June 2022 G-14642.71-006-R-Rev0_FINAL 26 7.0 MAIN TWIN ACCESS TUNNELS 7.1 Geotechnical Investigations Geotechnical investigations focussed on the main twin access tunnels have included fully cored drill holes at each end of the alignment. Lack of land access has prevented further geotechnical investigations along the alignment. Interpretation of ground conditions based on regional geological mapping information is presented in Section 7.1.1. 7.1.1 Regional geological model A conceptual geological model (GHD 2020 presented as Appendix B) summarises the findings of regional geological mapping from public sources and from OGNZL exploration mapping. The model identifies several volcanic formations that the twin tunnel alignment will encounter and infers the distribution along the alignment. However, given the lack of subsurface information, this model should be considered to have low precision and lacks geotechnical characterisation of the materials. The main twin access tunnels between Ventilation Shaft 1 and WUG have the following characteristics, with the distribution of geological units interpreted by GHD (2020): Total length of about 5500 m between Ch 1300 m and Ch 6800 m, with a heading direction of approximately 335°. Cover thickness above the twin tunnels is locally up to nearly 500 m, with a minimum cover of 120 m beneath Waiharakeke Stream at approximate Ch 5200 m. The cover beneath Mataura Stream and is about 225 m. The twin tunnels are expected to encounter Whitiroa Andesite from Ch 1300 m to approximately Ch 4400 m where a gently north dipping contact with overlying Waipupu Formation is inferred. The position of the contact between Waipupu Formation and the host rhyolite dome is unknown but inferred to be close to the orebody. Waiharakeke Dacite locally overlies Whiritoa Andesite and Waipupu Formation between Ch 2800 m and Ch 4200 m, several hundred metres above the tunnels. The twin tunnels are inferred to intersect the ENE trending Waiharakeke Fault at about Ch 5100 m, where there is about 250 m of cover. The Waiharakeke Fault is interpreted to be a north-dipping normal fault, with an associated zone of alteration. A zone of silicification is inferred between Chainage 6500 m and the end of the tunnel. 7.1.2 Subsurface investigations WNDD011 was a 700 m long fully cored borehole drilled from Willows Road Farm close to Mataura Stream and approximately parallel to the twin access tunnels (bearing of 301° and an inclination of 20° below horizontal). The position of WNDD011 is shown in Figure 5 and the log provides an indication of the expected ground conditions in this area. As described in Table 8, WNDD011 encountered alluvium and residual volcanic soils for the first 26 m beneath the Mataura Stream, then intervals of strong andesite, weak clay altered andesite and minor weak andesite breccia of the Waipupu Formation.
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