June 2022 G-14642.71-006-R-Rev0_FINAL 36 Both Option 2 and Option 4 have similar distribution of the various lithological units present. A simplified description of the tunnel geology for Option 2 and Option 4 is as follows: The geology at the Waihi Plant Site portal consists of andesite and ignimbrite as described in Section 8.1. The tunnel alignments are inferred to encounter moderately strong to strong andesite overlain by ignimbrite and young alluvium. It is possible that the alignment will cross the contact between the andesite and ignimbrite, such as within an old alluvial channel. However, we have seen no evidence that the contact reaches as low as the proposed tunnel alignment. At about 2200 m the tunnel is expected to encounter the Waihi Fault, which may comprise a zone of highly sheared, weak ground. Many of the logs do not provide detailed geotechnical characteristics of the materials that will be encountered by the tunnel. However, relevant data is available from the Willows Road farm site to characterise the ground conditions at that end of the tunnel extension where the tunnel will likely mainly encounter andesite and associated volcanic lithologies (refer to Section 6.0). 8.5 Key Geotechnical Issues The geological information that is currently available indicates that the tunnel alignment will encounter a suite of volcanic rocks including flows, breccias or pyroclastic materials. Based on the available drill core reviewed as part of this assessment, the layers appear to be in the order of tens to hundreds of metres thick and are oriented sub-horizontal or gently inclined. The strength of the material is difficult to determine based on the available drill core information provided by OGNZL near the portal. We have inferred that the andesite encountered at the Willows Connection end will be either the Whiritoa Andesite or the Waipupu Formation andesite, which are inferred to comprise geotechnically similar materials. The material in WNDD007 had a UCS of 11 MPa to 49 MPa, which is consistent with a weak to moderately strong rock. Limited information reviewed from the portal area appears to describe similar strength rock. The Waihi Fault is likely to be an east dipping normal (extensional) fault associated with local tectonic setting. Ground conditions in the vicinity of the Waihi Fault are likely to include weak materials and brecciated zones tens to hundreds of metres in width with local highly sheared clay gouge zones. Design of tunnel support is beyond the scope of this assessment. It is anticipated that tunnel support will mainly comprise pattern rock bolting and shotcrete installed as soon as practical after short excavations. Heavier support, including full shotcrete lining, with mesh and bolts will likely be required for areas of weak or highly fractured ground. As the tunnel extends deeper into the andesite it is anticipated that the tunnel support requirements will reduce. We anticipate that weak, high permeability zones may also be present within fault zones and on some subhorizontal to gently inclined layers of the suite of volcanic rocks. If ignimbrite is encountered by the tunnel, we anticipate this will comprise a weak material associated with high groundwater inflows. Further geotechnical investigations will be required to develop detailed design of the tunnel. These will need to address uncertainty in the geomechanical properties and the groundwater conditions of the materials that will affect the tunnel.
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