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that values from stability charts used for short-term analysis gave quick results relevant to the
stability of an embankment.
Spencer’s charts produced a factor of safety of 1.5 which was 1.5% higher than Bishop’s
simplified method (see Table 5.12). This was consistent with the results from Case 1, where
Spencer’s method calculated a suitably similar factor of safety and the value from Bishop and
Morgenstern’s stability charts was an overestimate (in this case by 22%) since no pore water
pressures were considered).
5.4.2
Undrained analysis
Table 5.11: Case 3 factors of safety from spreadsheets and SLOPE/W for undrained conditions
Fellenius
Janbu
simplified
Morgenstern
and Price
Spencer
General
limit
equilibrium
0.921
0.921
n/a
n/a
n/a
n/a
SLOPE/W
0.921
0.921
0.864
0.921
0.921
0.921
Spreadsheet
0.908
0.907
n/a
n/a
n/a
n/a
SLOPE/W
0.906
0.906
0.879
0.906
0.906
0.906
No. of
slices
5
20
Method
Bishop
simplified
Spreadsheet
0.906
Elevation (m)
15
10
5
0
0
5
10
15
20
25
30
35
40
Distance (m)
Figure 5.10: Most critical slip circle in undrained SLOPE/W analysis with 20 slices
Since the foundation had a greater cohesion and bulk unit weight (compared to the
embankment fill), the deep-seated failure only occurred in the weaker material of the
embankment, with a small section of local translational failure. The factor of safety had
improved, from the situation in Case 1, by approximately 20%, indicating that the effect of a
firm stratum as a foundation can improve the factor of safety. However since the failure
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