EGL Ref: 8983 23 June 2022 Page 20 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. 11.3. Initial Embankment and Impoundment The initial (starter) embankment with a crest of RL135 is located over the upstream half of the main embankment footprint and provides the initial tailings storage capacity for the facility. The approximate initial embankment layout is shown in Drawing 511 and the embankment and impoundment profile is shown in cross section in Drawings 516, 517 and 518. The proposed initial embankment profile has a buttress of material at its downstream toe which extends out to an initial downstream toe drain and shear key positioned for the RL145 embankment (Drawing 512, 516 and 517). This profile is currently proposed as it provides for a more efficient development of storage capacity between RL135 to RL145. The initial embankment abuts Storage 1A at its west extent and the rhyolite hills to the east. The impoundment is then formed against the initial embankment, East Stockpile ignimbrite toe bund and the slopes of the northern hills. The conditions encountered during the first discharge of tailings are different to the following lifts as no consistent layer of tailings is in place within the impoundment. Deposited tailings at Waihi have a relatively low permeability and this limits the amount of seepage from them. The underlying rock and surficial soil cover also do naturally provide some containment. The existing TSFs do not have a liner over the base of the impoundment and rely on hydraulic containment of the site and the cutoff and associated drain at the upstream toe of the embankment and subsurface drains to intercept and collect groundwater seepage. However, additional controls in the form of a lined impoundment are proposed for Storage 3 as an improvement over the previous facilities to limit seepage into the natural environment, and because the depth to rock for a cutoff is too deep to be practical. The existing TSFs have performed well. However, the proposed lining of the impoundment while still retaining the upstream cutoff drain provides an additional level of protection during this early stage of deposition and for the long term. As previously mentioned, the tailings consolidate to a low permeability soil over time which will offset any potential deterioration of the HDPE geomembrane. The earth liner (Zone A fill) will remain in perpetuity. The HDPE geomembrane will also limit seepage to the subsurface drains located beneath the liner. Drawings 621 and 622 show drain details. 11.4. Main Embankment West Abutment with Storage 1A The west side of the Storage 3 embankment abuts Storage 1A. The Storage 1A embankment is constructed of the same bulk material as the Storage 3 embankment and so the two embankment fills can be joined by stripping the outer capping layers, Zones H and G, from the Storage 1A embankment progressively as Storage 3 is raised so Zone D (PAF) material placed in Storage 3 is in contact with the Zone F (PAF) drainage layer of Storage 1A. The perimeter infrastructure for Storage 1A buried by Storage 3 (i.e., subsurface drain collection sumps/pumps and ring main) will be extended to the toe of Storage 3. The drains flowing to the collection sumps at the toe of Storage 1A that will be buried, will need to be extended through the sumps to a gravity outlet. The drains may need to be raised from the sumps to a level which is practical to construct a gravity outlet. This
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