4
4.1 Outflow volumes The outflow volume that is mobilised during a tailings dam failure usually depends on several factors. These include the geometry of the structure and its surrounds, the failure mechanism and previous weather conditions. In this assessment, calculation of the released dam breach volume will consider a conical surface geometry which is estimated by the tailings failure slope and dam breach height. A review of historical tailings failures summarised in Table 4.1 suggests failure slopes within the range of 6 to 20% with an average of approximately 13%. Based on this, a relatively conservative failure slope of 10% has been assumed for failure of the Upper Mundic Gully TSF. This is within the typical range mentioned by Rourke and Luppnow (2015), which indicates that post-failure tailings slopes are typically within the range of 5 to 18%.
Table4.1
Historical failure slopes
Name and location Year
Construction type
Failure mechanism Average slope (%)
Reference
El Cober, Chile
1965
Upstream Seismic liquefaction 6.1
Dobry & Alvarez (1967)
Mochikoshi No. 1 and 2, Japan Merriespruit, South Africa
1978
Upstream Seismic liquefaction 14.3, 16.7 (15.5 average)
Ishihara (1984)
1994
Upstream Overtopping
8.7
Blight & Fourie (2005)
Kayakari, Japan
2011
Downstream Seismic liquefaction 19.4
Ishihara et al. (2015)
Cadia, Australia
2018
Upstream Slumping
14.9
Jefferies et al. (2019)
In addition to post-failure slope, the volume of tailings outflow at each location has been estimated considering the following:
Embankment spatial alignment,
Base elevation estimated from natural ground level along the outside embankment toe,
Base of failure cone taken at the outside embankment toe.
Project No PS213278 Upper Mundic Gully TSF Stage 1 and 2 DBA and CCA Mount Morgan Gold Mine Heritage Minerals
WSP May 2026 Page 12
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