EGL Ref: 8983 23 June 2022 Page 22 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. 11.8. Impoundment against the Northern Slopes The northern slopes of the impoundment will need to be stripped and reworked to construct a HDPE liner up to RL135 and an earth liner above RL135. Where possible local surficial soils can be reworked, however, some overburden material may be required to line the full extent of the impoundment. Typical profiles are shown in Sections 7 and 8 on Drawing 521. 11.9. Impoundment over East Stockpile A Zone B earthfill liner with Zone C1 filter layer will be constructed over the East Stockpile, with the addition of a HDPE geomembrane up to RL135. The East Stockpile area is currently underlain by a Zone A liner. 11.10. Subsurface Drainage The subsurface drainage shown on Drawing 620 includes subsoil drains installed up the centre of the gullies, initial embankment upstream cutoff drain and initial toe drain, and a main embankment downstream toe drain. The subsurface drains intercept the groundwater flow immediately beneath the impoundment and the embankment liners to firstly control groundwater seepage during construction and to collect leachate that may seep through the liner. The drains are formed using drainage metal wrapped in geotextile to provide a filter for finer material. This type of drain has worked satisfactorily with the existing TSFs. A proposed improvement is to replace the geotextile over the top of the drain with a sandy gravel filter (Type A Drainage Metal) placed directly on top of the subsurface drainage metal (Type B Drainage Metal) to provide some redundancy against precipitate forming on the geotextile and clogging. ABS (acrylonitrile-butadienestyrene) pipes collect the seepage in the drainage metal. ABS pipes are proposed as they have performed well for the existing embankments. In the long term, if the pipes deteriorate or collapse, the gravel surrounding them will still provide drainage. Long term closure stability is not reliant on the drains. For closure design they are assumed to be blocked. 11.11. Impoundment Liner System For the base of the impoundment a geomembrane liner on top of a 0.6 m Zone A earth liner is proposed as the liner system. This is shown in Detail B in Drawing 517. The Zone A liner will be constructed on surficial soil or rock and over the subsoil drains. For the embankment up to RL135 a geomembrane liner is proposed over the upstream shoulder low permeability Zone B, as shown on Drawings 517 and 518. Zone B acts as a liner and is approximately 6 to 10m thick. A Zone B liner without geomembrane is proposed above RL135. The use of a geomembrane liner in addition to the Zone A earth liner provides a more robust design. Refer to Section 13.5.
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