June 2022 G-14642.71-006-R-Rev0_FINAL 22 6.4 Tunnelling Issues 6.4.1 Ground conditions The 1300 m long decline between the tunnel portal and the base of Ventilation Shaft 1 will have approximate dimensions of 5.5 m by 5.5 m and a gradient of 7H:1V. Ground conditions were investigated using borehole WNDD008 (1043 m long), which is approximately parallel to the proposed Willows Access Tunnel. This has been used to provide geotechnical interpretation, in addition to surface geological mapping by OGNZL. WNDD008 was logged by an engineering geologist from GHD and a summary of the geotechnical characteristics from the log is presented in Table 7. Table 7: Summary of findings from WNDD008 summarised from the GHD bore log. Drillhole interval (m) Geological description Geotechnical characteristics Comments 0.0 to 16.7 Sandy SILT Firm-stiff soil, low plasticity 16.7 to 41.5 TUFF Completely weathered, very weak rock 41.5 to 65.7 Porphyritic ANDESITE Highly - moderately weathered, weak rock 65.7 to 74.5 DACITE Moderately weathered, very weak to weak rock Possible fault gouge at 68.5 to 70.2 m 74.5 to 448.0 LAPILLI TUFF, TUFF breccia Moderately weathered, weak rock Andesite dykes/flows at 81.5 m (6 m core length), 102.0 m, (2 m core length), 113.5 m, (3 m core length), 120.5 m (7 m core length), 227.1 m (1 m core length) 448.0 to 519.0 Porphyritic ANDESITE Slightly weathered, Strong to very strong rock 8.0 m of completely weathered and very weak andesite for between 448.0 – 456.0 m 519.0 to 622.5 Porphyritic ANDESITE Moderately strong, moderately weathered rock 14.25 m of completely weathered very weak andesite between 608.25 – 622.5 m 622.5 to 724.6 Porphyritic ANDESITE Slightly weathered, Strong to very strong rock 14 m of highly weathered, very weak andesite between 710.6 – 724.6 m 724.6 to 785.0 LAPILLI TUFF Moderately weathered, moderately strong rock Andesite dyke/flow at 735.5 m (4 m core length)
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