Rory McNeill Reference No. 20148384_7407-012-LR-RevA_DRAFT OceanaGold Corporation 08 September 2021 7 unconfined compressive strength of 11 MPa to 49 MPa, which is consistent with a weak to moderately strong 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. The biggest uncertainties in the geotechnical conditions along the proposed tunnel alignments will be around the geotechnical characteristics of the ignimbrite (pyroclastic material) and how much of the alignment will encounter that material. The most obvious risks relate to the potentially very low strength of this material and associated need for heavy support and the potential for high groundwater inflows. The geological conditions described above will likely lead to mixed face conditions in some zones along the tunnel. This will occur where the tunnel face transitions between different volcanic rock units bounded by subhorizontal contacts. The position of contacts that could lead to mixed face conditions are currently unknown. The tunnelling methodology and chosen alignment will need to take into account the potential for variable strength materials and mixed face conditions. Design of tunnel support is beyond the scope of this assessment. However, consideration will need to be given to the potentially low strength of the ignimbrites at shallow depth. It is anticipated that tunnel support will mainly comprise pattern rocks 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 and longer stretches of tunnel can be excavated before support is required. The estimated groundwater inflows are outside the scope of this assessment. We envision that the ignimbrites may generate high groundwater inflows because these materials can be highly porous. We anticipate that high permeability zones may be locally present within fault zones and on some subhorizontal to gently inclined layers of the suite of volcanic rocks. 6.0 SUGGESTED INVESTIGATIONS TO ADDRESS UNCERTAINTIES Given the lack of geotechnical subsurface information (strength, stiffness, jointing, abrasivity, geochemisty etc.) along much of the proposed tunnel alignments, targeted subsurface investigations, such as boreholes is considered advisable. A programme of laboratory testing would accompany the drilling to characterise geomechanical properties of the encountered materials. The area close to the portal has been determined to comprise andesite at shallow depth based on the investigations completed by EGL; however, some geotechnical characterisation of the andesite at the portal site would be worthwhile as previous work has not assessed the viability of this site as a portal. We suggest some drillholes should target the ignimbrite between the portal and Walmsley Stream, focussed on characterising the geotechnical characteristics of the ignimbrite. These holes would be less than 200 m deep and should include in situ testing to characterise the strength and falling head tests to measure permeability. Samples should also be taken for laboratory strength testing and material characterisation. These holes would also aim to characterise the underlying andesite. It would be useful to complete drill hole investigations around the Waihi Fault, as the current ground conditions in this area are relatively unknown and would be important to help determine the expected tunnelling conditions and support required. Borehole investigations of the Waihi Fault should be located near to the D R A F T
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