5.6 Decant Pond Deposition into the TSF will be conducted from the embankment to keep the decant pond well away from the embankment. A suitable return water pump system will be required and may comprise a floating pontoon pump or other suitable mobile system. Liner Requirements A liner is required on the upstream batter of the embankment to mitigate potential seepage through the embankment. A geomembrane liner will be required as no suitable low permeability clayey material is found on the mine site. The liner bedding will be selectively borrowed from finer mine waste, however as it is difficult to remove coarse particles, a more robust bituminous geomembrane membrane (BGM) liner (BGM, Coletanche ES4) is proposed. 5.7 The BGM will need to be extended upstream of the embankment and across the valley walls to mitigate seepage past the liner and embankment. A nominal 10mm width has been adopted at this stage, however this is subject to further investigations to confirm the lining requirements and extents.
5.8
Embankment Design and Geotechnical Considerations
The Upper Mundic Gully TSF is proposed to be constructed by reshaping the existing mine spoil dump and constructing an embankment across to top of the existing western dump to RL 370.9m AHD. The embankment will be tied into the existing western dump so that no part of the outer embankment will be exposed. Consequently, there is no requirement to rehabilitate the outer embankment of the TSF at closure as no part of the embankment will be accessible. This concept is displayed in Figure 28. During the detailed design phase, further engineering works are proposed to specify tie in details and quantify volumes of material that require removal and placement. The UMG TSF design is at concept stage and has not advanced to detailed design stage where geotechnical investigations are undertaken. If the EA amendment is approved, Heritage will be unable to construct the UMG TSF until the RPEQ certifies the issued for construction (IFC) design drawings. Undertaking the detailed design requires completion of the future works identified in Section 9 of this application, which will require geotechnical investigations. Under these conditions, and in accordance with the Manual for assessing consequence categories and hydraulic performance of structures, the RPEQ tasked with the detailed design of the UMG TSF must at that stage: ∕ confirm that the TSF is designed to prevent seepage risk and is structurally stable (including aspects of the TSF involving reshaping the Western Dump); and ∕ confirm that appropriate geotechnical investigations have been undertaken to support the design plan. Further, existing EA condition H8 provides a hold point whereby construction of the proposed UMG TSF can not occur until a certified design plan is submitted to the administering authority. The conditions states that “ Regulated structures must be constructed in accordance with a certified design plan that has been submitted to the administering authority, and such that the resulting structure will deliver the performance stated in that submitted design plan and that dam is compliant in all respects with this environmental authority ”. The design basis for the UMG TSF embankment is intended to match that of Sandstone Gully TSF whereby the use of selected rock fill, borrowed from the western dump, will be used as zone 3A material used in embankment construction. This design basis is pending further detailed design works and RPEQ confirmation. Placing selected rock from western dump into the UMG is a similar
Project number: 25B061
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