently improved. Following settlement of the tailings, a capping layer with a minimum thickness of 1 m of low permeability NAF weathered rockfill will be installed to provide a water shedding surface. The capping layer will be gently contoured to direct runoff towards the southern edge of the pit where the existing wetland is located.
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