EGL Ref: 9215 23 June 2022 Page 55 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. Storage 3 TSF. With Storage 1A it forms a valley which an embankment can be constructed across. This is the most efficient site for a downstream embankment dam within proximity of the existing mine infrastructure and the Process Plant. The Storage 3 design encroaches into a SNA. 9.2. Rock disposal options 9.2.1. Underground backfill Backfill of the underground stopes is currently done and will continue, however this does not provide anywhere enough storage for the scheduled rock production. 9.2.2. Pit backfill Backfill of Martha Open Pit or Gladstone Open Pit is possible but depends on sequencing of the pit excavations and timing of closure of the mines. The pits could be backfilled below the edge of the existing ground surface to allow surface water to saturate all the backfill rock to prevent ARD. Alternatively, if material is placed to a higher level above the ground water level capping and seepage collection could be provided to limit ARD from PAF rock as is done for the existing TSF embankments. Constraints associated with placing rock in Martha Open Pit include health and safety risks for staff and sterilisation of future ore resources that could be mined underground or by a future pit expansion. The preferred options for tailings and waste rock disposal require all available overburden for constructing new TSFs (GOP and Storage 3) and the option of backfilling either of these pits does not meet project scheduling requirements. 9.2.3. TSF embankments Embankments that form the TSFs require large volumes of fill that can be provided by the mine pit overburden material. The TSF embankments have been and will continue to be the principal locations for disposal of mine pit overburden material associated with the Martha Project. The embankments are zoned to allow for the safe long-term storage of PAF rock. 9.2.4. TSF capping A low-profile mound of rockfill could be placed over the tailings to provide a dry capping. This would differ from the current closure concept of a perimeter capping with wetland and pond. A disadvantage with a dry cap is that runoff flows from extreme flood events are much higher than if a pond is part of the closure strategy. The presence of a pond allows for significant attenuation of flood flows. 9.2.5. Existing stockpiles There are three existing rock stockpiles at the Development Site (Northern, Central and East) for storage of mine pit overburden material.
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