GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 21 3.3 Potential impact classification Given that the GOP TSF is an in-pit tailings storage, no credible breach mechanisms are expected to result in a dam breach scenario. This is further discussed in Section 5.6. Therefore, the PIC is deemed to be non-applicable, and the relevant design and operation protocol shall be based on the lowest PIC (or consequence category) according to each relevant standard (i.e., NZSOLD Dam Safety Guidelines and ANCOLD Guidelines on Tailings Dams). 3.4 Geometric constraints The final design will consider the following geometric constraints in and around the GOP: – Existing underground portal – Potential new underground portal – Nearby underground mining, both historic and future workings – Processing plant and associated infrastructure – Martha pit conveyor – Wetlands – Active landslides and other surficial features. 3.5 Design standards The design standards and technical references adopted for this project will be as follows: – New Zealand Society on Large Dams (NZSOLD) Dam Safety Guidelines (2015) – Australian National Committee on Large Dams (ANCOLD) Guidelines on Tailings Dams (2019) and related ANCOLD guidelines. – Relevant ICOLD Guidelines. – Global Industry Standard on Tailings Management (GISTM) 3.6 Identified risks The following risks were identified during an initial risk workshop and the design has addressed these: – In-rush risk to connected underground mine through current and historic workings (portals, historic drill holes etc). To eliminate the in-rush risk, GOP TSF will not receive tailings until all potentially connected underground mining is completed. Alternatively, a risk based-approach may be considered whereby identified controls may be implemented to reduce the in-rush risk to an acceptable level. – Historical adits that pass through the pit shell and daylight outside of the pit footprint or connect to underground workings, potentially providing a path for release. Those historical workings that provide a potential path for release will need to be mapped and remediated (e.g., backfilled, plugged) prior to filling of GOP TSF. – Closure within an acceptable timeframe. OGNZL have indicated the preference for a water shedding cover. The anticipated high rate of rise of tailings in the GOP TSF will potentially (a) make trafficability of the final tailings surface challenging, and (b) create significant post deposition settlement. This risk has been considered through consolidation modelling presented in Section 5.4. – Failure of the underdrainage system leading to instability and failure of the liner and backfill surface. This risk has been addressed through stability modelling in Section 5.3 and through providing added redundancy to the underdrainage system. – Failure of the backfill or subgrade leading to failure of the liner and delay to commissioning or high costs for re-work. This risk has been addressed through stability modelling in Section 5.3. – Tailings and/or water release due to landslide/rockfall into the storage causing a wave of tailings/water to overtop the pit crest, resulting in damage to the environment, property and potential loss of life. This risk is addressed in Section 5.5.1.
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