GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 32 no-fines coarse rockfill will be overlayed by a layer of selectively well-graded rockfill and sand to prevent fines ingress to the sump to reduce risk of sump/pump blockage. Water will be retrieved from the sump via an inclined borehole with an inline pump. A redundancy borehole will be installed adjacent to the primary borehole given the importance of the underdrainage system. Groundwater pumped to the surface will be returned to the GOP decant pond for recovery by the decant pump. 4.9 Closure plan The closure plan concept is illustrated in Drawing 12537997-C006 and C007 (Appendix A). The current plan for closure is to cap the tailings with a water shedding cover once a majority of the post-deposition settlement has been completed and the storage has been “topped up” with further tailings or PAF waste rock to minimise cover thickness. The capping layer will consist of low permeability NAF weathered rockfill, with a minimum thickness of 1.0 m. The final capped surface will be graded towards two outlets, one on the southern side of the GOP TSF near the wetlands, and one on the western side where the pit crest is lowest (Figure 20). The outlets will consist of trapezoidal channels cut into natural ground. The south-eastern spillway channel is important to restore premining flows to the down gradient wetlands. Each closure spillway channel has conservatively been designed to pass peak rainfall during the PMP event. The design flow was estimated using the Rational Method, with time of concentration calculated using the Kinematic Wave Equation (assuming sheet flow across the catchment). Table 6 summarises the spillway design criteria. Where the spillway is cut in weathered rock that may be susceptible to erosion, rip-rap (D50 = 300 mm) should be used to line and protect the spillway channel. The pit highwalls above the final tailings closure surface will be mined to an overall slope of 2.5H:1V for improved stability and visual aspects of the post-mining landform. Table 6 Spillway channel design criteria Design criteria Design value Design rainfall event PMP Design rainfall intensity (mm/hr) 215 (based on 15-minute storm) Peak outflow (m3/s) 10.8 Catchment area (Ha) 18 Run-off coefficient Conservatively, assume 100% run-off Channel grade (%) 3 Base width (m) 10 Side slopes (XH:1V) 2
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