June 2022 G-14642.71-006-R-Rev0_FINAL 16 6.2.2 Groundwater A groundwater assessment completed for the project (GWS 2020) indicates that groundwater flows are expected to be concentrated within the shallow regolith soils, which are underlain by lower permeability clay soils. 6.2.3 Key geotechnical issues The Tunnel Portal will be accessed by a road from the mine infrastructure area that will follow the eastern margin of the WRS. Construction of the access road will require significant excavation to create a road with appropriate gradient and a stable road formation. It is anticipated that the slope around the portal will require local support by a suitable retaining structure, potentially a gravity wall or soil nail/rock bolt reinforced mesh and shotcrete. Consideration will need to be given to potential slope instability above the portal and the provision of a containment or catch structure to manage the risk associated with landslide impact or blockage. 6.3 Ventilation Shaft 1 A 250 m deep vertical ventilation shaft (‘Ventilation Shaft 1’) is planned to be located on the Willows Road Farm property immediately to the south of the DoC boundary along the tunnel alignment. Geotechnical drill hole WNDD007 was advanced to a total depth of 231.7 m near the proposed vent shaft site in August 2020. Drill hole photos and logs have been prepared by GHD and the geotechnical conditions summarised in Table 4. Table 4: Summary of geotechnical conditions from log of WNDD007 (Shaft #1 site). Drillhole interval (m) Geological description Geotechnical characteristics Comments 0 – 7.2 Sandy SILT Low plasticity Pedolith (residual Waipupu Fm) 7.2 – 32.0 Porphyritic ANDESITE Highly weathered, weak – very weak rock 32.0 – 42.0 Porphyritic ANDESITE Moderately weathered, moderately strong rock Multiple crush zones 42.0 – 66.7 Porphyritic ANDESITE Highly weathered, weak rock 66.7 – 78.2 Porphyritic ANDESITE Moderately weathered, very strong rock 78.2 – 129.5 Autoclasticbreccia ANDESITE Moderately weathered, moderately strong rock 102.8 – 104.1 m Heavy clay alteration
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