EGL Ref: 8983 23 June 2022 Page 32 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. 13.2. Embankment seismic shakedown settlements Seismic induced volumetric shakedown settlements of the fill are estimated to be less than 0.2% (Ref. 51) of the depth of fill when subjected to the 10,000 year ground motion. This is because the embankment is rockfill compacted in 0.25 m to 0.5 m thick layers with an expected SPT-N of 35+ and the CSR is less than 0.4. As an estimate of the maximum potential volumetric shakedown settlement, 0.2% of 46 m is 9.2 cm. Shakedown volumetric settlement is in addition to any settlement of the crest due to shear displacements considered in Section 13.1. See Section 13.4 for freeboard scenarios. 13.3. Embankment consolidation settlement Monitored settlements of the installed embankment deformation monitoring points on the finished surfaces indicates fill settlements are small. Most of the settlements of Storage 1A are less than 0.3 % of the depth of embankment fill. On Storage 2 they are up to 0.7%. For Storage 3 a settlement ratio of 0.7% has been applied for an initial assessment resulting in 0.32 m (0.7% x 46 m above rock) post construction. Both the potential static embankment settlement and potential fill volumetric shakedown settlement are not likely to be critical in the long term. They are easily manageable by setting the level of the closure outlet channel to allow for any potential future settlements. Section 13.4 considers the effects of embankment settlement regarding freeboard. Any effects are easily managed in design and construction, to prepare the facility for closure. 13.4. Freeboard scenarios The likely freeboard scenarios are summarised in Table 9. Specific freeboard calculations are undertaken for the different situations in operation and for closure. No scenarios compromise freeboard and risk overtopping. Initial estimates indicate that for Storage 3, the top of the tails beach will need to be at least 2.5 m below the minimum crest level and the normal operation water level will need to be 3.5 m below the minimum crest level. This allows for storage of the IDF (from a 72 hour PMP) above the maximum normal water operation level, with 1.0 m of freeboard remaining. The current freeboard criteria in the resource consent condition is 1.0 m above the 72 hour PMP level, and it is also the minimum freeboard level under the IDF recommended by the NZDSG.
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