EGL Ref: 9049 23 June 2022 Page 25 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. a mitigation option to maintain groundwater levels in the younger volcanics. Alternatively, it may be possible to avoid the younger volcanics completely be refining the decline alignment and level so to stay completely within the andesite. Experience from the development of the initial section of the Favona Decline is that the additional settlements could be in order of 10 to 50 mm locally above the decline, if the decline remains in the andesite and assuming the younger volcanics are not drained. Additional settlement monitoring points are recommended in Moore Street, to provide a closer network of points. Piezometers in borehole P75 and P76 already monitor groundwater levels in this area, however, an additional piezometer is recommended closer to the location of the decline to monitor the current groundwater levels above the decline and any future effects. 6.1.3. Wharekirauponga Access Tunnel decline behind Boyd Road and Barry Road The location of the Wharekirauponga Access Tunnel decline (i.e. southern end of Wharekirauponga Access Tunnel) is above the Favona Underground. The andesite in this area is already dewatered and settlements and tilts have occurred, however, no dwellings are present in the area with greatest effects. There is potential for the Wharekirauponga Access Tunnel decline to intersect the younger volcanics (ignimbrites) which potentially connect with ignimbrites at Boyd and Barry Road. The potential for intersection is illustrated by the cross section along the start of the decline shown on Figure 21 (Ref. 40), where the extended path of the top of the decline is close to the base of the younger volcanics (ignimbrites and tuffs). This could dewater the younger volcanics, and there is a risk this could cause settlement effects at Boyd Road and eastern end of Barry Road. This risk needs to be mitigated by OGNZL. Mitigation includes grouting methods to limit groundwater inflows into the decline prior to construction and post-grouting if inflows continue at unacceptable levels. Re-injection of captured groundwater can also be used as a mitigation option to maintain groundwater levels in the younger volcanics. Alternatively, it may be possible to avoid the younger volcanics completely be refining the decline alignment and level so to stay completely within the andesite. No calculations of potential settlements have been undertaken for the decline. Any effects from the decline can be limited to the existing settlement and tilt area above Favona Underground, through the mitigation measures discussed above. Some settlement points over Favona Underground are starting to exceed the settlement trigger levels for Zone 5 (see Figure 59). Review of the settlement values over Favona Underground indicated an increase in settlement after the completion of mining at Favona in 2014. This is likely a delayed settlement response to the Correnso dewatering to RL705m. The settlement increased
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