GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 30 4.7 Tailings management strategy 4.7.1 Overview Tailings will be deposited via three deposition pipelines down the side of the pit. The rate of rise of the tailings will be approximately 8 m per year, assuming only a fraction of the Waihi tailings stream is directed at the GOP TSF (typically ranges between 20% and 50% of total tailings stream). Depositing a smaller fraction of the tailings is recommended to limit the rate of rise and improve consolidation and reduce post closure settlement. Given the relatively high rate of rise (e.g. compared to the site’s other TSFs), tailings deposition will be primarily subaqueous, meaning the settled tailings density is expected to be low and the post-filling settlements high. As the tailings settle, additional tailings can be deposited to “top up” the tailings level and reduce the final cover thickness. Regular density reconciliations through completion of bathymetric surveys will be important to confirm the design assumptions for rate of rise, as this will control the settled density and post-deposition settlement. 4.7.2 Tailings pipeline The tailings deposition pipeline will consist of three slotted HDPE spigots running down the side of the lined pit wall, one each in the north, east and west. Each spigot will be anchored at the top of the pit and supported at the base of the pit using a concrete cradle. The concrete cradle will be installed prior to lining. The purpose of the concrete cradle is to allow some movement and thermal expansion of the spigot pipe. A collar will be installed on the end of the pipeline as a ‘stopper’ to prevent the spigot floating free from the concrete cradle as the pit fills with tailings and supernatant water. The slotted pipe allows tailings to progressively flow from higher slots as the tailings level comes up and buries the spigot. Installation of the tailings pipeline will be in two stages due to limited access to the base of the pit to install the spigot into the concrete cradle. The pit will first be backfilled and lined to RL 1080 m. The spigots will then be carefully lowered down the lined face using a crane and guided into place by personnel who have rappelled to the base. The top of the spigots will be anchored at RL 1080 m, with the tailings pipeline running up the pit haul road to the process plant. The tailings will then be deposited up to RL 1080 m. Stage 2 will then line the pit to RL 1107 m before installing the spigots in a similar way to the first stage and then depositing tailings up to RL 1103 m. Alternatively, to avoid the need to rappel down the liner and to improve access to the base of the pit, a light vehicle access ramp may be built into the backfill surface, as sketched in Figure 19. This will allow access by a light vehicle or a small crane to the floor of the pit to assist with installing the tailings pipeline. Alternatively, the tailings pipeline could be installed along the access road rather than down the face of the pit. The inclusion of a light vehicle access ramp will also improve access for any erosion repair to the backfill surface that may be required.
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