EGL Ref: 9049 23 June 2022 Page 24 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. and north sides of Gladstone Hill due to dewatering caused by Favona and Trio Underground developments. The areas to the north are already dewatered. Settlement around the south and east side of Gladstone Hill can be expected to be of a similar magnitude. The Processing Plant contains the only structures close to the pit and has experienced settlement due to the Favona dewatering. It is noted that on the west side of Gladstone Hill is a surficial landslide feature in extremely weak ignimbrites and tuffs which has recorded up to 540 mm of settlement. These settlements can be expected to continue and are likely a combination of surficial downslope movement and drawdown settlement. A series of additional settlement monitoring points around the remaining Gladstone Hill are recommended to provide an understanding of the settlement effects close to the pit, noting that the pit is proposed to become a TSF on closure (Ref. 1). EGL recommends that the settlement zones around GOP and over Union Hill are adjusted to remove Zone 1. Zone 1 is no longer representative of the settlement levels in this area. Proposed adjustments are shown on Figure 66 and Figure 69. 6.1.2. Potential dewatering in the andesite by the MUG decline The Gladstone Open Pit will require the existing Favona portal (access to MUG) and associated decline to be relocated. OGNZL is considering two options, one of which will be developed to provide a replacement portal in GOP, and a short length of new decline to link that portal to the existing underground workings to provide access to MUG. The other is a portal established just north of the existing conveyor, with the new decline running under the southern end of Moore Street linking into the existing development under Union Hill. This assessment considers the option of a portal north of the conveyor. The alternative portal and decline option, sited within the Gladstone Open Pit, remains south of the existing conveyor, and is not overlain by any built infrastructure. Any related settlement effects on privately or publicly owned property would therefore be of lesser effect than the portal north of the conveyor. The area of the proposed MUG Decline option under Moore Street is partially depressurised in the andesite, possibly by the incline between Favona Underground and Trio Underground at RL823m, the Favona Underground, existing Favona Decline to the east, the existing Trio Decline to the South, or the dewatering of the interconnected Martha vein system. The proposed MUG Decline will be at RL1115m to RL1040m, a much shallower depth than the incline at RL823. The decline might intersect historic workings and also might intersect the top of the No. 9 fault/vein structure. The OGNZL geological model for the area indicates that the overlying younger volcanics (ignimbrites and tuffs) interface with the andesites at RL1074m. The proposed decline will come close to younger volcanics and historic workings which are interpreted to extend back to Moore Street, Boyd Road and Barry Road area. This presents a risk of dewatering the younger volcanics and this risk will 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
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