7 Stage 2 will largely involve the progressive construction of the TSF3 impoundment area and embankment upstream toe. Initially, a clean water diversion will be constructed around the Stage 2 TSF3 impoundment area. Multiple stages and alterations of the clean water diversions will be required as the impoundment area increases in size. SRP-TSF3-07 will be constructed below the impoundment area to capture runoff during this stage of work. The SRP has been sized to cater for a maximum catchment area of 7.5ha which is shown on ESCP-TSF3-00202. This SRP has been designed with a total volume of 2,250m3, 3% of the contributing catchment area. Dual manholes and outlet pipes will need to be incorporated into the SRP, as well as five T-bars to maintain the recommended discharge rate of 3L/sec/ha. Additional weight will be required to be placed in the base of the manholes to keep them securely in place in the event that the pond fills with water. The manholes will also be built into the pond’s internal embankment if possible. It is anticipated that with the construction of the embankment and progressive lining of the impoundment area with a geomembrane, a 3% storage volume will be sufficient. The embankment will be progressively constructed for the upstream toe forming a large impoundment volume. The embankment crest will be constructed to an RL of 116.6. This will construct an embankment downstream height of 3.6m and an upstream height of 3.1m. The calculated storage volume to RL 116.6 is 19,120m3. Any Figure 2: TSF3 embankment upstream toe (modified from drawing WAI-983-080-DWG-CI-0517, Rev D, prepared by Engineering Geology Limited). Once the embankment is constructed (ESCP-TSF3-002-03) water impounded against the embankment will be pumped to the SRP. This SRP will then only capture overland flow during rain events from the downslope batter of the embankment. The tailings impoundment is to be fully lined with an earth liner. Additionally, a 1.5mm HDPE geomembrane liner is proposed within the tailings impoundment and up to the initial starter embankment height (RL135) to further minimise tailings seepage. During this stage the geomembrane will be progressively installed. Embankment constructed upslope of SRP-TSF3-07 creating impoundment area.
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