EGL Ref: 8983 23 June 2022 Page 11 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. 5.0 GEOTECHNICAL INVESTIGATIONS AND SITE GEOLOGY Extensive geotechnical investigations have been undertaken for Storage 3. They have been undertaken progressively over 25 years in 1994, 1995, 1996, 1997, 2001, 2007, 2009, 2010, 2017, 2018 and 2020. The investigations are documented in the Geotechnical Factual Report (GFR) for the Storage 3 site (Ref. 16). Figures 5 to 9 show the locations of machine drillholes, hand auger boreholes, test pits and Cone Penetration Tests (CPTs) undertaken for Storage 3. Interpretation of the Storage 3 site geology has the benefit of the deep sterilisation hole (GT020) drilled in 2017. The hole was drilled to 455m at an incline of 50 degrees to horizontal, reaching a vertical depth of 348m, starting from Storage 3 site and extending under the Storage 1A embankment. This deep sterilisation borehole is consistent with the interpreted site geology that has been mapped across the Storage 3 site with other deep boreholes (AP21a to 80m and AP22a 70m). 5.1. Geology Overview Homunga Rhyolite profiles are encountered in all boreholes across the Storage 3 site and effectively forms the bedrock surface on which the Storage 3 embankment is founded. This bedrock surface is encountered at the surface to variable depths up to 125 m. Underlying this rhyolite profile are deeper dacite and andesite rock units. Overlying the three rock units are layers of alluvium and ash. The interpretation of these geological units is described in the following sections, generally from the youngest and shallowest to oldest and deepest. These units are further detailed in the GFR (Ref. 16). 5.2. Topsoil Across the site there is a layer of topsoil, which typically varies in thickness from 0.1m to 0.3m, refer to Figure 10. There are some locations where the thickness of topsoil or organic material is locally thicker, up to 0.7m, likely due to their position near small gully features. All topsoil will need to be stripped from the embankment, collection pond and stockpile footprints and stockpiled for rehabilitation. 5.3. Volcanic Ash There are three volcanic ash units across the site of which the Waihi Ash unit is the most common. The ash layers are typically found blanketing the hills and are usually absent from lower valley floors. Of the locations where ash was identified, the thickness ranged from minimal to 2.2m, with an average thickness of approximately 1.1m. The thickness and extent of ash across the site is indicated in Figure 11. The ash soils require careful conditioning to compact to the required specifications. The ash can be used for construction and will either be worked into the Zone A layer beneath the impoundment or embankment, or stockpiled as a source of material for rehabilitation. They could also be used for Zone B as was done for the initial embankment in Storage 1A, however, sufficient NAF material needs to be stockpiled for closure rehabilitation layers.
RkJQdWJsaXNoZXIy MjE2NDg3