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The factors of safety calculated by the Case 3 Spreadsheet and SLOPE/W were similar (< 1%
difference), which gave confidence in the use of the SLOPE/W suite to analyse more complex
cases in terms of drained analyses.
5.5
Case 4: Two layer soils with phreatic surface
Case 4 analysed the same embankment as Case 3 but now with a phreatic surface (the same
phreatic surface as Case 2).
Elevation (m)
15
10
Embankment
5
Foundation
0
0
5
10
15
20
25
30
35
40
Distance (m)
Figure 5.12: Case 4 embankment geometry showing two soils and phreatic surface as entered into SLOPE/W
5.5.1
Stability charts results
Table 5.13: Factor of safety from stability charts
Analysis
Undrained
Drained
Taylor
Bishop &
Morgenstern
Spencer
Janbu
0.90
n/a
n/a
0.94
n/a
1.64
1.50
n/a
The short-term factors of safety using Taylor’s and Janbu’s stability charts were the same as
were calculated for Case 3. These values gave close (< 4% difference) representations of what
was calculated in Table 5.14 using Bishop’s simplified method.
The factor of safety using Bishop and Morgenstern’s stability charts was 1.64 (15% higher
than Bishop’s simplified method values in Table 5.15). This factor of safety depended on the
depth to the phreatic surface (zw) which varied for each slice. the resultant factor of safety may
not necessarily be the most critical since the depth to the phreatic surface (zw) was estimated
to the midpoint. Further in situ testing to determine the depth to the phreatic surface could be
undertaken, however it would be difficult to determine the phreatic surface position compared
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