EGL Ref: 8983 23 June 2022 Page 29 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. could occur if appropriate design, construction, monitoring, and surveillance methods were not followed. However, by adopting appropriate design and construction methods, the risk of these failures is reduced to a very low and acceptable level. A preliminary review of potential failure modes for the proposed Storage 3 has been undertaken. A further detailed review is to be undertaken in detailed design. Potential failure modes that have been identified are summarised in Table 7. The potential failure mechanism is described along with the design mitigation and proposed dam safety monitoring. Table 7: POTENTIAL FAILURES MODES Failure Mode Initiating hazard Potential Failure Mechanism Design Mitigation Dam Safety Monitoring FM1 Normal operation Concentrated seepage within or beneath the embankment leads to internal erosion of embankment and high seepage pressure, resulting in piping failure of embankment Specify Zone B sufficiently wide to have low seepage gradients and very low risk of internal erosion. Supervision of construction and testing of embankment fill to ensure fill complies with specifications. Design water management processes to maintain a tailings beach under normal operating water levels. Visual inspections and seepage collection and monitoring for early detection of seepage and any increases in flow. FM2 Normal operation Weak layer formed during a pause in construction creating potential failure plane and instability. Specify reworking and testing of the embankment surface prior to placement of new material. Include a visual inspection and approval by Designer Visual inspections and monitoring of embankment deformation. FM3 Normal Operation Weak layer within the embankment creating potential failure plane and instability. Careful zoning and selection of material used to construct the new embankment with monitoring, testing and certification of the compacted fill. Visual inspections and monitoring of embankment deformation. FM4 Heavy Rainfall Extreme rainfall events cause rise in the decant pond water and overtop the embankment causing erosion on the face and abutments leading to failure of embankment The maximum decant pond level is limited so that the available air space above to the lowest embankment crest level can accommodate the PMP event with 1.0m freeboard. Visual Inspections and monitoring of decant pond volume and level relative to embankment crest level. FM5 Normal Operation Failure through hidden weak layer in the foundation. Investigate foundation to depth to determine any potential weak planes. Include any weak planes in analysis. Installation of inclinometers at toe of embankment prior to fill placement
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