EGL Ref: 9018 22 June 2022 Page 22 WAI-985-000-REP-LC-0006_Rev0.docx This report shall only be read in its entirety. TABLE 6: SUMMARY OF NRS SLOPE STABILITY ANALYSIS Design case FOS required Section 1 FOS Section 2 FOS Stockpile global static stability ≥1.5 1.50 2.27 Post-earthquake global static stability 1.2 1.21 1.24 Earthquake case EQ performance criteria Section 1 estimated slope displacement Section 2 estimated slope displacement 1 in 150 year earthquake slope displacement estimate Damage is minor and easily repairable -Max Operation 0.8 to 4 cm -Max Closure <0.5 cm -Negligible disp. Criteria can be met. -Max Operation <0.5 to 3 cm -Max Closure <0.5 cm -Negligible. Criteria can be met. 1 in 2,500 year earthquake slope displacement estimate No major instability. Damage is recoverable. -Max Operation 11 to 47 cm -Max Closure 0.6 to 5 cm -Minor disp. Criteria can be met -Max Operation 10 to 40 cm -Max Closure <0.5 to 4 cm -Minor disp. Criteria can be met 10.2. Seepage GHD (Ref. 6) assess that there will be very little change to groundwater flow and levels due to placement of the NRS, as most construction works are anticipated to be undertaken above the groundwater table. Existing rainfall recharge will be intercepted by the waste rock and the perimeter drains, however, this will be partially offset by seepage of leachate through the NRS soil liner (Zone A) and discharge to ground. For the purposes of seepage and water quality assessment GHD assumed that the Zone A liner will be under-drained, and that groundwater levels beneath the NRS will be at an elevation lower than that of the subsoil drains. 10.3. Northern Uphill Diversion Drain and stream diversion Previous consents have required the uphill diversion drains to be sized for a 1 in 10 year ARI storm event. The uphill diversion drain presented for this preliminary design has been sized for a 1 in 50 year event. At closure the upper reaches of the existing Northern Uphill Diversion Drain will be redirected to Storage 2, which will provide detention and control of flows in the drain. For the sections of the drain which are only required during operation we recommend that the conditions continue to require a 1 in 10 year flow. For sections of the uphill diversion drain that are to be permanent the drain should be designed to pass a 1 in 100 year flow. This will be able to rely on the detention provided by Storage 2 closure.
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