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The factors of safety were 3.5% below the minimum recommended by ANCOLD (2012) in
Table 3.1. Further investigation of the consequences of failure would be required to determine
whether this factor of safety was acceptable for a tailings dam built with these material
properties, or not. In reality, an embankment would not have homogenous, isotropic soil
properties; there would be variations within the embankment resulting in different local
factors of safety. The values used in this analysis were idealised assumptions deemed to
represent the soil properties for the entire embankment.
Bishop and Morgenstern’s factor of safety from their stability chart was approximately 30%
higher than the values found using limit equilibrium methods in Table 5.4, while Spencer’s
was within 4%.. This showed that Spencer’s stability chart gave a suitable factor of safety for
drained conditions.
Case 2: One layer soil with phreatic surface
Case 2 analysed the same downstream embankment as Case 1 but a phreatic surface was
included to determine the changes in behaviour of each method of analysis and their factors of
safety. The phreatic surface was assumed to occur three metres below the top of the
embankment at x = 0, y = 12; this was where the imagined level to which the retained pond
was allowed to rise. It was assumed that a drain would be placed at the toe of the embankment
so the phreatic surface ends at the toe (Fell et al., 2005).
Elevation (m)
Distance (m)
Figure 5.6: Case 2 embankment geometry with phreatic surface
To ensure the same phreatic surface location was used in SLOPE/W and the spreadsheets, the
pore pressure depth (z) for each slice was entered into the spreadsheet using the pore water
pressures (from SLOPE/W) divided by the acceleration due to gravity (9.81 m/s2).