EGL Ref: 9049 23 June 2022 Page 27 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. The average modelled settlements from potential vein or fault drawdown due to seepage into the tunnel is 85 mm. The additional tilt due to the Wharekirauponga Access Tunnel dewatering effects are very low (1:6,400 to 1:19,000) due to its depth and indicate any related potential for damage to structures is low. For this assessment of effects settlements from the Wharekirauponga Access Tunnel east of Waihi East are expected to be in the order of 10 to 100 mm, depending on the length of the tunnel and the geological structures it passes. If the shallow water table is not affected, tilts will be very small. EGL recommends that the Wharekirauponga Access Tunnel alignment remains in the andesite. Like Correnso Underground, a 40 m minimum offset from the interface between the top of the andesite and top of the tunnel workings could be required as a consent condition. Further investigation of the tunnel alignment would be required ahead of construction to confirm the offset is achieved. Increased settlement monitoring and groundwater monitoring is recommended over the tunnel alignment. The increased monitoring reflects good practice in managing any risks associated with potential dewatering of the andesite and fault structures along the back of Waihi East. Groundwater monitoring in the form of nested piezometers (like the P90 and P100 series holes in the area of Correnso Underground) is recommended at regular intervals along the tunnel alignment. Dewatering in the younger volcanics has been found in the past to be a good indicator of potential settlement issues (see Section 5.3.3). The No. 9 Fault structure extends from the MUG decline to the Wharekirauponga Access Tunnel behind Waihi East (See Figure 39) and passes beneath privately owned dwellings. It is assumed that it is partially dewatered by the RL823m incline between Favona Underground and Trio Underground. A piezometer borehole in this area is recommended. EGL recommends that Zones 2 and 3 are extended partially up SH2 as shown on Figure 66 and Figure 69 to allow for the additional settlement from the Wharekirauponga Access Tunnel at depth in the andesite rock. 6.1.5. Depressurisation around the Willows Access Tunnel and Wharekirauponga Dual Tunnel The Willows Access Tunnel and Dual Access Tunnel to the WUG will be in andesite. Where the tunnel is at depth, average settlements and tilts at the surface are expected to be small. An assessment of potential magnitude of settlement was made for the tunnel when it is at 120 m depth. Most of the depressurisation is within the andesites. The stiffness applied to the regolith (alluvium, ash, residual soil) and weathered surface of the andesite was 1,000 MPa (this is a simplification assuming that the shallow groundwater is perched above the andesite), and the stiffness of the unweathered andesites was assumed to be 8,400 MPa. The calculated maximum settlement was 10 mm, and the tilt was 1:15,000. This is a small settlement owing to localised
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