GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 24 Figure 12 Typical section of GOP TSF showing the backfill surface Figure 13 Isometric view of GOP TSF backfill surface (yellow = pit shell; brown = backfill surface) The backfill is assumed to come from the NRS or MOP4 cutback. Material in the NRS and from MOP4 cutback has typically run through the jaw crusher and has a maximum particle size of 250 mm. However, the particle size and quality of rock is expected to be highly variable depending upon where the material was mined. Where possible, priority should be given to selecting fresher rock (higher strength) for the rockfill above RL 1060 m. Weathered rockfill/earthfill should be placed in the base of the pit, where strength is generally less critical. The backfill may consist of either potentially acid forming (PAF) or non-acid forming (NAF) rock. However, a large proportion of the backfill is expected to be PAF given the scarcity of NAF on site. All PAF will be lime treated and is assumed to be treated at a similar rate recommended in AECOM (2019). This treatment rate is 0.2% to 0.4% for Martha Pit overburden, assuming a target lag time of 210 days before being covered. The sub-20 mm liner subgrade will consist of select rock from the mining operations, crushed and screened using a mobile crushing plant. The subgrade will also act as a drainage layer beneath the liner, to (a) capture leakage beneath the liner, and (b) limit any uplift pressures from beneath the liner. Prior to the liner being placed, there is a risk of erosion to the subgrade during high rainfall events. This risk will be mitigated by:
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