EGL Ref: 9018 22 June 2022 Page 8 WAI-985-000-REP-LC-0006_Rev0.docx This report shall only be read in its entirety. interpretation. They do not show the andesite at depth as this is too deep to affect the design of the NRS. 4.2. Topsoil Across the site is a layer of topsoil which typically varies in thickness from 0.1 m to 0.4 m. The variation in thickness is shown in Figure 10. There are some locations where the thickness of topsoil is locally thicker, up to 0.6 m, likely due to their position near gully features. In some areas fill may be encountered at the surface above the topsoil layer due to placement of material as part of the current mine operation. 4.3. Volcanic ash The NRS site is partially covered in a sequence of volcanic ash. The ash covering on the rhyolite hills to the east is highly variable. However, some inference can be made that the gullies appear to be absent of ash. This translates to the lower topography where natural overland flow paths appear to have eroded the ash cover. The coverage of ash based on the spot locations is shown in Figure 11. Indicative lines in Figure 11 have been drawn to illustrate areas where ash layers appear to be mostly continuous and where they are not. Of the locations where ash was identified, the thickness ranged from 0.2 to 6.5 m, with an average thickness of approximately 1.1m. At boreholes WRS1a and MW2C the ash layers appear to total 4 to 6.5m thickness, which is thicker than the rest of the site. It is possible some of these layers are alluvium. Three major ash units have been identified: Waihi, Hauparu and Rotoehu. The most widespread is the Waihi Ash Series. Further description about the different ash units is detailed in the GFR (Ref. 5). The volcanic ashes are normally well drained. However, the less cohesive units are prone to erosion. 4.4. Colluvium Layers of colluvium are typically encountered on the hills on the eastern side of the Northern Rock Stack. The colluvium may be encountered beneath a layer of ash. The colluvium is generally a slightly gravelly clayey silt. Strengths are generally firm to stiff. Of the locations where colluvium was identified, the thickness is 0.4 to 2.3 m. The typical area where colluvium is encountered is illustrated in Figure 12. 4.5. Surficial alluvium Alluvium covers the site below the ash soils. It is variable in nature (organic silt, clays and sands, gravels) and generally soft to stiff. The depth of the surficial alluvium layers is summarised in Figure 12. Deeper alluvial layers associated with older erosional surfaces and channels are considered separately in Sections 4.7. The lower terraces consist of stiffer (50-150 kPa) alluvium overlain by volcanic ash soil. The lowest lying areas consist of alluvium that can be soft with strengths as low as 12 kPa ranging up to stiff 100 kPa.
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