GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 19 Criteria description Criteria requirements Basis Static long-term FoS greater than 1.5 ANCOLD (2019) Static short-term FoS greater than 1.3 ANCOLD (2019) OBE 1 in 150 AEP PGA = 0.1g, Mw = 6.3 No damage to liner, only minor pit wall or external slope deformation. As provided by EGL (NZSOLD) SEE 1 in 10,000 AEP PGA = 0.39g, Mw = 6.9 Some deformations of pit wall and external slopes permitted, but no damage to liner and no release of contents permitted. As provided by EGL (NZSOLD) Water Management Liner Entire facility fully lined with geosynthetic liner up to RL 1107 m. Total liner area: 119,000 m2 As discussed with OGNZL Under liner drainage Yes. Consisting of an underdrainage blanket (liner subgrade) and a sump with borehole pump return to decant pond. Provide two sumps for redundancy. GHD proposed Decant Floating pontoon pump with pipeline either (1) run down face of the pit; or (2) run down access road built into backfill surface. GHD proposed Decant operation Decant water pumped to the process plant for re-use or to water treatment plant prior to discharge. Storm event removal: 30 days to remove 1:100 AEP 24-hour storm. Minimum operating depth: 2.5 m GHD proposed Annual average rainfall 2116 mm As provided by EGL Probable maximum precipitation (PMP) 1223 mm (72 hour) As calculated by EGL Design storage allowance Operational live pond (100,000 m3) plus PMP event. As discussed with OGNZL (Same expectation as existing TSFs) Freeboard Operating pond plus PMP plus 1 m Consistent with other TSFs on site. Surface water diversion drains 1 in 10 AEP GHD proposed Closure Closure concept Water shedding cover consisting of NAF cap over the tailings is preferred. Some PAF (from NRS) may be used initially to compensate for settlement prior to capping with NAF. OGNZL have indicated 10 to 15 year closure period would be acceptable in order to achieve this. As discussed with OGNZL Water management post-closure Spillway (broad-crested weir) cut through natural ground at the southern side of the TSF to restore pre-mining flows to natural wetlands. Design to PMP for closure. (ANCOLD 2019)
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