EGL Ref: 9215 23 June 2022 Page 24 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. At the upstream toe of the embankment any seepage from the tailings impoundment is intercepted with the upstream cutoff drain. The initial toe drain and the downstream toe drain also collect any seepage. 4.4. Storage 2 design and construction The embankment forming Storage 2 was originally designed in the 1980’s by Tonkin & Taylor Limited. Construction commenced in 1987 and it was commissioned in 1988. Since 1989 EGL has been the engineering firm responsible for the modifications to the design and construction of the facility as it has been raised to its current height. The design of the Storage 2 embankment was revised in May 1997 to allow for raising to RL156, from the previous design crest of RL152. This was completed in 2001. The crest is now approximately 56 m above the banks of the Ohinemuri River which are approximately RL100. As described in Section 4.3, Storage 2 is zoned. Figure 17 shows the cross-sectional profile of Storage 2. Storage 2 as the first embankment has more weathered rock and earthfill Zone D3 than Storage 1A. This is because the mine overburden from the higher elevations in the Martha Pit was more weathered. FIGURE 17: TYPICAL SECTION OF STORAGE 2 EMBANKMENT The upstream shoulder of the embankment slopes at an average grade of 1 m vertical to 2.8 m horizontal (1V:2.8H) up to RL140 and 1V:1.5H above this. Steeper slopes at higher elevations are possible because of the buttressing effects of the tailings against the upstream shoulder, which is not practical for the initial starter embankment. The downstream shoulder varies from 1V:2.25H to 1V:4.3H between berms. The berms are 4m wide and formed at approximately every 10 m elevation. An extensive network of underdrains and in-embankment leachate drains exists to intercept seepage from the tailings, leachate from the PAF waste rock that forms much of the embankment and groundwater seepage in the underlying ground beneath. Figure 18 shows a location plan of the underdrains.
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