4.3.2.1. 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 19. The failure modes selected for further
assessment are based on current industry guidelines and on engineering judgement.
Table 19. 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 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
4.3.2.2. Consequence Category Assessment
A CCA was prepared for Upper Mundic Gully TSF in accordance with the criteria outlined in ANCOLD
and DETSI. An overview of the methods is provided in full in Appendix C and summarised below:
4.3.2.2.1.
ANCOLD Guidelines
According to the ANCOLD [1] [2] guidelines, the following two consequence categories need to be
assessed as part of a TSF design, considering the risk to human life and severity of damage and loss
estimates:
1 - Dam Failure Consequence Category: consequence assessed against the impacts of a catastrophic
release of water and tailings following an embankment breach.
2 - Environmental Spill Consequence Category: assessment considering the effects of spilling of water
from the dam during a flood or extreme wet period.
Risk to human life is expressed either as Population at Risk (PAR) or Potential Loss of Life (PLL):
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Wulguru Technical Services Pty Ltd – Supporting Information to Amend an Environmental Authority
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