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Table 7.2: Factors of safety and failure type for a typical tailings dam downstream slope using varying assumptions on
the foundation properties and apparent cohesion.
Foundation
type
No foundation
2 m clay
2 m alluvium
1 m clay and
1 m sandstone
2 m sandstone
Foundation properties
ϕ′ (°)
γ (kN/m3)
-
30
28
-
Earth fill
cʹ (kPa)
Factor of
safety
5
2.279
Shallow toe circle
0
2.082
Shallow toe circle
5
1.914
0
1.846
5
1.826
0
1.759
19
18
30
19
5
2.052
36
21
0
1.901
5
2.179
0
2.070
36
21
Failure type
Deep toe circle
through foundation
Deep toe circle
through foundation
Deep toe circle
through foundation
Deep toe circle
through foundation
Deep toe circle
tangential to sandstone
Deep toe circle
tangential to sandstone
Deep toe circle
through foundation
Deep toe circle
tangential to
foundation
When the foundation was not included in SLOPE/W, the software has assumed a stiff stratum,
causing a shallow toe failure arc and relatively high factor of safety. This was also evident
where the clay and alluvium foundations had failure surfaces tangential to the base of the
assumed two metre thick foundation (since the foundations were weaker than the earth fill).
This may not be representative of a real failure surface since the properties and depth of the
foundation are unknown. If the foundation properties at more locations beneath the
embankment and soil properties below the foundation were known then a more realistic
failure surface could be determined. If foundation properties are not known then an
assumption should be made for analysis purposes, if using SLOPE/W, to avoid unrealistic
factors of safety being calculated.
The weathered sandstone had the densest bulk unit weight of all materials in the case study
embankment, it acted as firm stratum below the embankment and so the critical failure surface
did not penetrate through it for the cases of 1 m clay and 1 m sandstone, and 2 m of
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