GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 38 Figure 23 Stability of western pit rim 5.4 Tailings consolidation and settlement 5.4.1 Analysis overview Consolidation modelling was completed to provide an estimate of post deposition settlement and void ratio profile of the deposited tailings. The magnitude of post deposition settlement and final void ratio profile will have implications on the closure of the GOP TSF, including: – Unsafe/impossible to traffic the soft tailings surface for construction of a water-shedding cover resulting in delay in closing the TSF. – Excessive quantities of capping material required to account for large settlement while still maintaining the designed closure profile. The consolidation modelling approach proposed by Li et al. (2012) was adopted to model the consolidation behaviour of the tailings deposited in GOP TSF. This modelling approach was developed specifically for tailings deposition with high rate of rise, where large settlements are anticipated under the self-weight of the tailings. For the analysis, the following key assumptions were made: – The pit will be lined. Therefore, it is reasonably assumed that only one-way drainage to the top surface will occur and tailings will consolidate under a one-dimensional condition. – The scope of work does not include additional tailings laboratory testing of the WUG tailings, the consolidation properties were assumed based on the test data provided by OGNZL. – The additional water on top of the subaqueous tailings would have negligible effect on the consolidation of the tailings during post deposition periods. – A rate of rise of 7.82 m/year was adopted, which is equivalent to the GOP TSF filling in 5.5 years. An update of the consolidation modelling will be completed at the detailed design phase to confirm results based on updated laboratory testing (slurry consolidation and permeability tests) of the WUG tailings. 5.4.2 Summary of results The initial consolidation analysis was carried out to estimate the degree of consolidation as the pit is filled with tailings up to RL 1103.0 m. The average degree of consolidation was estimated to be approximately 84% after five and a half years of operation (assumed end of deposition). The consolidation analysis for post-deposition periods was carried out to estimate how long it would take for final consolidation settlement to occur. The estimated post-deposition settlement with time is shown in Figure 24. Based on the consolidation analysis, the GOP TSF would take approximately 40 years to achieve the final consolidation of the tailings from the time of final deposition. The estimated final settlement is approximately 2.8 m. However, approximately 75% of post-deposition settlement occurs in the first 10 years after deposition is ceased.
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