June 2022 G-14642.71-006-R-Rev0_FINAL 43 9.6 Ventilation Shaft Closure at Completion of Mining Given the location of ventilation shafts will be located on HDC paper road reserves in the Coromandel Forest Park, we have reviewed the options for permanent closure of the shafts following completion of mining. The options considered include construction of a structural cap on the shafts or backfilling the shafts with cement stabilised fill. These options are considered feasible and are described in Appendix D. 10.0 CONCLUSIONS Willows Access Tunnel Ground conditions that will be encountered by the tunnel will comprise a sequence of volcanic rock units including andesites, dacites and rhyolites, deposited as flows, tuffs and breccias. These units are inferred to be arranged as layers with contacts that are horizontal or gently inclined. Rock strength is interpreted to be variable and range from strong to very weak. Joint spacing is interpreted to be in the range of highly fractured to moderately widely spaced. Given the inferred variability of the ground, tunnelling conditions are expected to include mixed face conditions, spalling and swelling ground. While the measured mass permeability is relatively low, groundwater inflows into the tunnel are anticipated to be high in fault zones or within brecciated zones. The tunnel portal is located on the side of a narrow ridge at the base of a moderately steep slope. The site appears to be viable for a tunnel portal. The slope above the portal is affected by shallow slope instability and will require measures to ensure that portal security is maintained. Such measures should include management of surface water and shallow groundwater and local support measures such as shotcrete and bolts or a mass gravity wall. Tunnelling will utilise drill and blast methods. Tunnel support requirements are expected to be variable but will generally include pattern rock bolts and shotcrete, with zones of full shotcrete support in poor ground. The relative proportions of ground types cannot yet be confirmed given the lack of subsurface information throughout the tunnel alignment. Ventilation Shaft 1 Ventilation Shaft 1 will be approximately 250 m deep and located near the boundary between Willows Road farm and DOC reserve. Based on borehole WNDD007, Ventilation Shaft 1 will be constructed in andesite rock with strength ranging from very weak to strong. Discontinuities interpreted from bore logs and downhole geophysics comprise a number of prominent sets, including a gently east dipping fabric and moderately steeply dipping discontinuities dipping to the northwest and southwest. Groundwater is inferred to be located at a depth of about 10 m BGL and the ground is interpreted to have a rock mass permeability of about 10-7 m.s-1.
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