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parks, etc., then the required factor of safety should be high to ensure the consequence of
failure is acceptably low. Further investigation into the consequences of failure for this
tailings dam would be required to determine whether the factors of safety are sufficiently high
enough.
The analyses assumed that in SLOPE/W (2012) the entry and exit ranges were at the ground
surface (i.e. above the rock armour); it could be possible that the failure surface is within the
earth fill layer and passes through the toe of earth fill embankment before passing through the
rock armour. SLOPE/W (2012) does not allow the entry and exit ranges to be defined within
the embankment and it is not practical to investigate the stability without the rock armour
layer, since the weight of the rock armour at the toe increases the stabilising moment, thus
improving the factor of safety. Further analysis, for example, a finite element analysis, would
be suitable in this situation to analyse the earth fill embankment while assuming the rock
armour has no strength.
The depth and location of the bore holes suggest that the properties of the embankment and
foundation have been analysed after the dam has been constructed. The pocket of natural
alluvium at the trial pit beneath the rock armour, and the fact that there is limited foundation
data, suggests that little or no work was performed to increase the bearing capacity of the
foundation before the embankment was constructed. The slope stability analyses performed
using SLOPE/W have assumed that the tailings dam will not fail by bearing failure as a result
of a weak foundation. Therefore, the analysis may have shown that the slopes are stable but
bearing failure of the foundation could potentially lead to failure of the embankment.
Translational sliding between the embankment and foundation is unlikely due to the length
and subsequent weight of the embankment.
The analyses assumed the tailings material has the same bulk unit weight as water (9.81
kN/m3) and the same physical properties. Further investigation would be required to
determine whether the bulk unit weight was slightly different due to the chemical properties
of the tailings and what affect this would have on the stability.
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