EGL Ref: 9049 23 June 2022 Page 29 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. have no adverse effect on the farmland and forest area in which they would be located. Any local depression around the ventilation shaft will only affect very local surface water flows around the shaft and this can be managed with minor ground profiling, so surface water does not pond against the shaft. 6.1.7. Dewatering due to the development of WUG At WUG the drawdown of the deep groundwater will follow the underground mining development. The proposed mine development drives reach approximately RL850m (Waihi Mine Datum) at the East Graben Vein. The ground level above the East Graben Vein is between RL1260m and RL1120m. This is between 410 m and 270 m of potential drawdown. An estimate of settlement for 380 m of drawdown was undertaken for an assumed geological ground profile of rhyolitic pyroclastics, rhyolite flows, and andesite flows (see Appendix A). The model was extended to a depth twice the depth of the proposed underground and the stiffness parameters used for the pyroclastics were assumed to be similar to the younger volcanic profile east of Waihi and given a Young’s modulus of 1,000MPa. The stiffness of the rhyolite and andesite flows was assumed to be 8,700MPa, which is similar to the unweathered andesite at Waihi. The calculated settlement is 700 mm. This calculation is undertaken to provide and understanding of the order of magnitude of possible settlement. Actual settlements will vary across the site. For comparison, the maximum drawdown in Waihi in 2021 was around 450 m and the maximum settlement near to Martha Open Pit was 300 mm. However, this area had been subject to historic dewatering so the materials are already pre-consolidated. Further, the groundwater in the younger volcanic materials beneath Waihi East is only partially drawn down or depressurised, whereas for the calculation at WUG full drawdown has been assumed. The Favona Underground area was subject to dewatering on the order of 350 m. The total settlement at Favona is around 300 mm. Favona is less likely to have had previous dewatering consolidation, however, the younger volcanics are less thick and not fully dewatered. Based on the general order of magnitude of the settlement experience in Waihi, and the calculation undertaken and presented in Appendix A, for this assessment of effects we estimate settlements up to between 300 mm to 1,000 mm could be expected in the area of the WUG site. Due to the hilly terrain at the WUG site, ground settlements in the order of 300 to 1,000 mm are not expected to have any material effect on stream flows or the forest environment. The flattest stream grade is approximately 1 in 30 over a 500 m length at the north end of the East Graben Vein, based on available topographic information. For a maximum settlement of 1m, occurring over a distance in the order of 200 m (i.e. estimated minimum distance from maximum to no settlement, based on the dewatering depths), this would result in a shallowing or steepening of grade in the order of 1 in 200. For a stream at 1 in 30, a 1 in 200 grade change is very minor. A 1 in 200 tilt is also unnoticeable in a forested environment.
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