EGL Ref: 9018 22 June 2022 Page 26 WAI-985-000-REP-LC-0006_Rev0.docx This report shall only be read in its entirety. 15.1. Northern Rock Stack – stockpile and infrastructure The NRS will be an operational stockpile and will be closed as an engineering landform with rehabilitation to re-establish the land to either pasture or other vegetation. Assessment of civil engineering related aspects in regard to design layout, material quantities and sources, geotechnical stability and construction have been made by EGL in this report. The following effects have been considered by others: • Geochemical assessment of the overburden sources have been made by AECOM (Ref. 13). • Assessment of the groundwater quality effects have been made by GHD (Ref. 6). • Assessment of surface water effects have been made by GHD (Ref. 10). • Assessment of dust effects will be prepared by Beca. • Assessment of noise effects will be prepared by Marshall Day. Past resource consent conditions have set requirements specific to the sizing of key elements. The following recommendations are made regarding the key elements: • Uphill drain sizing The existing consent conditions require the uphill diversion drains to pass a 1 in 10 year flow. The design has already been discussed in Sections 9.6 and 10.3. • Collection pond sizing The collection ponds around NRS are sized to hold the runoff from a 1 in 10 year return period, 72-hour storm. This has already been discussed in Sections 8.4, 9.8 and 10.5. • Zone A base pad earth liner thickness Modelling undertaken by GHD (Ref. 6) has shown that a Zone A base pad earth liner thickness of 0.75 m provides adequate protection to groundwater. This is half the thickness of the Zone A base pad thickness used under the existing TSF embankments. There was surplus NAF material available in previous stages that allowed a conservative thickness for Zone A. We support the adoption of 0.75m for Zone A because there are now limited quantities of NAF material, and there are other important zones where it is required. • Zone G capping liner thickness The existing Zone G thickness is 1.5 m on Storage 1A and 2. EGL has queried if this thickness can be reduced or if Low-PAF could be used for the underside of this layer, allowing for a reduction in NAF thickness. This is to reduce the demand on NAF material. It is recommended that this criterion is amended to be a performance requirement such that it leaves open the possibility for other options. Any amendments would need to be confirmed in consultation with AECOM and GHD.
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