B-3 – Area 3 - Assessment of Environmental Effects 25 The currently available geological information indicates that the tunnel alignment will encounter a suite of different volcanic rock. 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 meters thick and are oriented sub-horizontal or gently inclined. The assessment identifies that it is likely that the material encountered will be either the Whiritoa Andesite or the Waipupu Formation Andesite, which are inferred to comprise geotechnically similar materials. Andesite rock material has low permeability so in such areas groundwater and surface water inflows are not anticipated to be significant. A portion of the tunnel is expected to encounter the Waihi Fault. 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. Groundwater inflows into the tunnel have the potential to be high in fault zones or within brecciated zones and will require management in such locations. This will be done using standard management techniques applied to this type of tunnelling activity. 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. 5.3 GEOCHEMISTRY AND ACID ROCK DRAINAGE An assessment of geochemistry matters relating to the Waihi North Project is provided in AECOM (2022), a copy of which is provided in Part H of these application documents. An overview of that assessment as it relates to Area 3 is provided below. Geochemical modelling confirms that in general, the andesite material which is likely to be encountered during the development of the Wharekirauponga Access Tunnel is likely to be elevated in mercury, antimony and arsenic. OGNZL has utilised a combination of methods to effectively manage this type of spoil at its Waihi operations to date, and it is proposed that management of rock sourced from the Wharekirauponga Access Tunnel will be similar and integrate into that existing management regime. These methods include (but are not limited to): Oxidation control – Control of oxygen flux to reactive sulphides, such as by deposition under water or through the application of low permeability layers; Geochemical control - Blending rock types or addition of neutralising materials to control pH and oxidation rates; and
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