3
Failure scenarios
3.1 Failure mechanisms Potential failure modes that could result in an embankment failure were identified as credible for the Sandstone 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
Overtopping
The DFS report [5] defined the water storage capacity of the proposed TSF as the greater between the Design Storage Allowance (DSA) and the 120-hour PMP volume for Stages 1 and 3. For Stage 5, the report indicates that a closure spillway will be constructed to manage PMP events. Therefore, overtopping was not considered a credible failure scenario for Stages 1, 3, and 5. 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.
Piping (internal erosion)
Slope instability / foundation failure
Project No PS213278 Sandstone Gully TSF Dam Break and Consequence Category Assessments Mount Morgan Gold Mine Heritage Minerals
WSP January 2025 Page 9
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