7.6.1 Lining system
A liner is required on the upstream batter of the embankment to prevent seepage through the embankment. A geomembrane liner will be required as no suitable low permeability clayey material is to be found on the mine site. The liner bedding will need to be selectively borrowed from finer mine waste, but as it may be impractical to remove coarse particles, a more robust bituminous geomembrane liner (BGM, Coletanche ES4) is proposed. The BGM will need to be extended 10 m beyond the abutments to mitigate seepage past the liner and embankment. A geotechnical investigation will need to be undertaken to assess the permeability of the natural ground and existing mine spoil dump to confirm the lining requirements and extents. Anchoring requirements for the BGM on the embankment and abutments will need to be to be assessed to minimise potential seepage underneath the liner system. As low permeability clay material required to suitably backfill the anchor trenches is expected to be in limited supply onsite, other options, such as a flowable sand fill, may need to be considered. Highly permeable materials may be removed from the basin foundation, where this material is shallow. A cutoff trench or similar may be required where deeper horizons of permeable materials are encountered.
7.6.2 Instrumentation
Piezometers or monitoring bores will be installed within the embankment and foundation to monitor seepage.
7.7 Spillway
An operational spillway will not be provided for Stage 1, as this would require significant excavations to be undertaken into the reservoir perimeter. Sufficient water storage has been allowed for in Stage 1 to store the PMP event, as indicated in Section 6, which is in accordance with ANCOLD [10] requirements as follows: A spillway should always be provided – even where "no spill" design requirements are in force, to guard against overtopping in extreme, or unexpected, events that could lead to failure of the dam embankment. The only exception to this guideline is where extreme flood storage (storage specifically provided to contain an extreme event) is provided for within the structure and that overtopping is inconceivable. A spillway will be required for Stage 2 to enable the design event to be passed with further studies required to assess suitable locations to discharge flows. The spillway design would be undertaken based on geotechnical investigations (refer Section 7.8) and as part of the Upper Mundic Gully TSF detailed design.
7.8 Geotechnical investigations
Geotechnical investigation should be conducted across the floor and sides of the gully and the mine spoil dump to:
— Identify geohazards and site features (e.g. faulting, shear zones, preferential seepage paths) — Assess foundation lithology and material characterisation — Enable development of a geological ground model — Assess potential for seepage beneath the TSF embankment and through the abutments — Calibrate the stability and seepage modelling to be undertaken as part of the detailed design of the TSF — Assess lining and erosion protection requirements for the spillway excavation.
The investigation should include the following:
— Geological mapping — Test pitting to confirm the depth of weathered rock – expected to be near the surface of the gully
Project No PS213278 Upper Mundic Gully TSF Concept Design to RL 390 m Mount Morgan Project Heritage Minerals
WSP October 2025 Page 15
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