June 2022 G-14642.71-006-R-Rev0_FINAL 25 6.4.2 Key geotechnical issues The following geotechnical characteristics are expected for the Willows Access Tunnel: The first 100 m of tunnel has a heading direction of about 260° and from chainage 100 m the tunnel heading trends approximately 315° to the first ventilation shaft. Cover thickness above the tunnel is in the range of 120 m to 220 m. The tunnel is inferred to mainly encounter Whiritoa Andesite (comprising phyrric andesite and dacite flows and domes with tuff breccias and lithic crystal tuff). However, the contact with the underlying Waipupu Formation andesite (comprising andesite flows, dacite, locally with quartz phenocrysts; minor tuff breccia, crystal tuff and lacustrine sediments) is interpreted to dip about 20° to the east and cross the tunnel a few hundred metres from Ventilation Shaft 1. Waipupu Formation was not identified in WNDD008. Geological conditions encountered in WNDD008 are summarised in Table 7. A number of lineaments inferred from topographic expression cross the tunnel, with a prominent northeast trend, though these have not correlated with observations of faulting in the core of WNDD008. Core from WNDD008 suggests that about half of the length of the Willows Access Tunnel will encounter very weak, soil-like material (‘poor rock’) and half will be moderately strong rock or better (‘good rock’). Artesian groundwater conditions were encountered in WNDD008 and significant groundwater inflows are expected to occur in some intervals of the decline. In particular, the inflows are anticipated to be significant from within strong, fracture rock units, faults and from permeable unit boundaries. It is anticipated that the Willows Access Tunnel will be excavated using drill and blast techniques.
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