EGL Ref: 8983 23 June 2022 Page 21 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. will have the effect of raising the level of effective drainage provided by the drains. Eventually Storage 3 will buttress the toe of Storage 1A in this area. 11.5. Main Embankment across lower valley floor A cross section of the main embankment across the lower valley floor is shown in Drawing 516. Where the main embankment crosses the lower valley floor sensitive redeposited rhyolite soils within a paleo gully constrain the downstream toe position of the embankment. The embankment is positioned so undercut of these sensitive soils is limited to approximately 20 m depth (the sensitive material reaches 34 m depth downstream). The extent of excavation will need to be confirmed during construction. The excavated area will be backfilled with structural fill before the construction of the embankment and drainage systems. This structural fill material will be NAF. The collection pond for Storage 3 is positioned at the toe of the main embankment in the lower valley floor as this is the practical location to collect runoff by gravity. 11.6. Main Embankment over the toe of east rhyolite ridge To the east of the paleo gully the ground rises gradually towards the toe of the hills of the east ridge. The positive topography is the surface of a rhyolite lava flow deposit over 80 m deep (Boreholes AP21a and AP22a). The embankment downstream toe is positioned on this rhyolite flow. The downstream toe is to have a 50 m width (minimum) excavation down to rock with a downstream toe drain installed at the upstream extent of the cut. The depth of weathering of the rhyolite surface varies, and the expected depth of cut is between 4 m to 7 m. Further testing of the strength of the material will be required for detailed design to confirm the likely depth of excavation with confirmation during construction. The rhyolite is covered by 2 m to 3 m of ash and this material will be stockpiled and used for construction of the Zone A liner of the impoundment and embankment. 11.7. Main Embankment against east rhyolite ridge The east rhyolite ridge is a weathered rhyolite flow deposit. It is covered in a surficial layer of colluvium and ash soils. The embankment’s downstream toe sits against the ridge line and eventually wraps across a gully and inside the property boundary to the northeast. The layout is shown on Drawing 510. As shown in cross section in Drawings 518 and 519, the embankment profile is truncated at its downstream toe with the surface of the Zone A base liner sloping upstream. Seepage through the embankment will therefore flow back towards the upstream toe of the embankment. As the embankment ridge line rises towards the east there is an opportunity to place weaker Zone D3 material in the embankment in this area due to the favourable geometry. Indicative locations are shown on Drawings 518 and 519.
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