EGL Ref: 9049 23 June 2022 Page 16 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. 4.5.2. Wharekirauponga Access Tunnel GWS (Ref. 36) assessed the groundwater effects of the Wharekirauponga Access Tunnel from the Processing Plant to the start of the Wharekirauponga Dual Tunnel at Willows Farm. The tunnel will cause depressurisation of the andesite where not already depressurised by underground working. Where the Wharekirauponga Access Tunnel (decline) is passing Favona Underground in the andesite the ground water is indicated by monitoring to have been drawn down. No further drawdown effects are expected over the length of the decline in andesite. Any sections of the decline which intersects the younger volcanic (ignimbrite) deposits can be expected to draw down the shallow groundwater system, which is not currently affected by the Favona Underground. OGNZL will use grouting methods at these sections of the decline to limit shallow groundwater inflow from the ignimbrites or undertake investigations to confirm any effects would not extend to reach any properties. The proposed Wharekirauponga Access Tunnel corridor east of Waihi East is expected to pass at depth through andesite rock and therefore no notable drawdown of the overlying perched groundwater system in the younger volcanics is expected. Further investigation is required to confirm the tunnel will pass Waihi East within the andesite. GWS highlighted (Ref. 36), the younger andesite is present in the mid-section of the tunnel alignment due to being in a down thrown block that is bounded by regional scale faults, and that it is expected that there will be fracture zones associated with these faults and that ground conditions will be weaker than the general andesite rockmass. Where the tunnel passes these faults zones or veins in the andesite they may already be partially drained. EGL recommends installation of piezometers to determine the groundwater situation prior to the tunnel being drilled. These piezometers will follow the standard practice for Waihi East where piezometer tips are positioned in the shallow alluvium, deeper younger volcanics, weathered andesite and deeper andesite. They are also recommended to allow interpretation of the response of the groundwater system to tunnelling. If dewatering of the andesite does not draw down the groundwater in the perched younger volcanics or shallow alluvium, then pre-grouting ahead of the tunnel can be optimised. If there is drawdown in the shallow groundwater systems, the tunnel will have to be pre-grouted with a curtain anywhere the tunnel is likely to draw considerable groundwater, which is most likely to be around faults or veins. If the weathered surface of the andesite was not present, much more extensive pre-grouting could be required, however, this is unlikely given the wider spread drawdown of andesite and monitoring to date in Waihi. 4.5.3. Willows Access Tunnel GWS (Ref. 36) assessed the groundwater effects for the decline for the Willows Access Tunnel and Willows Ventilation Shaft based on ground investigations by Golder (Ref. 38) and GHD (Ref. 28). The site has relatively steep topography sloping down to the Mataura Stream. The wells indicated at
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