GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 35 (approximately 58 m3/day) and deep groundwater (approximately 1,194 m3/day), upwelling from the deep groundwater system. On complete closure of the GOP TSF, when the GOP TSF drainage system is no longer operated (GOP TSF closure scenario in GHD (2021a)), groundwater discharge from the GOP TSF (approximately 50 m3/day) is estimated to be predominantly towards the Ohinemuri River (OH6). This dominant flow path results from the presence of the relatively permeable young volcanics on the western pit face at an elevation that allows contact with the saturated waste rock. Water being discharged from the GOP TSF is modelled to be primarily groundwater upwelling through the waste rock, and rainwater infiltrating the cover and migrating above the tailings towards the pit rim. Only a relatively small proportion of water is expected to be influenced by tailings (approximately 10%), due to the low permeability of consolidated tailings relative to emplaced waste rock. 5.2 Surface water 5.2.1 Water balance The GHD Water Balance Model (WBM) (outlined in GHD 2021b) incorporates both GOP sump water (during development) and tailings pond water (during filling). The model assumes that both sump and decant water is pumped to the nearby water treatment plant (WTP) for processing before discharge into the Ohinemuri. The WBM estimates that sufficient capacity within the WTP and discharge (subject to various upgrades and consent variations) together with adaptive management procedures such as prioritisation of decant pumping will allow the GOP TSF to operate within the assumed normal operating water levels with allowance for the appropriate freeboard and contingency storage volume. 5.2.2 Surface water diversion drains During construction, management of surface water run-off will be important to minimise damage (erosion) of the liner sub-grade and interference with installation of the liner. A perimeter drain will be constructed on the bench above the construction front to intercept run-off and divert away from the work front and safely convey down the pit wall to the base of the pit to a sump for recovery. The perimeter drainage network will be designed during the detailed design phase. 5.3 Stability 5.3.1 Analysis overview Stability analyses were completed for the following aspects of the GOP TSF: 1. Stability assessment of the constructed backfill surface including: a. An infinite slope assessment of the thin, outer-facing liner sub-grade material. b. A global stability assessment of the backfill using the two-dimensional, limit-equilibrium based software package Rocscience Slide. 2. Stability assessment of the portion of pit crest on the western side that may act as an embankment due to the natural topography falling away (Figure 21). The assessment of the pit rim was completed using the twodimensional, limit-equilibrium based software package Rocscience Slide. The stability analyses were based on generally assumed parameters for rockfill and natural ground, with some guidance on the pit wall geotechnical parameters provided by the values recommended by PSM (2021). Correlations from literature were used to develop parameters for the backfill material.
RkJQdWJsaXNoZXIy MjE2NDg3