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5.5.3
Drained analysis
Table 5.15: Case 4 factors of safety from spreadsheets and SLOPE/W for drained conditions
Method
Bishop
simplified
Fellenius
Janbu
simplified
Morgenstern
and Price
Spencer
General
limit
equilibrium
Spreadsheet
1.434
1.333
n/a
n/a
n/a
n/a
SLOPE/W
1.438
1.326
1.335
1.439
1.439
1.439
Spreadsheet
1.426
1.477
n/a
n/a
n/a
n/a
SLOPE/W
1.426
1.340
1.349
1.427
1.426
1.427
No. of
slices
5
20
1.426
Elevation (m)
15
10
5
0
0
5
10
15
20
25
30
35
40
Distance (m)
Figure 5.14: Most critical slip circle in drained SLOPE/W analysis with 20 slices
The failure surface in Case 4 was similar to that in Case 3, however the factor of safety had
decreased by approximately 3.5% with the addition of the phreatic surface. The slight
difference in the shape of the failure surface was due to the foundation layer consisting of soil
with a greater bulk unit weight. It was found previously that the foundation layer increased
the factor of safety, however, now that the phreatic surface was also included, the factor of
safety decreased, showing that the inclusion of water in the embankment reduces stability.
5.6
Summary
The deterministic analysis of the four baseline cases assumed that the soil was homogenous
and isotropic giving the embankment constant properties. In reality, this would not be true due
to variations in the materials: the data used in any analysis would have to be idealised as sets
of values used to describe the average soil properties (namely cohesion, bulk unit weight, and
angle of shearing resistance). Nevertheless, the baseline cases were used to identify
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