EGL Ref: 9049 23 June 2022 Page 19 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. rather than a rapid response to changes in the drawdown rate. However, Favona Underground dewatering has affected groundwater levels and settlement in its vicinity, at different rates than dewatering around the Martha vein system. Dewatering at Favona Underground from 2005 to 2008 was notably faster than dewatering for Martha Open Pit over the same period. The dewatering of Favona Decline and Martha System are shown together on Figure 4 and Figure 59. Settlements in Zone 5 at Favona (Figure 59) were also at a greater rate than the rest of Zone 5 (Figure 58) over this time. Notably this rapid drawdown of groundwater at Favona Underground did not affect other areas of Zone 5, which were likely already drawdown to lower levels or were not connected to the Favona vein system. There was a significant change in pit water level from November 2010 to November 2011. In general, settlement markers (refer Figure 53 to Figure 61) either showed no noticeable or only a very small increase in the rate of settlement. Where there was a change in rate of settlement it occurred either immediately or within one month following the change in pit water level. However, the rates of settlement reduced to their original levels in approximately six months. Notably settlements continued at Favona until 2014 when underground mining of stopes was completed (see Figure 59). At this point the settlement rate slowed till November 2017. Ground above Favona Underground (Figure 59) has shown a more notable settlement rate when water levels were further drawn down from Correnso Underground, approximately 75m, between November 2017 to January 2019. This indicates that there is now likely some connection between the andesite around the Martha veins and the Favona veins or underground. Dewatering of the Martha veins through the Martha Underground may result in further settlement at Favona, however, further monitoring data is required to confirm. Settlements show a slower response to pit dewatering with increasing distance from the Martha Open Pit and underground mines, as shown by settlement measurements in Zones 1, 2 and 3 (refer to Figure 53, Figure 54, and Figure 55). At these locations settlements are much smaller and the rate of settlement is more uniform with time. Smaller settlements are expected because groundwater drawdown away from the Martha Open Pit and underground mines is less and the rate of settlement is more uniform with time due to the slower rate of depressurisation of the andesite rock mass away from depressurised veins. Settlements have continued to occur, albeit at very slow rates (typically 0.1-1 mm per year), during periods when mine water levels have not changed significantly (July 1998 to January 2000 and November 2006 to November 2010). The most likely explanation is ongoing drainage of the andesite rock mass, with slow depressurisation of the rock away from the vein system occurring due to its low permeability. Condition 7 of resource consent 12860 (and previously condition 10 of the Water Permit 971286) requires that tilt be assessed based on the six-monthly settlement survey data and reported in the Annual Dewatering and Settlement Monitoring report (Ref.6). Tilt gradients are generally very low, less than 1:1,000, in the area of the Waihi township. Tilt of less than 1:1,000 is well below the gradient that would give rise to concern for structures. Over Favona Underground there have been tilts with gradients greater than 1:1,000, however there are no structures in this area, so is not of concern.
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