Phillip Jones Reference No. G-01483.84-008-LR-Rev0_FINAL OceanaGold Corporation 17 June 2022 4 availability and suitability of the pump and confirm that the pump selected will fit in the drift development openings. A waste rock or concrete wall barricade would be required to retain the first lift of backfill, which would be poured until it is slightly above the drift brow elevation in the shaft (i.e., above the drift back elevation). This would be used to retain the first lift of backfill until it cures enough to support subsequent lifts. Subsequent lifts, or a continuous pour, would be completed upon initial curing of the first lift, typically after a minimum of 7 days of curing. Backfilling would progress until surface elevation is reached. The volume of cemented backfill required varies depending on the depth of the shaft. For a 5.5 m diameter shaft, approximately 23.7 m3 of fill per metre of shaft height is required. A sketch of this design option as a section is presented in sketch 7 in Attachment B. An example of a specialised high pressure pump in included in Attachment C. Consideration for Sloping Ground Given the general topography at the WKP site, it is anticipated that shafts may be constructed beneath sloping ground. Consideration will need to be given of the impacts of sloping ground on the design of a shaft opening and post closure capping. It is likely that some cut and fill will be required to create a level site large enough to construct surface structures needed for operation of the shaft during mining. Sketch 7 shows a concept design for a shaft opening in sloping ground. This option includes the use of precast concrete blocks to form a retained edge to the working platform. Design of the shaft opening for capping options (1&2) will also need to take into account potential for slope instability or surface water flow. Maintenance and Monitoring We assume that a design life of at least 100 years will be required to meet stakeholder expectations of mine shafts at closure. We believe a structural capping solution, such as the options illustrated in sketches 3 to 7 can be constructed to achieve a 100-year design life using conventional materials and approaches. Should a longer design life be required consideration will need to be given to use of more durable materials or capping Options 1 and 2, such as stainless steel. The consequence of failure of a shaft cap are potentially high, given the possibility of people or wildlife falling into the open shaft. Potential failure modes include degradation of the structural elements and erosion and collapse of the soil or rock around the shaft collar. For capping Options 1 and 2 we would expect that regular inspections (perhaps annually) would be carried out to determine whether the shaft closure materials are undamaged and functioning. Should any evidence be observed of erosion around the shaft cap consideration would need to be given to a repair strategy. We would expect that vegetation around the shaft cap would need to be evaluated to identify and mitigate any potential risk of damage to the capping material. Once constructed, Option 3 backfill is not anticipated to require regular monitoring or collar maintenance. Should an adverse event occur within the area of the shafts, e.g., earthquake, fire, or other natural hazard, we would expect the site to be inspected and potential for damage and need for repair be assessed.
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