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0.735
Elevation (m)
15
10
5
0
0
5
10
15
20
25
30
35
40
Distance (m)
Figure 5.3: Case 1 most critical slip circle in undrained SLOPE/W analysis with 20 slices
The range of failure surfaces is shown in Figure 5.4 with the red arcs showing the lower
factors of safety.
Elevation (m)
15
10
5
0
0
5
10
15
20
25
30
35
40
Distance (m)
Figure 5.4 : Distribution of failure circles analysed (GEO-SLOPE, 2007)
The factors of safety calculated in the spreadsheet and in SLOPE/W were all less than one
which suggested that it would not be possible to construct this embankment due to its inherent
instability. Figure 5.3 shows that the undrained failure surface was a deep-seated failure
resembling a mid-point circle, as predicted by Taylor’s and Janbu’s stability charts. The
failure surface contained the largest possible mass within the defined entry/exit points and
elevation which is a result of the undrained cohesion of 20 kPa corresponding to a very soft to
soft soil (AS1726-1993; BS 5930-1999). On comparison, the undrained values from stability
charts were approximately 10% less than those calculated using limit equilibrium methods in
Table 5.3. This difference was attributed to the simplicity of the stability charts as a method.
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