EGL Ref: 9049 23 June 2022 Page 28 File: WAI-985-000-REP-LC-0050-Rev0.docx This report shall only be read in its entirety. depressurisation in the tunnel at depth and the stiffness applied to the andesite. The stiffness of the andesite could be less. For example, if the stiffness was half the settlements would approximately double. This would still be a small settlement. As a comparison the settlement calculated for the Wharekirauponga Access Tunnel in the andesite rock mass was 8 mm at the surface (see Section 6.1.4), for a tunnel depth of approximately 160 m. For the Wharekirauponga Access Tunnel the potential for the tunnel to drain multiple veins of fault structures and result in more widespread dewatering of the andesite was considered and this indicated 85 mm of settlement, with very low tilt. It is likely that the access tunnel will pass through vein and fault structures and settlements could be higher. To conclude, settlements from the Wharekirauponga Dual Tunnel and Willows Access Tunnel to WUG are expected to be in the order of 10 to 100 mm, and if the shallow water table is not affected tilts will be very small. There are no buildings near to the access tunnel which could be affected by the effects of access tunnel and any settlement or tilt will not have any noticeable effect on farmland or forest environments. A network of piezometers has already been installed at Willows Farm. EGL recommends that the town settlement monitoring network is extended up Willows Road and Highland Road to the privately-owned dwellings at the end of these roads. No settlement monitoring is required above the decline and access tunnel alignments, as it passes under farmland and forest. Any settlement is not expected to have any effects at the surface for these land uses. 6.1.6. Dewatering around ventilations shafts Potential settlements around the vent shafts located at Willows Road and WUG can be expected due to construction or long-term seepage into the vents. The seepage could be from local drawdown of local perched water tables, and drawdown of deeper groundwater. Settlement effects are expected to be greatest close to the ventilation shafts where the drawdown profile is the greatest. Simplified models of groundwater drawdown were developed by GWS, and EGL applied selected stiffness profiles to two profiles (see Appendix A). The two stiffness profiles considered were for the shaft at Willows Farm (Shaft 1 – 200 m deep) and the first shaft out at WUG (Shaft 2 – 320 m deep). Shaft 2 is the deepest at WUG and is the only shaft with current borehole information. The applied maximum drawdowns at the ventilation shafts were 50 to 65 mbgl. The calculated settlement was 75 mm at Shaft 1 and 140 mm at Shaft 2, with tilts between 1:8,200 and 1:1,400. These settlements are based on initial profiles, which pass through materials with rock stiffnesses. If the other ventilation shafts are located where soil is present the settlements could be greater. Based on the initial calculations, and observed settlements from dewatering adjacent to Martha Pit, settlements around the ventilation shafts are expected to be on the order of 50 to 300 mm. These settlements would
RkJQdWJsaXNoZXIy MjE2NDg3