EGL Ref: 9018 22 June 2022 Page 17 WAI-985-000-REP-LC-0006_Rev0.docx This report shall only be read in its entirety. 9.3. Subsurface drainage The subsurface drainage shown on Drawing 772 includes subsoil drains installed up the centre of the gullies and around the embankment slope toe. The subsurface drains intercept the groundwater flow immediately beneath the Zone A base earth liner to control groundwater seepage during construction and to collect any leachate that may have seeped through the liner. The drains are formed using drainage metal wrapped in geotextile to provide a filter for finer material. A sand filter (Type A Drainage Metal) is proposed along the top of the subsurface drainage metal (Type B Drainage Metal) to provide some redundancy against precipitate forming on the geotextile and clogging. ABS (acrylonitrilebutadiene-styrene) pipes collect the seepage in the drain metal. ABS pipes are proposed as they are very strong and have performed well in the existing embankments forming Storage 1A and 2. In the long term if the pipes deteriorate and crush the gravel surrounding them will still provide drainage. 9.4. Stockpile base liner system The stockpile embankment is to be underlain by a 0.75 m thick Zone A earthfill liner. Previously the Zone A base liner was 1.5 m thick beneath the East Stockpile and Storage 1A embankment. However, this additional thickness has been shown to provide only minor benefit over 0.75 m thickness. Using a 0.75 m thickness reduces the demand on NAF material, saving it for other critical locations, like capping of the embankments. 9.5. Leachate collector drainage Leachate collector drains are installed over the top of the Zone A base liner within the embankment. The drains are constructed from Type C Drainage Material which is a gravelly cobble. Drains are proposed at the slope toe and up the gullies on top of the Zone A base earth liner as shown in Drawing 770. 9.6. Northern Uphill Diversion Drain and stream diversion The extension of the Northern Uphill Diversion Drain and stream diversion is to be constructed before the foundations are stripped in the area of the TB1 stream. The clean water diversion minimises the amount of water needing to be treated for sediment and, once PAF material is being placed, treatment for potential contaminants. The uphill and perimeter drains are shown in plan on Drawing 0715 and in cross section in Drawings 0730, 0731, 0732, to 733. 9.7. Surface water Runoff from the ground above the NRS is intercepted and diverted by the uphill diversion drain that is to be constructed. This includes diversion of the stream TB1 which is a tributary of the Ohinemuri River.
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