GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 37 ൌtanሺ∅ሻ/ tanሺ ሻ where is the friction angle and is the slope of the surface. This calculation assumes the liner sub-grade material remains freely draining. Given the inter-bench slope of the backfill is 1.5H:1V (approximately 33.5), the FoS was calculated to be 1.4. This is considered acceptable as the slope is temporary and the FoS will improve as the GOP TSF is filled with tailings. The global stability assessment critical slip surface is shown in Figure 22. The calculated minimum FoS of 1.8 is considered acceptable. Figure 22 Global stability results for backfill surface 5.3.3.2 Pit crest The stability of the pit crest was analysed to assess whether there was credible failure mode that justified completing a dam breach assessment. The assessment focused on the western side of the pit where the natural topography meant that the pit rim may have been considered an embankment. The pit rim was conservatively assumed to have lower strength material and had properties more representative of an extremely/completely weathered rock. Results in Figure 23 show a calculated factor of safety equal to 4.7, for the critical slip surface that impacts the crest of the backfill. While there is no guidance on assessing the credibility of failure modes compared to calculated factors of safety, the following observations are made: – The calculated factor of safety is high, and the identified slip surface appears improbable. – The portion of pit crest analysed has a relatively flat slope and the height is low compared to most traditional embankments. Therefore, instability of the western crest leading to release of containment was not considered a credible failure mode.
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