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dam failure incidents per year. This equates to an annual probability of failure between 1 in
700 (0.14%) and 1 in 1750 (0.06%).
Types of failure
An analysis of potential failure methods, shapes, and locations is required to determine the
stability of an embankment. The Australian Government LPSDP (2007) cites a summary of
the main causes of failure from ICOLD Bulletin 121 as “…a lack of control of the water
balance, lack of control of construction and a general lack of understanding of the features
that control safe operations”. The most common types of failure are “…slope instability,
earthquake loading, overtopping, inadequate foundations and seepage” (DITR, 2007).
This thesis focuses on analysing slope instability failure mechanisms (predominantly
rotational sliding), however it is important to note that there other causes of failure that can
contribute to embankment instability. Davies (2002) states that “each and every failure is
entirely predictable in hindsight”. Hence, the importance of understanding the different failure
types, and what causes a tailings dam embankment to fail, is demonstrated.
Rotational sliding
In two-dimensional analysis, the failure surface can be approximated by a circular arc, often
called a slip circle. There are an infinite number of possible failure arcs (shown in Figure 2.7),
however, failure will occur at the most critical slip circle with the lowest factor of safety
(Horn, 1960). This occurs when the shear stress (𝜏𝑚𝑜𝑏 ) along the failure circle is equal to, or
greater than, the shear strength of the soil (𝜏𝑓 ), i.e. when the factor of safety is less than 1. The
shear strength resisting slope movement must be greater than the shear stress created by the
self-weight (W) of the sliding mass for the slope to be stable (Sivakugan, 2007).
Figure 2.6: Failure surface