GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 36 Figure 21 Pit crest stability section (shown in red) 5.3.2 Material parameters The material parameters adopted for the analysis are summarised in Table 7. The material parameters adopted for the rockfill have been derived from published correlations by Leps (1970). The following assumptions were made in deriving the materials: – Coefficient of uniformity (Cu) is greater than 4. – Target relative density of greater than 60% is achieved. – Given the friction angle of rockfill generally decreases with increasing normal stress, a maximum normal stress of approximately 700 kPa (assuming 40 m maximum depth of rockfill) was used when deriving the friction angle. This is considered a conservative assumption, particularly for the liner sub-grade, which will be under low stress during the critical case (i.e., with no tailings). – Average rockfill assumed for the backfill as defined by Leps (1970). This assumes that overburden is selectively placed as backfill on the side slopes and highly weathered material with low strength is rejected or used in the base of the pit. Parameters for the parent rock were based on strengths derived by PSM (2021) for Andesite Class 3 (refer Section 2.3.2). Conservative parameters were adopted for the shallow surface materials when assessing the stability of the pit rim. Table 7 Parameters adopted for the stability analysis Material Unit weight (kN/m3) Friction angle, () Cohesion, c (kPa) Parent rock (pit walls) 22.5 50 140 Ignimbrite 22.5 25 40 Backfill 20 42 0 Liner sub-grade 20 42 0 5.3.3 Summary of results 5.3.3.1 Backfill The infinite slope assessment was completed to confirm the stability of the thin, outer-facing liner sub-grade material. For an infinite slope, the factor of safety (FoS) is equal to:
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