GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 25 1. Limiting the amount of time that the subgrade is exposed by incrementally lining the face as the backfill is placed (i.e., backfill to first bench, line, backfill to second bench, line, etc.). 2. Diverting run-off using bunds, drains, sumps and pumps. 3. Developing repair methods (if required) which do not require significant bulk earthworks to cutback erosion gullies or rilling such as repair with small equipment / long reach excavator for subgrade replacement with sub- 20 mm or alternate materials such as cement stabilised fill or shotcrete. A geosynthetic liner is recommended for lining the GOP TSF. The type of geosynthetic liner shall be confirmed during detailed design and the decision shall be based on the following criteria: – Impact resistance and puncture strength given the potentially rough and uneven sub-grade. – Demonstrated experience being installed at steeper slopes (i.e., steeper than 2H:1V). – Longevity of the liner. – Speed of installation. The liner will be anchored in a trench at each bench to limit the strain on the liner, given the relatively high pit face. Table 5 summarises the material quantities for the GOP TSF. Table 5 GOP TSF bulk quantities Material Description Material quantity Material source Backfill Bulk earth and rockfill (PAF or NAF); maximum 250 mm particle size. 2,490,000 m3 NRS and/or MOP4 cutback Liner subgrade Sub-20 mm crushed and screened rock produced from select high quality rock (with high permeability) from the mining operations or Martha Phase 4 pit cutback. 131,000 m3 Manufactured material from select ROM rock Liner Geosynthetic liner (e.g., bituminous geomembrane, HDPE or PVC) 119,000 m2 Liner supplier 4.4 Construction aspects Backfill and liner subgrade is proposed to be placed in horizontal layers of maximum 1 m thickness. Material will be loaded near the processing plant and hauled to the base of the pit using haul trucks. The haul trucks will dump the material where a dozer will spread into 1 m lifts. Placement of the external facing (sub-20 mm sub-grade) will be placed steeper than design to enable compaction to the edge of the lift. An excavator will be used to trim the batters to the design slope of 1.5H:1V. Compaction using rollers will be required for the external facing, liner subgrade. However, the continued passing of loaded haul trucks may provide acceptable compaction for the backfill. This will need to be confirmed by completing a trial lift prior to construction, otherwise rollers will also be required to compact the backfill. The liner will be installed incrementally as each 10 m bench of backfill is constructed. The minimum crest width of the backfill platform will be 10 m wide (at the final bench) but will generally be much wider. This will provide plenty of width for traffic and for safe construction of the liner. The general liner installation procedure is illustrated in Figure 14.
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