4
Dam break characterisation
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 Sandstone Gully TSF. This is within the typical range mentioned by Rourke and Luppnow [11], which indicates that post-failure tailings slopes are typically within the range of 5 to 18%.
Table 4.1
Historical failure slopes
Name and location
Construction type
Failure mechanism Seismic liquefaction Seismic liquefaction
Failure average slope (%)
Year
Reference
El Cober, Chile
1965 Upstream
6.1
Dobry & Alvarez [12]
Mochikoshi No. 1 and 2, Japan
1978 Upstream
15.5
Ishihara [13]
Merriespruit, South Africa
1994 Upstream
Overtopping
8.7
Blight & Fourie [14]
Seismic liquefaction
Kayakari, Japan
2011 Downstream
19.4
Ishihara et al. [15]
Cadia, Australia
2018 Upstream
Slumping
14.9
Jefferies et al. [16]
In addition to the 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. Furthermore, in addition to the tailings outflow, all decant water above the lowest intersection point of the decant pond and the tailings failure cone surface will be mobilized. An indicative diagram depicting the failure volume is presented in Figure 4.1 and the estimated cone footprints for the stages are presented in Figure 4.2, Figure 4.3, and Figure 4.4.
Project No PS213278 Sandstone Gully TSF Dam Break and Consequence Category Assessments Mount Morgan Gold Mine Heritage Minerals
WSP January 2025 Page 14
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