EGL Ref: 9049 23 June 2022 Page 13 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. was reached in about 1996. This contrasts with the continuing drop in groundwater levels in the deeper andesite rock (P2-1, P2-2 and P8-1). Groundwater level changes and the associated consolidation of the varying thickness of the younger volcanic deposits, which are weaker and more compressible than the andesite, is responsible for much of the settlement that occurred in the early stages of dewatering close to the Martha Open Pit, however, as dewatering has transitioned to being predominantly in the andesite, compression of the andesite has become the more notable contributor for settlement. The andesites host the mineralised veins. The veins are therefore located within the deep andesite groundwater system. The veins are directly affected by dewatering in Martha Open Pit and underground mines and respond rapidly to mine dewatering. The adjacent andesite rock responds slower than the veins due to its low permeability. The veins have good connection due to both their high permeability nature and the intersecting historical mining drives/adits and shafts. Dewatering effects associated with the Martha Open Pit are abruptly attenuated to the north and west of the mine because of faulting. Dewatering extends to the south-west and east of Martha Pit. There is a connection between Martha, Correnso and Trio groundwater systems. The Favona vein groundwater system was originally thought to be hydrologically separate from Martha-Correnso-Trio groundwater system because of the groundwater levels in the andesite. Now the construction of Favona to Trio incline and decline provides a connection of the two systems above RL823m (Ref. 9). Some settlement (10 to 50 mm) had been observed at Favona prior to its development, hence the effects of deep dewatering of the andesite close to Martha Pit were having some minor effect at Favona. Development of the Favona Underground caused significant local drawdown of groundwater in the andesite as indicated by piezometer P64-a on Figure 46, which is now dry below the screen depth 184 m below ground level (mbgl). The Favona Underground drives have reached RL766m (Figure 4), approximately 330 to 360 mbgl. Drawdown of the upper younger volcanic material in this area did not occur, as confirmed by piezometer P64-i (screen depth 30.5 mbgl) (Figure 46). However, some local drawdown of groundwater levels in the younger volcanics may have occurred temporarily during the ventilation shaft construction (Ref. 39). Piezometers P64-d was also drawn down dry during the development of Favona decline, with a screen depth at 53.6 mbgl. Drawdown has also occurred in the andesites south toward Gladstone Hill where the Moonlight Underground was developed. The dewatering reduces further south towards Gladstone Hill (Ref. 27) as shown on Figure 39 (Ref. 9) (see borehole GLD004) where the andesite groundwater levels at depth are close to the level of the Ohinemuri River. 4.4. Martha Underground dewatering The Martha Underground dewatering will occur before and during the WNP. The effects may therefore occur concurrently with WNP. The Martha Underground mine (Project Martha) extends to RL500m. Therefore, the water level will need to be lowered by 205 m below the previously held (Correnso) dewatering level (RL705m). Dewatering for Martha Underground is another 40 m deeper (RL500m) than historic mining. Dewatering will be achieved by the installation of stage pumping chambers within the mine that pump into the existing Correnso
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