3
Failure scenarios
3.1
Failure mechanisms
Potential failure modes that could result in an embankment failure were identified as credible for the Upper Mundic Gully TSF, as summarised in Table 3.1. The failure modes selected for further assessment are based on current industry guidelines and on engineering judgement.
Table 3.1
Summary of failure mechanisms assessed
Failure mode
Rationale for the selection of failure mode
The design criteria proposed for the Upper Mundic Gully TSF defines the water storage capacity as the greater between the Design Storage Allowance (DSA) and the 120-hour PMP volume for both stages. Considering the significant storage containment allowance, overtopping failure was not considered a credible failure scenario for Stages 1 and 2. Seepage through the embankment can occur due to chemical breakdown of the liner or defective liner material or construction, and failure of the under liner collection system. Consequently, piping failure driven by a high hydraulic gradient along soil/pipe interfaces in the embankment should be considered. Based on this, piping failure is further considered in the assessment as credible under a fair-weather condition. There are several failure mechanisms that could lead to slope instability or foundation failure, such as seismicity, liner failure leading to saturation of the embankment, reduction in strength of material, poor construction, and changes in geometry (e.g. unauthorised embankment modifications). Slope instability failure was not considered in this assessment due to the low likelihood of failure based on the design profile, and because the failure scenarios described above are expected to have similar or worse consequences than failure due to slope instability.
Overtopping
Piping (internal erosion)
Slope instability / foundation failure
3.2
Failure location
In accordance with the criteria described in Section 2.3, the failure locations for both Stages 1 and 2 were selected to maximise the tailings release. Figure 3.1 and Figure 3.2 display the Upper Mundic Gully TSF failure locations selected for this assessment for each embankment layout. These locations were chosen because they align with the highest section of the embankment and are expected to generate a larger release volume compared to other locations.
Project No PS213278 Upper Mundic Gully TSF Stage 1 and 2 DBA and CCA Mount Morgan Gold Mine Heritage Minerals
WSP JUly 2025 Page 9
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