EGL Ref: 9049 23 June 2022 Page 34 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. • EGL recommends that specific geotechnical and hydrogeological investigations along the Wharekirauponga Access Tunnel length east of Waihi East are undertaken to confirm the geotechnical and permeability conditions, and groundwater pressure profile, and to confirm the level of the tunnel to achieve the minimum offset to the younger volcanics. This can include surface drilled investigation boreholes, drilled boreholes ahead of the tunnel and/or geophysical surveys. Surface borehole drilling mitigations and recommendations • Where possible boreholes drilled from the surface should be sited away from structures to minimise settlement risk associated with leakage of shallow groundwater to deeper systems. • Where possible boreholes should avoid intersecting underground workings. • Grouting of all future surface-drilled holes to a depth below the top of the andesite. General tunnel development mitigations and recommendations • Avoid intercepting existing drillholes with the tunnel operation wherever possible. • Avoid intercepting historical workings with the tunnel wherever practicable. • Drilling ahead of the tunnelling operation, to inspect for geological structures or areas of ground that may make notable water inflows into the tunnel. • Mitigation of sustained groundwater inflows through the use of grouting methods. Dewatering and ground settlement monitoring recommendations Monitoring of groundwater levels and ground settlements is required to check settlements are within estimates and there are no unforeseen effects. The dewatering and settlement monitoring plan will be updated for the Waihi North Project. Recommendations in regard to dewatering and ground settlement monitoring are summarised below: • Continued monitoring of dewatering and settlement as required by the expired Mining Licence and resource consent 124860. • The settlement monitoring zones and settlement trigger limits previously proposed for Project Martha are adjusted over the Favona Underground and locally around Gladstone Open Pit and Wharekirauponga Access Tunnel Alignment for the Waihi North Project. • The dewatering and settlement monitoring plan needs to be updated for the Waihi North Project. • Groundwater monitoring piezometers provide the best early warning of potential adverse ground settlement effects such as tilt. This was evident in the situation which occurred in Gladstone Road related to borehole CDG008 as described in Section 5.3.3. EGL recommends that additional piezometers are installed along with
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