June 2022 G-14642.71-006-R-Rev0_FINAL ii Summary Preliminary geotechnical investigations were undertaken to support a prefeasibility study and consent application for the tunnelling and underground mine design of the Waihi Underground Project (WUG). The project will include the development of an underground mine targeting the EG vein and T-Stream vein that are part of the Wharekirauponga ore body. Access to the underground mine site will be achieved by tunnelling from near Waihi, and the mine will be located within a Department of Conservation (DoC) administered reserve. Three specific tunnels will be required: the main twin access tunnels from Ventilation Shaft 1 located within Willows Road Farm; the Willows Road Access Tunnel, which will access the main tunnels from a portal on Willows Road Farm, and; the Waihi Plant Access Tunnel, which will provide access to the main tunnels from the Waihi Processing Plant Site. The available information includes extensive fully cored exploration drillholes at the Wharekirauponga ore body, a number of fully cored boreholes at the Willows Road Farm site and exploration drillholes from the vicinity of Waihi. The drillholes at Willows Road Farm were specifically undertaken to provide geotechnical information to assess the tunnel and shafts in that area. However, limited geotechnical characterisation of the rock mass at the WUG orebody and twin access tunnels has been completed to date due to limited access within the DoC reserve. A 400 m deep drillhole (WKP-P05D), located on the eastern side of the WUG orebody, has provided information about the geotechnical and hydrogeological characteristics in that area. Geotechnical assessment of the Plant Access Tunnel has been based on regional geological information, exploration drilling near the proposed alignment and geotechnical investigations carried out near the plant site. Key findings of the investigations include the following: Extensive investigation drilling along the Willows Road access tunnel alignment encountered a combination of weak, highly weathered andesite or tuff and strong to very strong andesite. Given the low strength of some of the rock, significant support requirements are anticipated for about 50 % of this tunnel length. No specific design of tunnel support has been completed as part of this study. No subsurface investigations have been completed between the Mataura Stream and WKPP05D on the eastern side of the WUG orebody. Design of tunnel support and groundwater inflow control will rely on investigation drilling ahead of the tunnel face. Groundwater levels have been measured at shallow depths in the boreholes, while piezometric measurements in boreholes suggest a general downward groundwater gradient. Rock mass permeability, inferred by hydrogeological consultant GWS, is generally low (average value of 2.5 x 10-8 m/s). Zones of higher permeability are inferred in highly fractured brittle units, around faults and the contacts between volcanic rock units. Rock strength encountered in a borehole close to the location of Ventilation Shaft 1 ranges from very weak to strong. We infer that substantial support will be required and that construction by Alimak raise is likely to be the most appropriate construction method.
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