EGL Ref: 9049 23 June 2022 Page vi File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. o Specific geotechnical and groundwater investigations to assist in determining the ground conditions and groundwater profile between the decline and closest properties. o Monitoring of groundwater levels and ground surface settlement to detect possible effects. o Development of Trigger Action and Response Plans (TARPS) that set triggers and specific actions including mitigation measures if trigger levels are exceeded. o Mitigation of groundwater inflows into the tunnel decline (as required) during construction with grouting methods. o Mitigation of shallow groundwater drawdown by the injection of captured groundwater back into the ground. Wharekirauponga Access Tunnel development mitigations and recommendations • EGL recommends that the Wharekirauponga Access Tunnel east of Waihi East is restricted to a minimum tunnel depth of 130 m, or 40 m (Ref. 7) minimum vertical offset from the top of the andesite/younger volcanic interface, whichever is deeper. The minimum depth reduces the risk of tilt at the ground surface and the minimum vertical offset reduces the risk of dewatering the younger volcanics which underlie Waihi East. This was a condition of the Correnso Underground mine which is directly beneath Waihi East that has been shown to be successful. • 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.
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