EGL Ref: 9049 23 June 2022 Page 23 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. interface has a lower permeability than the overlying ignimbrites and fresher andesite rock beneath. The modelled drawdown of the groundwater in the ignimbrites by GHD is up to 4 m over the vein position. Depressurisation in the andesite is concentrated around the vein with a steep draw down profile modelled down to RL1005m or 80 m below the current water level in the vein. The stiffness values used in the settlement calculations were selected by EGL. A stiffness value (young modulus) of 1,000 MPa was initially selected for ignimbrite profile which is the stiffness value used at Waihi East (Ref. 7) for softer rocks typical of the younger volcanics (for the purpose of estimating settlement). 2,000 MPa was used for the weathered andesite and 8,400 MPa for the slightly to unweathered andesite. The calculations are summarised in Appendix A. For the vein dewatering associated with Gladstone Open Pit excavation, the average modelled settlement was 36 mm, and the additional tilt effects were very low at 1:4800. Approximately 10 to 20 % of the calculated settlement is attributable to settlement in the ignimbrites and the remaining 80 to 90 % in the andesites. For the assessment of effects, settlement south of the Ohinemuri River can be expected to be in the order of 10 to 50 mm with very low additional tilts. The settlement points (1.12A, 1.22, 1.25) south of GOP were reviewed against the trigger limits for monitoring plan zonation (Figure 62). The settlement in November 2021 in Zone 3 is between 45 and 75 mm. The settlement trigger limit for Zone 3 is 95 mm. The settlements in November are 47 to 78 % of the trigger level, with 73 % of the currently proposed drawdown in the Waihi vein systems undertaken. The settlement (at points 1.26 and AP100) south of GOP in November 2021 in Zone 4 is approximately 110 mm (Figure 63). The settlement trigger limit for Zone 4 is 160 mm. The settlements in November are 69 % of the trigger level, with 73 % of the currently proposed drawdown in the Waihi vein systems undertaken. Notably, settlements south of GOP have followed a relatively consistent trend with time as the Martha and Favona vein systems have been progressively dewatered, and the level of groundwater drawdown proposed for GOP is much less. The vein is indicated to have limited extent (Ref. 37), ending approximately at the Ohinemuri River. Overall, the extent of potential additional settlement from GOP is expected to diminish towards Heath Road and settlements associated with Martha dewatering are expected to continue. For the Heath Road area it is proposed that the settlement zonation remains the same (Zone 3 and 4) and additional monitoring points be added near to the existing dwellings to provide forward warning of any unforeseen settlement effects. GLD004 piezometers are already in place close to the end of the Gladstone vein on the north side of Ohinemuri River to provide indication of dewatering effects in the ignimbrite and andesite. EGL recommends that additional settlement monitoring points are also included in Clarke Street to monitor for any effects southwest of GOP. Settlements can also be expected close to GOP from dewatering of the younger volcanics and andesite close to the existing Gladstone Hill. Past settlements in this area have been on the order of 120 to 200 mm on the west
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