June 2022 G-01483.84-017-R-Rev0 Because the 24 6.5 Stability Assessment Slope stability analysis has been undertaken using the limit equilibrium computer program Slide2 from Rocscience™. The analyses have been undertaken for circular and non-circular (block style) failure modes. The Spencer solution method has been used for the analyses of the circular potential failure surfaces. The Janbu simplified method has been used for the analyses of potential block failure surfaces. 6.6 Design Criteria There are no New Zealand standards or guidance for the design of WRS. However, we have adopted stability acceptance criteria of minimum design FoS of 1.3. We consider this appropriate for the short design life of the structure. In our experience, this is a commonly used approach in mines in New Zealand for temporary material storage. 6.7 WRS Cross-Sections Two sections have been analyzed for stability thought the WRS: Section A-A’ and Section B-B’ (Appendix B). Section A-A’ extends east to west through the center of the WRS that follows the gully and intersects the pond at the base of the WRS. Section B-B’ extends east to west on the northern flank of the WRS to illustrate the change in foundation topography of the WRS. The critical potential slip surface analyzed is at the toe of the WRS near the pond in Section A-A’. 6.8 Results of Stability Analyses The results of the stability analyses are summarized Table 9 and Table 10. Results are provided for critical failure surfaces (i.e., those with the lowest Factor or Safety (FoS)). 6.8.1 Static Stability Analyses – Short-Term Case The results of the slope stability analysis has been presented in two scenarios. Scenario 1 assumes that the topsoil has been stripped from the entire WRS footprint but that the underlying weak soils (comprising firm clays) remain in place. The results indicate FoS ranging from 0.97 – 1.14 for Section A-A’ and 0.78 – 1.32 for Section B-B’ for both potential circular and non-circular failure surfaces and assuming the two groundwater conditions described above. The modelling suggests that under these conditions, the WRS would not meet the design acceptance criteria. The toe area, where the foundation geometry slopes steeply and unfavourably towards the toe, is the least stable part of the WRS. Scenario 2 incorporates removal of the weak soils beneath the toe of the WRS. The FoS for the potential circular and non-circular failure surfaces and two groundwater conditions increases the stability to 1.08 – 1.34 for Section A-A’ and 1.11 – 1.45 for Section B-B’. The results suggest that FoS of 1.3 can be achieved (refer Figures A5, B1 and B5) if the surficial weak soils are stripped prior to placement of WRS materials and adequate drainage is accommodated in the toe area to prevent excessive pore pressures from developing in the foundation materials. Further geotechnical investigation of the toe area of the WRS and more detailed stability analysis should be undertaken during detailed design to identify the areal extent and depth of excavation that is required to ensure that the design stability acceptance criteria are met.
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