7
Concept design
The Upper Mundic Gully TSF concept design drawings are presented in Appendix E.
7.1 Site access
Current access to the Upper Mundic Gully area is through a narrow dirt light vehicle track. The design of an access road suitable for construction equipment and a traffic management plan will be required to be included in detailed design.
7.2 Foundation preparation
Foundation clearing and grubbing of the following areas will be required to prepare the Upper Mundic Gully TSF footprint, including: — Embankment abutments and footprint to provide a suitable tie in surface and a competent liner placement surface — Basin of Upper Mundic Gully TSF to identify and remove unsuitable material, including soft spots and permeable zones that may transfer water under the liner, and provide a suitable surface for liner placement.
7.3 Upper Mundic Gully rock-slope stability
The Upper Mundic Gully basin is underlain by Arenite and Diorite, as discussed in Section 2.1, and the relatively steep terrain is indicative of competent rock within the basis slopes, noting that high strength igneous rock is observed in outcropping (refer Section 3 and Appendix B). Rim instability is not expected to present a significant issue noting observed geological conditions and that tailings will add weight to the basin slopes as it is deposited, however, a geohazard assessment can be conducted as part of geotechnical investigations (refer Section 7.8). The decant pond is to be managed to a limit volume, which will reduce potential for an elevated phreatic surface and drawdown to occur simultaneously. The upgradient landform and catchment at the final Stage 2 crest elevation (RL 390 m) is limited and therefore unlikely to generate significant sediment flows, however, any potential erosion sediment loss which occurs during operation would be captured by the TSF. Rim and basin instability and erosion risks during construction mitigation would be managed using standard construction erosion and sediment controls developed prior to construction.
7.4 Tunnel plug
There is a stormwater tunnel inlet, located below and through the waste rock dump, immediately downstream of the embankment. This will need to be plugged prior to embankment construction to mitigate seepage from the TSF flowing though the tunnel. A tunnel portal plug will be inserted, which will comprise a 1.5 m concrete plug located in competent rock, approximately 5 m past the tunnel inlet. Mine spoil should then be pushed into the tunnel to backfill the inlet to support the embankment (DWG 006 in Appendix E).
7.5 Embankment staging
The Upper Mundic Gully TSF is proposed to be constructed in two stages:
— Stage 1 – Reshape of existing mine spoil dump and construct starter embankment (RL 360 m) — Stage 2 – Raise embankment across top of spoil dump to top of gully ridgeline (RL 390 m).
Project No PS213278 Upper Mundic Gully TSF Concept Design to RL 390 m Mount Morgan Project Heritage Minerals
WSP October 2025 Page 13
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