EGL Ref: 8983 23 June 2022 Page 12 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. 5.4. Colluvium Layers of colluvium are typically encountered on the hills. The thickness and extent of colluvium is shown in Figure 12. The colluviummay be encountered below or above the ash layers and consists of ash material mixed with reworked rhyolite materials. The colluvium layers are typically sandy or gravelly silt with some cobble size material. In some locations it may also be slightly clayey. The thickest layer of colluvium encountered was 3.3m thick to the northeast of the site. 5.5. Alluvium Alluvium is encountered over the floor of the valley and in hillside gullies. Alluvium can also be found on the positive topography at the toe of the steeper hills and is expected to be encountered on the terrace area where the main Storage 3 stockpile is located. The thickness and extent of alluvium is shown in Figure 12. The alluvium is derived from the erosion of the surrounding rhyolite hills and transportation of material in surface water runoff and streams. The nature of the alluvium varies across the site from gravelly and sandy, to clayey. There are also some buried organic or peaty layers in the northeast and in the lowest valley floor. 5.6. Rhyolite – Tuff and Lava Flows An extensive field investigation has been undertaken to determine the extent, depth and nature of the bedrock profile. The deep GT020 borehole encountered rhyolite from 125 m depth up to the surface. The base of the profile is the remnants of lava flows grading upwards into a welded tuff (air fall deposits) closer to the surface. This borehole found the rhyolite unit sits on an epiclastic (volcanic unit of variable material) unit over dacite rock. Deep boreholes AP21a and AP22a were specifically drilled to prove the rhyolite bedrock profile is competent to depth at the downstream toe of the embankment. AP21a was drilled to a depth of 80 m and AP22a to 70 m. They encountered a competent rhyolite lava flow profile from close to the ground surface to depth. This lava flow profile is part of the rhyolite flows which form the East Ridge. The embankment abuts into it and it is a competent foundation for the embankment. The boreholes indicate that the top surface of the rhyolite is weathered, and it will need to be undercut at the toe of the embankment slopes. Refer to Sections 11.4 and 11.5. Generally, to the south of the proposed embankment position the rhyolite rock surface deepens and is covered by a complex mix of rhyolite tuffs, pyroclastic flows, gaseous lava flows and reworked rhyolite deposits. Figure 13 shows a site geology plan indicating different tuffs and lava flows. The reworked rhyolite deposits are particularly sensitive to strength loss with notable loading from a large embankment. It is recommended that the embankment avoids these deposits, or they are undercut and replaced with structural fill. A Paleo Gully undercut area is indicated in Drawing 0513 and is further discussed in Section 11.4.
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