June 2022 G-01483.84-017-R-Rev0 Because the 21 6.2 Geotechnical Assessment of WRS Stability The site of the preferred WRS was investigated with one fully cored borehole (WFBH010) and six test pits (WFTP033 to WFTP038). The test pits and borehole indicate that the site is underlain by surficial soils to depths of up to 4.5 m consisting of about 0.3 m of topsoil overlying firm to hard sandy/gravelly clays. These surficial soils include insitu highly weathered volcanic rock, weathered volcanic ash and colluvium derived from those materials. The thickness of the surficial soils increases towards the east down the gully with less than 1 m of soil material overlying weathered rock in the west of the footprint. For the stability evaluation we have adopted a generalized geological model with thickness of 1 m of firm clay representing weathered volcanic ash and colluvium, overlying 4.5 m of very stiff to hard clay representing the highly weathered andesite. The underlying bedrock is known from borehole drilling in the area of the WRS location. Bedrock comprises moderately strong Whitiroa Andesite. The model assumes that topsoil has been removed. The tunnel has been investigated using several cored boreholes including WNDD007 at Ventilation Shaft 1, WNDD008 and WNDD011, which were drilled sub parallel to the tunnel alignment. The characteristic properties of the tunnel spoil have been estimated from logs, core photographs and laboratory testing of core from these boreholes. 6.3 Material Properties The material properties of the tunnel spoil and foundation materials in the footprint of the WRS have been characterized as summarized in Table 6. The drained strengths for the surficial soils have been estimated using the available descriptions and laboratory testing. Moderately strong andesite rock has been characterized using modified Hoek-Brown criterion for jointed rock masses based on the assumptions summarized in Table 7. The review of available information indicates that about 50 % of the Willows Access Tunnel will encounter soil materials (including weak rock that will readily break down during tunnelling, transport and placement) and fine grained soils. The remaining 50 % is expected to comprise moderately strong or strong rock that will form bouldery gravel tunnel spoil with relatively minor fines. The strength parameters presented in Table 7 are believed appropriate for such tunnel spoil materials placed as uncompacted fill. We anticipate that ground conditions and tunnel spoil between Ventilation Shaft 1 and WUG will typically be significantly better (i.e., more rock dominated) than the Willows Access Tunnel, but there is currently only minor surface geological information available along that alignment.
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