Phillip Jones Reference No. G-01483.84-008-LR-Rev0_FINAL OceanaGold Corporation 17 June 2022 2 − A 130 m deep return air raise (RAR – Shaft 5) to exhaust air from the exploration tunnels. Shaft sizes are anticipated to be in the range of 3 m to 5.5 m diameter; however, for this preliminary design exercise we have assumed all will have a diameter of 5.5 m (Sketch 1 and 2, 3 Attachment B). Owing to the lack of surface access and the rock mass conditions, the shafts are anticipated to be constructed using a bottom-up technique from the mine, such as an Alimak raise, with support being installed progressively. Ground Conditions Geotechnical drilling investigations have been completed at the Willows Road farm site, including the proposed location of Ventilation Shaft 1, and extensive exploration drilling has undertaken at the WKP orebody. However, there is currently no subsurface geotechnical information available from the proposed sites of the four shafts described above within the DOC reserve. However, based on a high-level geological model (GHD 2020) summarised from publicly available data, airborne geophysical investigations and surface mapping completed by OGC, the ground conditions for the shafts are expected to comprise: Late Miocene to Pliocene volcanic rocks including andesite and rhyolite. The rocks are anticipated to comprise gently inclined layers metres to tens of metres in thickness. The rock mass quality is anticipated to vary from very weak to strong rock, including andesitic lava flows, airfall tuffs and breccias and potentially thin sedimentary layers. Groundwater is anticipated to be within ten to twenty metres of the ground surface and the rock mass permeability is likely to vary from low to high. However, in the vicinity of the orebody and future underground mine, the groundwater may be effectively drained. Shaft Closure Options There are many factors that influence the optimal choice of shaft closure that is required at any of the five shaft sites, including accessibility, topography, surface water runoff and subsurface ground conditions, including potentially aggressive soils. We have prepared sketches of various options for shaft capping, including reinforced concrete or a prefabricated structural element and level ground or sloping ground situations and a sketch of bottom-up placement of cemented backfill. Detailed design of the shaft capping for each location will need to take into account the above factors, final size of the shaft opening and the presence of operational elements such as egress structures, ventilation equipment, utility ducting, etc. However, the general design features of the shaft caps will comprise the features presented in the design sketches.
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