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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.
Table3.1
Summary of failure mechanisms assessed
Failure mode
Rationale for the selection of failure mode
Overtopping
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.
Piping (internal erosion) Seepage through the embankment can occur due to chemical breakdown of the liner or defective liner material or construction, and failure of the underliner 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.
Slope instability/ foundation failure
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.
Project No PS213278 Upper Mundic Gully TSF Stage 1 and 2 DBA and CCA Mount Morgan Gold Mine Heritage Minerals
WSP May 2026 Page 9
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