B-7 – Area 7 - Assessment of Environmental Effects 43 The TSF3 embankment will be raised in stages as required to provide for both: the receipt of overburden material mined from GOP and Martha Open Pit Phase 4 (MOP4); and the storage of tailings as necessary from the Waihi North Project and any other consented mining activities. The proposed starter embankment is 135 m RL (Figure 27). This provides a starter impoundment of approximately 21 m depth (crest RL135 to impoundment floor RL114). The embankment will be 26 m high, from crest to downstream toe. The embankment will be fully lined with an earth liner. The 135 m RL impoundment area will be lined with a 1.5 mm HDPE geomembrane underlain with a Zone A earth liner. The plan area of lining is approximately 17 ha. Benches will be constructed to anchor the geomembrane liner on the slopes as constructed. Tailings underdrains could be installed on top of this liner, however they would not be opened until a suitable thickness of tails has covered the drains. The starter impoundment will provide approximately 700,000 m3 of tails storage allowing for the Probable Maximum Precipitation (“PMP”) with 1 m freeboard. Assuming an initial density of 1.0 t/m3 due to rapid discharge and shallow tails depth, the starter tails storage is estimated at 700,000 tonnes. The initial toe drain and key cut to rock is set for a 145 m RL embankment geometry shown in Figure 28. This provides for faster construction of crest height without a large proportion of material having to be placed over the full extent of final RL155 embankment footprint. From the RL145 profile to RL155 profile, the fill material will need to be placed to buttress the downstream slope out to the final downstream toe position (set for a potential 177 m RL embankment). This results in an overall flatter profile in the embankment downstream slope profile. This profile allows for management of a range of material strengths within the embankment profile. The TSF3 embankment will be constructed with low permeability liners and capping to limit oxygen and water ingress to potentially acid-forming (“PAF”) used in the embankment construction. Limestone may also be placed in the embankment to minimise any acid generation potential. This approach has been successful for TSF1A and TSF2. Some externally sourced aggregate and other construction materials will be required to supplement the construction of TSF3. This includes materials such as drainage metal, sand, roading metal, rockfill lining, concrete, drains, sumps, piping, and geomembrane. This material will be transported to the site via Baxter Road, and it is estimated that approximately 100 to 130 truck and 1100 to 1300 truck and trailer loads will be required over the life of the project.
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