GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 9 2.2.2 Landslides near Gladstone pit. Two landslide features have been observed by OGNZL staff adjacent to the GOP (Figure 3). These features were investigated by Engineering Geology Ltd (EGL) and documented in a memorandum (EGL 2021). The observations from the investigation are summarised as follows: – A large active landslide, with a well-defined geometry, is evident on the western side of Gladstone Hill. The slide is approximately 60 m high and the movement is inferred to be translational. EGL noted that the failure is assumed to be occurring either within the surficial volcanic ash units or rhyolite tuff. The failure is attributed to low shear strength of these materials, natural slope of the ground (initial static shear stress) and the transient generation of pore water pressure after intense rainfall. – A second landslide is observed on the north eastern side of Gladstone Hill. The landslide is 35 m high with translational movement observed. The mechanism of failure is similar to the western landslide. Surface evidence indicated the landslide is not active. Figure 3 Landslides and other surficial features at the GOP (EGL 2021) The implications of EGL’s findings, regarding the GOP mining and TSF operations, are as follows: – Mining of the GOP will reduce the height of the slope of the western side of Gladstone Hill. This would result in a decrease in the failure mobilising forces, and thus, an improvement in the stability of the landslide. The entire north eastern landslide is removed by the mining of GOP. – The operation of GOP as a TSF may rewet the toe of the existing landslide, with the potential to cause further instability. This risk is considered negligible given the use of a liner and underdrainage in the GOP TSF design. – The presence of the landslide should not affect the proposed GOP TSF. However, design work should consider the presence of the weak ground that is the cause of the landslides. This weak ground is inferred to be at a relatively shallow depth and is likely to only affect the upper reaches of the GOP. – Figure 3 also shows historical workings that intercept the pit and potentially daylight outside of the pit footprint. These historical workings will need to be identified and remediated (through plugging) prior to filling of the GOP TSF to minimise the risk of release.
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