the embankment. The concept engineering to support the tunnel plug is displayed in Figure 31 (refer to
Appendix D for engineering drawings).
Figure 31. Proposed Tunnel Portal Plug Design Section
Piezometers are proposed to be installed within the embankment to monitor the phreatic surface and
detect potential seepage/ discharge of contaminants. The locations of the piezometers are to be
finalised during the next design phase and are typically installed following construction of the Stage 1
embankment.
4.3.8. Flood Potential
A flood potential impact study was undertaken to determine impacts that might occur in the event of a
catastrophic failure of the proposed Upper Mundic Gully TSF. A copy of the flood impact study is
provided in the design report in Appendix D and summarised below.
Given the total volume capacity of both assessed stages of the TSF, a flood included overtopping failure
was deemed as non-credible. The following scenarios were identified as critical in terms of
consequence and adopted in the assessment:
• Sunny day piping failure of the Stage 1 embankment layout; and
• Sunny day piping failure of the Stage 2 embankment layout,
Sunny day piping failure (sometimes referred to as Seepage/internal erosion) is typically triggered by
seepage flows concentrated along a path of high hydraulic conductivity. These seepage paths can be
caused by cracking, combustion, animal activity, high hydraulic gradients, or relict structures in the
foundation or embankment materials. The shear forces exerted by the escaping water can enlarge the
seepage path until a portion of the embankment collapses. This results in a breach through overtopping
and erosion of the caved area. A piping failure event may occur at any time and may not be a direct
result of a storm event. Figure 32 presents an overview of overtopping and piping failure configurations.
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Wulguru Technical Services Pty Ltd – Supporting Information to Amend an Environmental Authority
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