GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 31 Figure 19 Proposed light vehicle access road down the backfill face 4.8 Water management strategy 4.8.1 Decant The decant will consist of a floating pontoon pump, with a pipeline either run down the face of the pit or along the access road (refer Section 4.7.2). If the pipeline is run down the face, the pipeline will be anchored at the pit crest using a similar anchor system as the tailings spigots. The decant water will be pumped to the Processing Plant for re-use or to the Water Treatment Plant prior to discharge to the environment. The typical “live” operating pond was assumed to be 100,000 m3 in the water balance and freeboard calculation. The GOP TSF should not be used to store excess water beyond the typical operating pond as this will reduce the flood storage capacity, increasing the risk of overtopping. After a significant rain event, the decant pond level should be reduced to the typical operating volume within 30 days. Sizing of the decant pumping system should be completed at the detailed design phase. 4.8.2 Over liner drainage GHD considered an over liner to improve consolidation of the tailings by allowing downward and radial drainage. However, an over liner has been excluded from the design for the following reasons: – Experience shows that the tailings adjacent to tailings underdrainage systems can become orders of magnitude lower in permeability (relative to the remaining tailings), effectively “blinding off” the drain making the underdrainage system ineffective. – Extracting the water from the underdrainage system would have proved challenging and would have required a bore through the liner to pump the water to the surface introducing additional risk due to liner penetrations. 4.8.3 Under liner drainage The under-liner drainage will rely on the higher permeability of the liner sub-grade, relative to the permeability of the pit walls. The under-liner drainage will (a) capture any leakage through the liner, and (b) reduce the uplift pressure on the underside of the liner from potential groundwater inflow while the TSF is empty. Intercepted seepage/groundwater will percolate down through the backfill to the base of the pit. A sump will be installed beneath the liner to (a) maintain zero pressure on the liner, and (b) maintain saturation of the rockfill beneath the liner (which may be PAF). The drainage sump will consist of a zone of no-fines, coarse rockfill. The
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