EGL Ref: 8983 23 June 2022 Page 31 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. 13.0 DESIGN ASSESSMENT 13.1. Embankment geotechnical stability Slope stability analyses for the proposed Storage 3 RL155 embankment have been carried out using limit equilibrium methods outlined in Appendix B. The stability design criteria are outlined in Section 10.1. The analyses have been undertaken for operation and closure groundwater levels for the final profile. Two critical cross sections were analysed. These were Section 1 and Section 2 shown in plan view in Drawing 0513 and in cross section in Drawing 0516 and 0517. Section 1 is through the Paleo Gully and Section 2 is through the end of the rhyolite (lava flow) rock ridge that extends from the east abutment, west along the toe of the proposed embankment. Section 1 and Section 2 also represent different situations in terms of ground works for stability, with the Paleo Gully requiring a full undercut and replacement with engineered fill, and Cross Section 2 requiring instead a 50 m wide downstream shear key cut and initial embankment key cut for stability. The stability assessment considered varying slide surfaces, including through the foundations, Zone A base pad liner and within the embankment. The assessed factor of safety (FOS) values for each loading condition are summarised in Table 8 below. Refer to Appendix B for the calculations and analysis outputs. All NZDSG criteria are meet. Table 8: SUMMARY OF EMBANKMENT SLOPE STABILITY ANALYSIS Design case FOS required by NZDSG Section 1 FOS Section 2 FOS Operational/construction static stability – peak strengths ≥1.5 2.4 2.3 Long term post closure static stability – peak strengths ≥1.5 2.4 2.2 Post-earthquake strength conditions – static stability 1.2 2.3 1.9 Required by NZDSG Section 2 EQ. Slope Disp. Section 2 EQ. Slope Disp. OBE 1 in 150 year Earthquake Slope Displacement Estimate Minor easily repairable damage Less than 0.5cm – Negligible Less than 0.5cm – Negligible SEE 1 in 10,000 year Earthquake Slope Displacement Estimate No release of contents Between 2 and 11 cm – Relatively minor Between 4 and 19 cm - Relatively minor The effect of earthquake displacements on freeboard are considered in Section 1.4.
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