EGL Ref: 9049 23 June 2022 Page 33 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. 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 decline (as required) during construction with grouting methods. o Mitigation of shallow groundwater drawdown by re-injection of captured groundwater back into the ground. Wharekirauponga Access Tunnel decline development mitigations and recommendations • The Wharekirauponga Access Tunnel decline from the WUG portal to Barry Road could potentially dewater the shallow younger volcanics (ignimbrites and tuffs) and cause settlement effects at Boyd Road and eastern end of Barry Road. EGL recommends that OGNZL mitigate this risk through: o Detailed assessment of the decline tunnel alignment to confirm options and risks. ▪ Avoidance of the younger volcanics may be possible through refinement of decline alignment and level so to stay completely within the andesite. 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.
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