SGME T-02 Memorandum
Conduct a soil investigation to determine if the tailings material presents any hazards to human health or ecological receptors and inform the cover design; Conduct seepage and groundwater assessments to determine if the structure is potentially polluting to the receiving environment; Installation of a cover over the tailings to prevent tailings erosion and to reduce potential for capillary rise; Installation of a closure spillway capable of conveying runoff generated by the PMP event; and Verifying that the nominal factor of safety for long-term drained (steady state seepage at normal operating level) from ANCOLD has been achieved. 1.3.3 Landform Design The geometry of the TSFs, being a valley impoundment that will ultimately fill the natural depression in the ground, does not lend itself to a cellular TSF design. Given the staged approach to TSF construction and considering that Heritage Mineral are authorised currently for one TSF, there are limited opportunities for progressive rehabilitation of this facility. The final tailings surface of the TSF is to be developed as a shedding cover so that no water is stored on the TSF following deposition. Tailings deposition will be used to regrade the tailings beach towards the closure spillway at the end of the TSF life to facilitate runoff flows towards and over the spillway. 1.3.4 Drainage Structures at Closure Design of both TSFs has assumed that no emergency spillways will be constructed as the embankment is raised, but rather that the freeboard provided above the tailings beach will be able to accommodate the runoff generated by a PMF for each stage raise of the embankment. As the TSF will be constructed within a naturally occurring valley, the TSF will need to be established to a final RL that will allow for the conveying of water from a spillway to the surrounding environment. At the time of TMP development the final location of the closure spillways for both TSFs can not be accurately determined as the final landform height will only be determined when the staged embankment rises during operations are complete. The location of the spillway will consider: Geotechnical conditions and erosion likely to be generated by runoff from higher frequency storm events, as well as the design storm event; The water quality of spillway flows and the receiving environment; and Licensed release points. 1.3.5 Cover System Conditions of the EA require the installation of a a dry cover store and release capping system on the Sandstone Gully TSF following cessation of deposition. It is expected that this condition will also apply to Upper Mundic Gully TSF following approvals. Concept design work completed by Golder Associates (2016) on the Sandstone Gully TSF provides the following key features of the cover design: Progressive deposition during latter stages of the life of the TSF to form a water shedding surface, enabling incidental rainfall to be shed from the landform which will reduce the potential for erosion to occur; Compaction of the reshaped surface of the TSF to minimise incidental rainfall from seeping into unsaturated tailings; and Installation of crest bunds to minimise overtopping and reduce erosion potential. Application of cover systems to TSFs at the site is limited by the lack of suitable sources of material, given the widescale impact to land associated with historical mining activities. Best practice as informed from the PRCP Guideline identifies that an EA holder, over the course of operations, should plan for the conservation of soil material to facilitate rehabilitation planning. As the Mount Morgan mine is situated on a highly disturbed former mine site there has been limited opportunities for the State and previous operators to stockpile topsoil and
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