June 2022 G-14642.71-006-R-Rev0_FINAL 30 7.1.3.2 Groundwater GWS (2020) indicates that average rock mass permeability is relatively low (2.5 x 10-8 m/s) based on preliminary investigations. Higher permeability zones are anticipated to be present within fault zones and some subhorizontal to gently inclined volcanic layers. Consequently, minor groundwater inflows are expected in most of the tunnel length, with high inflows in localised zones. Mitigation of high inflows will be by grouting and lining, although estimating the length of treatment is not possible due to the lack of exploratory information. Inflows in low inflow zones will be collected at sumps and pumped to the surface for treatment at the existing water treatment plant. It is anticipated that exploratory drilling ahead of the tunnel face will be required to identify inflow rates along the alignment. Consideration should be given to potential elevated groundwater temperatures affecting the underground works. Hydrothermal alteration has widely affected the rock mass at the site, including placement of the WUG orebody. We are not aware of any evidence of on-going hydrothermal processes affecting the area; however, some inflows of hot groundwater are possible. We understand that high ground temperatures affected construction of the Kaimai rail tunnel. Consideration should also be given to water quality adversely affecting ground support and underground infrastructure. GHD (2022) has identified that there is a high probability of the rock mass generating acidic groundwater, which could degrade steel support. 7.1.3.3 Tunnelling conditions The geological conditions described above will likely lead to mixed face conditions in some zones along the tunnel. This will likely occur where the tunnel face transitions between different volcanic rock units bounded by subhorizontal contacts parallel to the tunnel direction. The position of contacts that could lead to mixed face conditions are currently unknown and the tunnelling methodology will need to take this into account. The detailed design stage will need to consider the generally low strength of the rock mass, high groundwater inflow and stress conditions. Squeezing ground may be present in areas of very weak ground. It is anticipated that tunnel support will mainly comprise pattern rock bolting and shotcrete installed shortly after excavation. Heavier, full shotcrete lining, with mesh and bolts will likely be required for areas of weak or highly fractured ground. Given the lack of subsurface information along much of the tunnel alignment, drilling ahead of the tunnel face as described by Mitchell Daysh 2022 will be important for predicting ground conditions ahead of the tunnel face for support and design of groundwater management, as well as providing geological exploratory information.
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